research paper black holes

Black Holes

Black holes are some of the most fascinating and mind-bending objects in the cosmos. The very thing that characterizes a black hole also makes it hard to study: its intense gravity. All the mass in a black hole is concentrated in a tiny region, surrounded by a boundary called the “event horizon”. Nothing that crosses that boundary can return to the outside universe, not even light. A black hole itself is invisible.

But astronomers can still observe black holes indirectly by the way their gravity affects stars and pulls matter into orbit. As gas flows around a black hole, it heats up, paradoxically making these invisible objects into some of the brightest things in the entire universe. As a result, we can see some black holes from billions of light-years away. For one large black hole in a nearby galaxy, astronomers even managed to see a ring of light around the event horizon, using a globe-spanning array of powerful telescopes.

Center for Astrophysics | Harvard & Smithsonian scientists participate in many black hole-related projects:

Using the Event Horizon Telescope (EHT) to capture the first image of a black hole’s “shadow”: the absence of light that marks where the event horizon is located. The EHT is composed of many telescopes working together to create one Earth-sized observatory , all monitoring the supermassive black hole at the center of the galaxy M87, leading to the first image ever captured of a black hole. CfA Plays Central Role In Capturing Landmark Black Hole Image

Observing supermassive black holes in other galaxies to understand how they evolve and shape their host galaxies. CfA astronomers use telescopes across the entire spectrum of light, from radio waves to X-rays to gamma rays. A Surprising Blazar Connection Revealed

Studying the infall of matter — called “accretion” — onto black holes, using NASA’s Chandra X-ray Observatory and other telescopes. In addition, CfA researchers use cutting-edge supercomputers to create theoretical models for the disks and jets of matter that black holes create around themselves. Supermassive Black Hole Spins Super-Fast

Hunting for black hole interactions with other astronomical objects. That includes “disruption” events, where black holes tear stars or other objects apart, creating bursts of intense light. Black Hole Meal Sets Record for Length and Size

Observing clusters of stars to find intermediate mass black holes, and modeling how they shape their environments. A Middleweight Black Hole is Hiding at the Center of a Giant Star Cluster

Hunting for and characterizing stellar mass black holes, which can include information about their birth process and evolution. NASA's Chandra Adds to Black Hole Birth Announcement

The Varieties of Black Holes

Black holes come in three categories:

Stellar Mass Black Holes are born from the death of stars much more massive than the Sun. When some of these stars run out of the nuclear fuel that makes them shine, their cores collapse into black holes under their own gravity. Other stellar mass black holes form from the collision of neutron stars , such as the ones first detected by LIGO and Virgo in 2017. These are probably the most common black holes in the cosmos, but are hard to detect unless they have an ordinary star for a companion. When that happens, the black hole can strip material from the star, causing the gas to heat up and glow brightly in X-rays.

Supermassive Black Holes are the monsters of the universe, living at the centers of nearly every galaxy. They range in mass from 100,000 to billions of times the mass of the Sun, far too massive to be born from a single star. The Milky Way’s black hole is about 4 million times the Sun’s mass, putting it in the middle of the pack. In the form of quasars and other “active” galaxies , these black holes can shine brightly enough to be seen from billions of light-years away. Understanding when these black holes formed and how they grow is a major area of research. Center for Astrophysics | Harvard & Smithsonian scientists are part of the Event Horizon Telescope (EHT) collaboration, which captured the first-ever image of the black hole: the supermassive black hole at the center of the galaxy M87.

Intermediate Mass Black Holes are the most mysterious, since we’ve hardly seen any of them yet. They weigh 100 to 10,000 times the mass of the Sun, putting them between stellar and supermassive black holes. We don’t know exactly how many of these are, and like supermassive black holes, we don’t fully understand how they’re born or grow. However, studying them could tell us a lot about how the most supermassive black holes came to be.

Black holes can seem bizarre and incomprehensible, but in truth they’re remarkably understandable. Despite not being able to see black holes directly, we know quite a bit about them. They are …

Simple . All three black hole types can be described by just two observable quantities: their mass and how fast they spin. That’s much simpler than a star, for example, which in addition to mass is a product of its unique history and evolution , including its chemical makeup. Mass and spin tell us everything we need to know about a black hole: it “forgets” everything that went into making it. Those two quantities determine how big the event horizon is, and the way gravity affects any matter falling onto the black hole.

Compact . Black holes are tiny compared to their mass. The event horizon of a black hole the mass of the Sun would be no more than 6 kilometers across, and the faster it spins, the smaller that size is. Even a supermassive black hole would fit easily inside our Solar System.

Powerful . The combination of large mass and small size results in very strong gravity. This gravity is strong enough to pull a star apart if it gets too close, producing powerful bursts of light. A supermassive black hole heats gas falling onto it to temperatures of millions of degrees, making it glow brightly enough in X-rays and other types of radiation to be seen across the universe.

Very common . From theoretical calculations based on observations, astronomers think the Milky Way might have as many as a hundred million black holes, most of which are stellar mass. And with at least one supermassive black hole in most galaxies, there could be hundreds of billions of supermassive black holes in the observable universe.

Very important . Black holes have a reputation for eating everything that comes by, but they turn out to be messy eaters. A lot of stuff that falls toward a black hole gets jetted away, thanks to the complicated churning of gas near the event horizon. These jets and outflows of gas called “winds” spread atoms throughout the galaxy, and can either boost or throttle the birth of new stars, depending on other factors. That means supermassive black holes play an important role in the life of galaxies, even far beyond the black hole’s gravitational pull.

And yes, mysterious . Along with astronomers, physicists are interested in black holes because they’re a laboratory for “quantum gravity”. Black holes are described by Albert Einstein’s general relativity, which is our modern theory of gravity, but the other forces of nature are described by quantum physics. So far, nobody has developed a complete quantum gravity theory, but we already know black holes will be an important test of any proposed theory.

The first image of a black hole in human history, captured by the Event Horizon Telescope, showing light emitted by matter as it swirls under the influence of intense gravity. This black hole is 6.5 billion times the mass of the Sun and resides at the center of the galaxy M87.

What do black holes look like?
What happens to space time when cosmic objects collide?
The Energetic Universe
The Milky Way Galaxy
Extragalactic Astronomy
Stellar Astronomy
Theoretical Astrophysics
Einstein's Theory of Gravitation
Radio and Geoastronomy

Related News

Cfa astronomers help find most distant galaxy using james webb space telescope, astronomers unveil strong magnetic fields spiraling at the edge of milky way’s central black hole, black hole fashions stellar beads on a string, m87* one year later: proof of a persistent black hole shadow, unexpectedly massive black holes dominate small galaxies in the distant universe, unveiling black hole spins using polarized radio glasses, a supermassive black hole’s strong magnetic fields are revealed in a new light, nasa telescopes discover record-breaking black hole, new horizons in physics breakthrough prize awarded to cfa astrophysicist, cfa selects contractor for next generation event horizon telescope antennas, dasch (digital access to a sky century @ harvard), sensing the dynamic universe, champ (chandra multiwavelength project) and champlane (chandra multiwavelength plane) survey, telescopes and instruments, einstein observatory, event horizon telescope (eht), large aperture experiment to explore the dark ages (leda), the greenland telescope, very energetic radiation imaging telescope array system (veritas).

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Black Holes

Black holes are the most extreme manifestation of the force of gravity. When enough matter, whether from the remnants left at the end of the life of a massive star, or gas at the center of a galaxy, is compressed into a small enough volume, the force of gravity (the mutual attraction, pulling everything together) becomes so strong that no other force can match it. The matter is pulled together and eventually becomes so compact and dense that its gravitational force becomes so strong that nothing can escape it, not even light. At that point it is said to have formed a black hole, and the point of no return --- the distance around it from which nothing can escape --- is called the event horizon. Scientists at KIPAC are working to understand many aspects of black holes: how they formed, how they grew, the exact processes by which energy is released as material falls into them, and the process by which some black holes are able to launch jets.

Black holes with masses comparable to that of the Sun (or, stellar mass black holes) are scattered through the Milky Way and neighboring galaxies, and are formed when the most massive stars come to the ends of their lives. In addition, supermassive black holes, a million to a billion times more massive than the Sun, sit in the centers of galaxies. This includes Sagittarius A*, which is found at the heart of our own Milky Way galaxy. Black holes are difficult to observe directly, since no light can escape from within the event horizon. Fortunately, when matter falls into a black hole, it becomes superheated and produces an intense source of light before it crosses the horizon.

Out of Darkness... Light

We can observe stellar mass black holes in our galaxy when they feed on material from a companion star in an X-ray binary. When gas falls into a supermassive black hole at the center of a galaxy, it lights up active galactic nuclei (AGN) or quasars, the most powerful continuous sources of light we can see from the farthest reaches of the Universe. On top of impressive light shows, some black holes are able to launch streams of material into jets. Jets travel close to the speed of light; some can even span great distances, reaching far out of their host galaxies. KIPAC scientists use observations of black holes and their jets taken by X-ray telescopes (Chandra, Swift, XMM-Newton and NuSTAR), the Fermi gamma-ray telescope, and a wide range of optical and radio telescopes. These observations are then coupled with theoretical models and computer simulations to map out and understand the extreme environments around various types of black holes.

The X-rays that are emitted from a corona of energetic particles close to the black hole shine down on the accretion disk of gas spiralling in, allowing KIPAC astronomers to map out the extreme environment just outside the event horizon. In radio galaxies and radio-loud AGN, jets that can tap into the energy of a spinning black hole emanate vast distances out of the host galaxy, carrying vast amounts of kinetic energy into the surrounding gas. At the same time, powerful winds carry energy out into the galaxy. When jets point toward Earth, the AGN is called a blazar. Blazars are strongly variable in all observable bands of the electromagnetic spectrum, and simultaneous observations with instruments sensitive to different wavelengths of radiation are critical for studying them. KIPAC scientists study the gamma-ray emission from blazars to understand jets and their relation to the black holes and the accretion disks, as well as the contributions blazars have made to the evolution of the Universe as a whole.

Monsters Lurking in the Centers of Galaxies

The total energy output from the supermassive black holes in AGN and quasars is comparable to the total energy holding the constituent stars of their galaxies together. This means that the supermassive black holes in the centers of galaxies must have played an important role in the formation and growth of the galaxy, through a process known as AGN feedback. As galaxies grew bigger from the gas falling into them from outside, and the supermassive black holes at their centers grew with them, the energy output from the black holes would have been able to push gas away from the galaxy, slowing its growth, and controlling the formation of stars throughout the galaxy.

KIPAC scientists study how the supermassive black holes in the centers of galaxies interact with their surroundings. By studying not just the processes by which energy is released by black holes, but how AGN evolve over time, and the population of supermassive black holes that are found in different galactic environments, they are working to piece together how black holes grew with their host galaxies and the role they played in governing the growth of structure in our Universe.

Visualization of a simulated black hole with jets. (Visualization: Ralf Kaehler Simulation: Jonathan McKinney, Alexander Tchekhovskoy, Roger Blandford.)

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Combined X-ray and optical observations on left and simulated gas column density on right, with inset of artist's impression of active supermassive black hole

How do supermassive black holes get super massive?

UNIVERSITY PARK, Pa. — By combining forefront X-ray observations with state-of-the-art supercomputer simulations of the buildup of galaxies over cosmic history, researchers have provided the best modeling to date of the growth of the supermassive black holes found in the centers of galaxies. Using this hybrid approach, a research team led by Penn State astronomers derived a complete picture of black-hole growth over 12 billion years, from the Universe’s infancy at around 1.8 billion years old to now at 13.8 billion years old.

The research comprises two papers, one published in The Astrophysical Journal in April 2024, and one as yet unpublished that will be submitted to the same journal. The results will be presented at the 244th meeting of the American Astronomical Society, held June 9 through June 13 at the Monona Terrace Convention Center in Madison, Wisconsin. The results were featured during a press conference that was livestreamed and is available to view at the AAS Press Office YouTube page .

“Supermassive black holes in galaxy centers have millions-to-billions of times the mass of the Sun,” said Fan Zou , a graduate student at Penn State and first author of the papers. “How do they become such monsters? This is a question that astronomers have been studying for decades, but it has been difficult to track all the ways black holes can grow reliably.”

Supermassive black holes grow through a combination of two main channels. They consume cold gas from their host galaxy — a process called accretion — and they can merge with other supermassive black holes when galaxies collide.

“During the process of consuming gas from their hosting galaxies, black holes radiate strong X-rays, and this is the key to tracking their growth by accretion,” said W. Niel Brandt , Eberly Family Chair Professor of Astronomy and Astrophysics and professor of physics at Penn State and a leader of the research team. “We measured the accretion-driven growth using X-ray sky survey data accumulated over more than 20 years from three of the most powerful X-ray facilities ever launched into space.”

The research team used complementary data from NASA’s Chandra X-ray Observatory, the European Space Agency’s X-ray Multi-Mirror Mission-Newton (XMM-Newton), and the Max Planck Institute for Extraterrestrial Physics’ eROSITA telescope. In total, they measured the accretion-driven growth in a sample of 1.3 million galaxies that contained over 8,000 rapidly growing black holes.

“All of the galaxies and black holes in our sample are very well characterized at multiple wavelengths, with superb measurements in the infrared, optical, ultraviolet, and X-ray bands,” Zou said. “This allows for robust conclusions, and the data show that, at all cosmic epochs, more massive galaxies grew their black holes by accretion faster. With the quality of the data, we were able to quantify this important phenomenon much better than in past works.”

The second way that supermassive black holes grow is through mergers, where two supermassive black holes collide and merge together to form a single, even more massive, black hole. To track growth by mergers, the team used IllustrisTNG , a set of supercomputer simulations that model galaxy formation, evolution, and merging from shortly after the Big Bang until the present.

“In our hybrid approach, we combine the observed growth by accretion with the simulated growth through mergers to reproduce the growth history of supermassive black holes,” Brandt said. “With this new approach, we believe we have produced the most realistic picture of the growth of supermassive black holes up to the present day.”

The researchers found that, in most cases, accretion dominated black-hole growth. Mergers made notable secondary contributions, especially over the past 5 billion years of cosmic time for the most-massive black holes. Overall, supermassive black holes of all masses grew much more rapidly when the Universe was younger. Because of this, the total number of supermassive black holes was almost settled by 7 billion years ago, while earlier in the Universe many new ones kept emerging.

“With our approach, we can track how central black holes in the local universe most likely grew over cosmic time,” Zou said. “As an example, we considered the growth of the supermassive black hole in the center of our Milky Way Galaxy, which has a mass of 4 million solar masses. Our results indicate that our Galaxy's black hole most likely grew relatively late in cosmic time.”

In addition to Zou and Brandt, the research team includes Zhibo Yu , graduate student at Penn State; Hyungsuk Tak , assistant professor of statistics and of astronomy and astrophysics at Penn State; Elena Gallo at the University of Michigan; Bin Luo at Nanjing University in China; Qingling Ni at the Max Planck Institute for Extraterrestrial Physics in Germany; Yongquan Xue at the University of Science and Technology of China; and Guang Yang at the University of Groningen in the Netherlands.

Funding from the U.S. National Science Foundation, the Chandra X-ray Center, and Penn State supported this work. The work was also made possible by the sharing of the IllustrisTNG simulation results with the scientific community.

Black Holes

Center for Astrophysics | Harvard & Smithsonian scientists participate in many black hole-related projects:

Observing clusters of stars to find intermediate mass black holes, and modeling how they shape their environments. A Middleweight Black Hole is Hiding at the Center of a Giant Star Cluster

Hunting for and characterizing stellar mass black holes, which can include information about their birth process and evolution. NASA's Chandra Adds to Black Hole Birth Announcement

The Varieties of Black Holes

Black holes come in three categories:

Black holes can seem bizarre and incomprehensible, but in truth they’re remarkably understandable. Despite not being able to see black holes directly, we know quite a bit about them. They are …

What do black holes look like?
What happens to space time when cosmic objects collide?
The Energetic Universe
The Milky Way Galaxy
Extragalactic Astronomy
Stellar Astronomy
Theoretical Astrophysics
Einstein's Theory of Gravitation
Radio and Geoastronomy

Related News

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Two black holes colliding and resulting gravitational waves

Credit: Texas Advanced Computing Center

Black holes have long inspired the imagination yet challenged discovery. However, from a combination of theory and observation, scientists now know much about these objects and how they form, and can even see how they impact their surroundings.

So, how does one study a region of space that is defined by being invisible?

Theorists can calculate properties of black holes based on their understanding of the universe, and such discoveries have come from a range of great thinkers, from Albert Einstein to Stephen Hawking to Kip Thorne. However, despite being so powerful, it's hard to see something that does not emit photons, let alone traps any light that passes by.

Now, nearly a century after scientists suggested black holes might exist, the world now has tools to see them in action. Using powerful observatories on Earth, astronomers can see the jets of plasma that black holes spew into space , detect the ripples in space-time from black holes colliding , and may soon even peer at the disc of disrupted mass and energy that surrounds the black hole's event horizon, the edge beyond which nothing can escape.

Event Horizon Telescope

Sagittarius a*.

Credit: Event Horizon Telescope Collaboration

First image of the black hole at the center of the Milky Way

This is the first image of Sagittarius A*, or Sgr A*, the supermassive black hole at the center of our galaxy. It's the first direct visual evidence of the presence of this black hole. It was captured by the Event Horizon Telescope (EHT), an array which links together eight existing radio observatories across the planet to form a single Earth-sized virtual telescope. The telescope is named after the "event horizon", the boundary of the black hole beyond which no light can escape.

Although we cannot see the event horizon itself, because it cannot emit light, glowing gas orbiting around the black hole reveals a telltale signature: a dark central region, called a "shadow," surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is 4 million times more massive than our sun. The image of the Sgr A* black hole is an average of the different images that the EHT Collaboration has extracted from its 2017 observations.

News Release

We got it! Astronomers reveal first image of the black hole at the heart of our galaxy

This result provides overwhelming evidence that the object is indeed a black hole and yields valuable clues about the workings ...

2022 EHT Panelists PDFs

EHT Press Conference Panelists (PDF, 1.36 MB)

Public Q&A Panelists (PDF, 2.25 MB)

Credit: National Science Foundation/Keyi "Onyx" Li

Download a high-resolution version of this image with no labels (JPEG, 1MB)

While Sgr A* is the supermassive black hole in the center of our own galaxy, the supermassive black hole M87* resides more than 55,000,000 lightyears from Earth.

Credit: Keyi "Onyx" Li/National Science Foundation; Lia Medeiros, Institute for Advanced Study

The supermassive black holes M87* and Sgr A* are not even in the same galaxy, but if it were possible to place them next to each other, Sgr A* would be dwarfed by M87*, which is 1,500 times more massive.

Science Matters

Image of Sgr A*, the black hole at the center of our galaxy

At the center of our very own Milky Way galaxy, scientists long suspected that there was a supermassive black hole, and they named this black hole Sagittarius A* (Sgr A*, pronounced "sadge-ay-star").

Related PDFs

EHT Black Hole Poster (PDF, 754.37 KB)
Sgr A* black hole color palette (PDF, 915.99 KB)
NSF Press Conference Poster (PDF, 3.07 MB)

EHT virtual backgrounds

virtual background for eht with telescopes and starry background

Credit: C. Padilla, NRAO/AUI/National Science Foundation

virtual background eht Sgr A* Black Hole

2019 EHT Event

On April 10, 2019, the U.S. National Science Foundation hosted scientists from the Event Horizon Telescope Collaboration at a press conference in Washington, D.C. and revealed the world’s first image of a black hole.

Astronomical experiments at geographical South Pole

Credit: Dr. Daniel Michalik

The event was the North American pillar of a simultaneous, global announcement, with NSF hosting due to its pivotal role in the discovery, having spent two decades investing in researchers, radio telescopes, and facilities that anchored the project. The content below tells the story of that image, how it was captured, and how it was revealed.

Astronomers capture first image of a black hole

National Science Foundation and Event Horizon Telescope contribute to paradigm-shifting observations of the gargantuan black hole.

2019 Related PDFs

EHT Poster w/ Black Hole Image (PDF, 565.79 KB)
EHT Factsheet (PDF, 265.91 KB)
NSF Press Conference Panelist Bios (PDF, 759.69 KB)
History of NSF Support for VLBI (PDF, 295.57 KB)

For the full suite of images, animations, explanatory videos, and other multimedia content from the press event, see the Images, video, and educational resources page

Return to top

NSF’s Laser Interferometer Gravitational-wave Observatory (LIGO)

Credit: T. Pyle/Caltech/MIT/LIGO Lab

A century ago, Albert Einstein predicted gravitational waves, ripples in the fabric of space-time that result from the universe's most violent phenomena. In 2016, NSF researchers using one of the most precise instruments ever made—the NSF Laser Interferometer Gravitational-wave Observatory (LIGO)—announced the historic first detection of gravitational waves, the violent remnant of black holes colliding more than 1.3 billion years ago.

LIGO Special Report LIGO Caltech

Andrea Ghez and the UCLA Galactic Center Group

Star makes closest approach to black hole in Milky Way

Credit: Nicolle R. Fuller, National Science Foundation

The Galactic Center Group studies the black hole at the heart of the Milky Way and how it impacts its surroundings, a multi-decade effort to better understand how galaxies formed and evolved. In 2020, Ghez shared the Nobel Prize in Physics for her discoveries, which confirmed the presence of a black hole at our galactic center.

The Galactic Center Group

NSF's National Radio Astronomy Observatory (NRAO)

Discovery of cool, interstellar gas around black hole

Credit: NRAO/AUI/National Science Foundation; S. Dagnello

NSF’s NRAO manages several powerful radio telescopes that capture unprecedented images of the cosmos, including plasma jets and other evidence of black holes.

NSF’s NOIRLab

Credit: NOIRLab/NSF/AURA/P. Marenfeld

NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab) is the United States' flagship center for ground-based, nighttime optical and infrared astronomy. NSF's NOIRLab manages five observatories and centers located across the globe: Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), Gemini Observatory, Kitt Peak National Observatory (KPNO) and the Vera C. Rubin Observatory, once it becomes operational.

NSF’s Green Bank Observatory

Credit: National Science Foundation/GBO 20; photo by Jill Malusky (available under Creative Commons

NSF's Green Bank Observatory enables leading edge research at radio wavelengths by offering access to telescopes, facilities, and advanced instrumentation to the global scientific and research community.

Green Bank Observatory

South Pole Telescope

Credit: Dr. Keith Vanderlinde

The South Pole Telescope is a 10-meter-diameter microwave / millimeter / sub-millimeter telescope located at the U.S. National Science Foundation's Amundsen-Scott South Pole Station, which is the best currently operational site on Earth for mm-wave survey observations due to its stable, dry atmosphere.

South Pole telescope

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Published: 17 January 2024

A small and vigorous black hole in the early Universe

Roberto Maiolino ORCID: orcid.org/0000-0002-4985-3819 1 , 2 , 3 ,
Jan Scholtz 1 , 2 ,
Joris Witstok ORCID: orcid.org/0000-0002-7595-121X 1 , 2 ,
Stefano Carniani ORCID: orcid.org/0000-0002-6719-380X 4 ,
Francesco D’Eugenio 1 , 2 ,
Anna de Graaff ORCID: orcid.org/0000-0002-2380-9801 5 ,
Hannah Übler ORCID: orcid.org/0000-0003-4891-0794 1 , 2 ,
Sandro Tacchella ORCID: orcid.org/0000-0002-8224-4505 1 , 2 ,
Emma Curtis-Lake ORCID: orcid.org/0000-0002-9551-0534 6 ,
Santiago Arribas ORCID: orcid.org/0000-0001-7997-1640 7 ,
Andrew Bunker ORCID: orcid.org/0000-0002-8651-9879 8 ,
Stéphane Charlot ORCID: orcid.org/0000-0003-3458-2275 9 ,
Jacopo Chevallard 8 ,
Mirko Curti ORCID: orcid.org/0000-0002-2678-2560 10 ,
Tobias J. Looser ORCID: orcid.org/0000-0002-3642-2446 1 , 2 ,
Michael V. Maseda ORCID: orcid.org/0000-0003-0695-4414 11 ,
Timothy D. Rawle ORCID: orcid.org/0000-0002-7028-5588 12 ,
Bruno Rodríguez del Pino 7 ,
Chris J. Willott ORCID: orcid.org/0000-0002-4201-7367 13 ,
Eiichi Egami ORCID: orcid.org/0000-0003-1344-9475 14 ,
Daniel J. Eisenstein ORCID: orcid.org/0000-0002-2929-3121 15 ,
Kevin N. Hainline 14 ,
Brant Robertson ORCID: orcid.org/0000-0002-4271-0364 16 ,
Christina C. Williams ORCID: orcid.org/0000-0003-2919-7495 17 ,
Christopher N. A. Willmer ORCID: orcid.org/0000-0001-9262-9997 14 ,
William M. Baker ORCID: orcid.org/0000-0003-0215-1104 1 , 2 ,
Kristan Boyett ORCID: orcid.org/0000-0003-4109-304X 18 , 19 ,
Christa DeCoursey 14 ,
Andrew C. Fabian ORCID: orcid.org/0000-0002-9378-4072 20 ,
Jakob M. Helton 14 ,
Zhiyuan Ji 14 ,
Gareth C. Jones ORCID: orcid.org/0000-0002-0267-9024 8 ,
Nimisha Kumari ORCID: orcid.org/0000-0002-5320-2568 21 ,
Nicolas Laporte ORCID: orcid.org/0000-0001-7459-6335 1 , 2 ,
Erica J. Nelson ORCID: orcid.org/0000-0002-7524-374X 22 ,
Michele Perna 7 ,
Lester Sandles ORCID: orcid.org/0000-0001-9276-7062 1 , 2 ,
Irene Shivaei 14 &
Fengwu Sun 14

Nature volume 627 , pages 59–63 ( 2024 ) Cite this article

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Early universe
Galaxies and clusters

An Author Correction to this article was published on 17 May 2024

This article has been updated

Several theories have been proposed to describe the formation of black hole seeds in the early Universe and to explain the emergence of very massive black holes observed in the first thousand million years after the Big Bang 1 , 2 , 3 . Models consider different seeding and accretion scenarios 4 , 5 , 6 , 7 , which require the detection and characterization of black holes in the first few hundred million years after the Big Bang to be validated. Here we present an extensive analysis of the JWST-NIRSpec spectrum of GN-z11, an exceptionally luminous galaxy at z = 10.6, revealing the detection of the [Ne iv ] λ 2423 and CII* λ 1335 transitions (typical of active galactic nuclei), as well as semi-forbidden nebular lines tracing gas densities higher than 10 9 cm −3 , typical of the broad line region of active galactic nuclei. These spectral features indicate that GN-z11 hosts an accreting black hole. The spectrum also reveals a deep and blueshifted CIV λ 1549 absorption trough, tracing an outflow with velocity 800−1,000 km s −1 , probably driven by the active galactic nucleus. Assuming local virial relations, we derive a black hole mass of \(\log ({M}_{{\rm{BH}}}/{M}_{\odot })=6.2\pm 0.3\) , accreting at about five times the Eddington rate. These properties are consistent with both heavy seeds scenarios and scenarios considering intermediate and light seeds experiencing episodic super-Eddington phases. Our finding explains the high luminosity of GN-z11 and can also provide an explanation for its exceptionally high nitrogen abundance.

A possible direct exposure of the Earth to the cold dense interstellar medium 2–3 Myr ago

No massive black holes in the Milky Way halo

Assembly theory explains and quantifies selection and evolution

GN-z11 was recently observed with JWST. The analysis of the NIRCam images revealed an unresolved nuclear component and a disc-like component with a few 100 parsec (pc) radius 8 . A first NIRSpec spectrum was presented in ref. 9 , which found it to be consistent with star formation, although the presence of an active galactic nucleus (AGN) was not excluded. Here we explore the latter scenario using a deeper spectrum of GN-z11.

Figure 1a shows the detection of the [Ne iv ] λλ 2422,2424 doublet. As NeIV requires photons more energetic than 63.5 eV, this line is an unambiguous AGN tracer 10 , 11 , 12 and not seen in star-forming galaxies, not even those hosting the Wolf–Rayet (WR) stars 13 .

Dashed lines indicate the rest-frame wavelengths of the lines at z = 10.603. a , [Ne iv ] λλ 2422,2424 doublet; b , NIII] multiplet, illustrating the detection of the resolved N iii ] λ 1754 emission; c , NIV] doublet, showing the absence of [N iv ] λ 1483 despite the strong N iv ] λ 1486; d , CIV blueshifted absorption trough and redshifted resonant emission, compared with the CIV P-Cygni profile observed in low-metallicity, young star-forming galaxies (stack: orange dashed line; most extreme case: orange dotted line), showing inconsistency with the latter. e , CII/CII* λλ 1334,1335 doublet (seen in emission, without P-Cygni, only in type 1 AGN); f , Expected flux of the NIV1718 line in the case that NIV]1486 was associated with WR stars. In a – c , e and f , the continuum is subtracted, whereas in d the continuum is normalized to one. The grey dotted lines indicate the noise level (1 σ ).

We also detect CII* λ 1335 emission (Fig. 1e ). This line is commonly observed in AGN 14 , 15 , 16 . In star-forming galaxies, this line is generally totally undetected; when detected, it is extremely faint and always associated with deep CII λ 1334 resonant absorption 17 , not seen in GN-z11.

The [N iv ] λ 1483, N iv ] λ 1486 doublet is very sensitive to the gas density (and insensitive to ionization parameter, metallicity and shape of the ionizing spectrum). Figure 1c shows the detection of the semi-forbidden N iv ] λ 1486 line (critical density 4.7 × 10 9 cm −3 ) and the non-detection of the forbidden [N iv ] λ 1483 line (critical density 1.5 × 10 5 cm −3 ), which indicate densities much higher than 10 5 cm −3 . Specifically, the various photoionization models shown in Fig. 2a (see the Methods for details) illustrate that the upper limit on the doublet ratio requires densities greater than about 10 6 cm −3 , which are incompatible with the densities of the ionized interstellar medium (ISM) that are typically in the range of 10−10 3 cm −3 , and only rarely approach a few times 10 4 cm −3 (ref. 18 ).

A large range of Cloudy models ( Methods ) are compared with the values observed in GN-z11. Models with metal-poor ( Z neb = 0.1 Z ⊙ ) and metal-rich ( Z neb = 1 Z ⊙ ) gas are shown with solid lines and dashed lines, respectively (colour-coded according to the ionization parameter U ), in the scenario in which either an AGN (filled symbols demarcating different black body temperatures for the accretion disc, T AGN ) or stellar populations (open markers for various ages, t * ) is responsible for the incident radiation field. a , [N iv ] λ 1483/N iv ] λ 1486 flux ratio. b , Ratio of N iii ] λ 1754 to total flux of the multiplet. The black dashed lines and blue-shaded regions (in decreasing darkness for 1 σ , 2 σ and 3 σ confidence level as indicated) show the observed fractional contribution of N iii ] λ 1754 and upper limit on [N iv ] λ 1483/N iv ] λ 1486 obtained for GN-z11, indicating that the gas emitting these lines has high density ( n H ≳ 10 9 cm −3 at 3 σ ). The light-green-shaded areas highlight the range of densities typical of the broad line regions (BLRs), whereas the grey-shaded regions highlight the range of densities typical of the ionized interstellar medium (ISM).

Even stronger constraints come from the NIII] multiplet (Fig. 1b ). This is contributed primarily by four semi-forbidden lines at 1,748.6 Å, 1,749.7 Å, 1,752.2 Å and 1,754.0 Å (and the much weaker 1,746.8 Å). All of these have high critical densities (>10 9 cm −3 ), but the 1,748.6 Å and 1,754.0 Å transitions have the highest critical density of 10 10 cm −3 (note that atomic physics requires fixed flux ratios F 1754 / F 1748 = 1.05 and F 1746 / F 1752 = 0.14; Methods ). The 1,754.0 Å line is well resolved compared with the rest of the multiplet, and its intensity is well constrained to be 0.25 ± 0.04 of the total intensity of the multiplet. This high ratio can be achieved only when both the 1,749.7 Å and 1,752.2 Å transitions are suppressed relative to the 1,748.6 Å and 1,754.0 Å because of the very high density. As shown in Fig. 2b , when compared with the expectations from a wide range of photoionization models, the observed ratio requires a density higher than 10 9 cm −3 at 3 σ (higher than 10 10 cm −3 at 2 σ ). These high densities are completely inconsistent with any HII regions in any star-forming galaxy but are fully in the realm of the broad line regions (BLRs) of AGN, which are characterized by extremely high densities (about 10 9 −10 15 cm −3 ).

Therefore, the most plausible explanation is that GN-z11 hosts an AGN and that these semi-forbidden lines observed in its spectrum are mostly emitted by the associated BLR.

It may seem puzzling that the spectrum of GN-z11 does not seem to show the typical ‘broad lines’ seen in type 1 quasars and AGN, with widths of thousands km s −1 . However, the width of the broad lines scales quadratically with the black hole mass, hence in the case of ‘small’ black holes the broadening is expected to be substantially smaller. Moreover, there are classes of type 1 AGN that have broad lines with widths less than 1,000 km s −1 : these are the so-called Narrow Line Seyfert 1 (NLSy1), the permitted lines of which are broader than their forbidden lines, but not by a large factor, and in many cases reaching a width of only a few 100 km s −1 (ref. 19 ), and which are inferred to have small black holes (about 10 6 M ⊙ ) (ref. 20 ). This seems to be the case of GN-z11, in which the semi-forbidden (NIII], NIV]) and permitted lines (MgII) all have widths between 430 km s −1 and 470 km s −1 , whereas [NeIII] has a significantly narrower width (340 ± 30 km s −1 ), hence coming from the host galaxy (either from HII regions or from the narrow line region of the AGN). We note that the interpretation of some permitted and semi-forbidden lines, such as the Balmer lines and CIII] is made complex by the fact that these are also generally contributed to by the ISM photoionized by star formation in the host galaxy.

The spectrum of GN-z11 also shows a deep (EW rest ≈ 5 Å) and blueshifted absorption trough of the CIV λλ 1548,1550 doublet (Fig. 1d ). Deep CIV absorption is sometimes observed in young stellar populations, but the depth observed in GN-z11 would require high metallicities, typically solar or super-solar 21 . This is in contrast with the metallicity inferred from the nebular lines of GN-z11 ( Z ≈ 0.1 Z ⊙ ) (ref. 9 ). To illustrate more quantitatively the inconsistency with the stellar-wind origin, the orange dashed line in Fig. 1d shows the stacked spectrum of local galaxies with metallicity around the value inferred for GN-z11, resampled to the NIRSpec grating resolution: the stellar trough is much shallower than that observed in GN-z11 and with a completely different shape. Apart from the stellar origin, this deep CIV absorption is seen also in lower redshift star-forming galaxies and associated with galactic outflows 22 . However, in these cases the outflow velocities are only of a few 100 km s −1 ( Methods ), whereas for GN-z11 the CIV trough traces a much faster outflow of around 800−1,000 km s −1 . A more plausible explanation of the deep blueshifted trough of CIV is that GN-z11 is part of the class of broad absorption line (BAL) quasars, which are characterized by deep absorption of blueshifted CIV by up to several thousands km s −1 . Actually, GN-z11 would fit in the ‘mini-BAL’ category, with velocities between 500 and 2,000 km s −1 , more common in lower luminosity AGN, or in the ‘narrow’ (approximately 1,000 km s −1 ) absorption line (NAL) quasars category 23 . The spectrum also shows a clear CIV redshifted emission, which is probably tracing the receding component of the outflow. As CIV is a resonant line, this is the counterpart of the redshifted Lyα identified in ref. 9 (consistent shift and width).

In sum, the detection of [NeIV] and CII*, the extremely high gas density matching those of the AGN BLRs, and the presence of a deep, blueshifted absorption trough of CIV tracing a high-velocity outflow are all consistent with the scenario in which GN-z11 hosts an accreting black hole, that is, an AGN, specifically what would be called NLSy1 and (mini-)BAL/NAL AGN.

In the Methods , we also discuss other diagnostics, such as the ratio of UV transitions (for example, CIII]/CIV, CIII]/HeII) and the upper limits on high ionization lines (NV λλ 1238,1242 and [Ne v ] λ 3426), are fully consistent with the AGN scenario.

Some works have suggested that GN-z11 may host a population of WR stars 24 . The HeII λ 1640 line shows a potentially broad profile, as shown in Extended Data Fig. 1b (although the wings are mostly in the noise). This, if confirmed, could come from the inner region of the BLR but could also trace the presence of a WR population. However, various other features are inconsistent with the main contribution from WR stars. Specifically, in the case of WR stars, the NIV λλ 1483,1486 doublet, if present, is always accompanied by an even stronger NIV λ 1718 line, with a prominent P-Cygni profile, which is not seen at high confidence in the spectrum of GN-z11 (ref. 13 ) (Fig. 1f ); [NeIV] and CII* are never seen associated with WR stars 13 ; when present, the NIII] multiplet has a much weaker λ 1754 component 18 . Therefore, if WR stars are present in GN-z11, then they must co-exist with the AGN and are unlikely to play a dominant part in the excitation of the observed nebular lines.

Assuming local virial relations, the black hole mass can be estimated from the line widths and continuum luminosity. As discussed in the Methods , we estimate a black hole mass of about 1.6 × 10 6 M ⊙ . In the Methods , we also discuss potential uncertainties and caveats in the determination of the black hole.

We infer a bolometric luminosity of the AGN of 10 45 erg s −1 ( Methods ), which is a factor of about 5 higher than the Eddington limit (with an uncertainty of a factor of 2). Super-Eddington accretion is generally inferred for NLSy1s and is one of the scenarios proposed to rapidly grow supermassive black holes in the early Universe 4 , 25 .

Figure 3 shows how the black hole mass in GN-z11 would have evolved at earlier cosmic epochs if accreting at the Eddington rate, or at the super-Eddington rate estimated at the time of observation. For comparison, the grey-shaded areas show the range of possible black hole seeds scenarios: black holes resulting from the direct collapse of primordial clouds into seeds with masses in the range of about 10 4 −10 6 M ⊙ , the so-called direct collapse black holes (DCBHs); rapid merging of stars and black holes in dense, nuclear star clusters; accretion onto Population III black hole seeds or even normal stellar remnants 1 , 9 JWST galaxies hosted massive black holes? Mon. Not. R. Astron. Soc. 521, 241–250 (2023)." href="#ref-CR3" id="ref-link-section-d48084151e1981">3 , 4 , 5 , 6 . Many of these semi-analytical models and cosmological simulations could reproduce the mass of GN-z11 at z = 10.6 (refs. 7 , 25 , 26 , 27 ). The solid and dashed lines show the evolutionary tracks for some of them (described more extensively in the Methods ). These can be broadly divided in models assuming heavy seeds (DCBH), whose accretion is limited to the Eddington rate, and intermediate mass (stellar clusters) or light (stellar remnants) seeds experiencing episodes of super-Eddington accretion. It is interesting to also note that GN-z11 evolving at sub-Eddington rate can easily result into the supermassive black holes (10 7 −10 9 M ⊙ ) observed in quasars at z = 6–7, as predicted by many models.

The black hole mass inferred for GN-z11 is shown with the large golden symbol. The red-shaded region indicates the evolution expected in the case of super-Eddington accretion at the level inferred for GN-z11. The darker-blue-shaded region shows the black hole mass evolution assuming Eddington-limited accretion, whereas the lighter-blue-shaded region shows the case of evolution in the case of sub-Eddington accretion (between 0.1 and 1 the Eddington rate). The horizontal grey-shaded regions indicate the range of black hole seeds expected by different scenarios. Solid and dashed lines indicate the evolutionary tracks of various simulations and models 7 , 25 , 35 that can reproduce the GN-z11 black hole mass, with different seeding and accretion rate assumptions, as detailed in the Methods . The small grey symbols indicate the black holes measured in quasars (QSOs) at z ≈ 6–7.5 (refs. 1 , 2 ) (whose representative 1 σ error bar is shown in the top left), most of which can originate from a progenitor such as the black hole in GN-z11.

However, it is possible that the local or low- z scaling relations do not apply for AGN at such early epochs. If we disregard the local virial relations and instead assume that the black hole in GN-z11 is accreting at the Eddington rate, then the black hole mass would be 10 7 M ⊙ . A black hole with this mass is more difficult to account for, but achievable in models assuming heavy seeds and episodes of super-Eddington accretion 7 , 26 , 27 .

Taking the stellar mass of the extended disk-like component measured in ref. 8 ( M * = 8 × 10 8 M ⊙ ), it is possible to locate GN-z11 on the M BH – M star relation. As shown in Fig. 4 , GN-z11 is placed above the local relation, although marginally consistent within the scatter. An early evolution above the local M BH – M star relation is what is expected from models invoking DCBHs and/or super-Eddington accretion 9 JWST galaxies hosted massive black holes? Mon. Not. R. Astron. Soc. 521, 241–250 (2023)." href="/articles/s41586-024-07052-5#ref-CR3" id="ref-link-section-d48084151e2120">3 , 4 , 25 .

The location of GN-z11 (large golden symbol) is compared with local galaxies as indicated by the small red symbols and their best-fit relation (black solid line and uncertainty traced by the grey-shaded region) 36 . The grey symbols show the values estimated for quasars (QSOs) at z ≈ 6−7 (ref. 37 ), although in these cases the galaxy mass is inferred from dynamical tracers. The blue symbols are AGN at z > 4 for which the black hole and galaxy stellar mass has been measured with JWST data ( Methods ) using the same calibration as in ref. 36 for consistency.

We note that the exceptionally high nitrogen abundance inferred for GN-z11 (specifically, high N/O) 9 becomes much less problematic in the AGN scenario. To begin with, several ‘nitrogen-loud’ AGN have already been found both at low and high redshifts (including NLSy1) 28 , 29 , 30 , 31 . So GN-z11 is not a very peculiar system in this context. Second, the mass of the BLR in AGN is very small 32 :

Specifically, the H γ luminosity observed in GN-z11 ( L H γ = 1.7 × 10 42 erg s −1 , even assuming the extreme case it is not contributed by the NLR and HII regions) implies a mass of the BLR of only a few solar masses. It would take just one or two supernovae to enrich such a small mass to solar or super-solar metallicity, especially within the small physical region associated with the BLR (about 10 −2 pc for GN-z11) (ref. 33 ). These could be supernovae from supermassive stellar progenitors, with high nitrogen yields 34 . However, even without invoking exotic scenarios, given the accelerated metal enrichment of such a small, central region, it is also possible that secondary, recycled nitrogen production can occur within a timescale of a few tens Myr (especially given the very fast cooling times at such high densities, which allow star formation to quickly occur out of cooled SN ejecta).

We finally discuss our results within the context of the recent JWST findings of an excess of exceptionally luminous galaxies at high redshift. GN-z11 is one of the first of such hyperluminous galaxies at high- z to be spectroscopically confirmed, and for which such a detailed spectroscopic analysis has been feasible. The AGN scenario revealed by our analysis provides a natural explanation for the exceptional luminosity of GN-z11. If this is representative of the broader class of luminous galaxies discovered at high- z , then it would greatly alleviate the tension with models and simulations.

Observations and data processing

The data presented in this paper are part of the JADES survey 38 and, specifically, obtained through programme ID 1181 (principal investigator D. Eisenstein). GN-z11 was observed in two epochs: the first one on UT 5 and 7 February 2023 and the second one on UT 4 and 5 May 2023. The February observations were already presented in ref. 9 . We refer to that paper for a detailed description. Briefly, the spectroscopic data were obtained with four different configurations of the NIRSpec micro-shutter array (MSA) 39 , 40 , 41 , using a three-shutter nodding pattern. Four different dispersers were used to cover the 0.6–5.3 μm wavelength range: the low-resolution prism mode (exposure time of 6,200 s, per configuration), and three medium-resolution gratings (3,100 s each, per configuration), which provide a nominal spectral resolution of R ≈ 1,000 for a uniformly illuminated slit 39 . However, the highly compact light profile of GN-z11, with respect to the width of the slit, results in a substantially higher effective resolution. To estimate the effective resolution, we forward model the morphology of GN-z11 through the NIRSpec instrument for the grating dispersers, finding that the resolution ranges between 1,100 and 2,100. Four MSA configurations were used (two pointing and two dither positions). The May observations were similar, but in this case they consisted of three consecutive dithers (with three different MSA configurations), each with three nods, resulting in an on-source exposure of 2.7 h for the prism, each of the three medium-resolution gratings and also with the high-resolution grating G395H/290LP. Unfortunately, at the location of GN-z11 on the MSA the latter spectrum is heavily truncated at wavelengths longer than 4.1 μm, hence all strong optical emission lines are not observed with this grating.

By combining the two sets of observations, the total exposure time is 9.6 h with the prism and 6.15 h with each of the medium-resolution gratings.

The data processing is also described in ref. 9 , and we refer to that paper for a detailed discussion. Here we only mention that we used the pipeline developed by the ESA NIRSpec Science Operations Team and the NIRSpec GTO Team. Most of the processing steps in the pipeline adopt the same algorithms used in the JWST Science Calibration Pipeline 42 . Different from the official pipeline, the final one-dimensional (1D) combined spectra are obtained by combining the 1D individual spectra rather than performing the extraction process in the combined two-dimensional spectra. This step guarantees that the final 1D spectra are well flux calibrated for slit losses. In the combination process, we also applied a 3 σ -clipping algorithm and excluded bad pixels based on the data quality files provided by the pipeline. The extraction of 1D spectra in the individual exposures is also optimized on the basis of science. In this paper, we adopt a three-pixels (0.3″) extraction along the slit, as it improves the signal-to-noise ratio (S/N) of the spectrum (for point sources). Finally, the GTO pipeline provides spectra beyond the nominal wavelength range for the spectral configuration G140M/F070LP by taking into account the transmission filter throughput in the flux calibration processing step. The extended spectra cover the wavelength range of 1.27–1.84 μm.

Moreover, here we combine the grating spectra in their overlapping ranges, which increases the S/N in those regions. The combined spectra in these regions were resampled to 8 Å around the NIV doublet, not to affect resolution, and to 12 Å around the CIV, as in this case higher S/N is required on the continuum to properly trace the CIV absorption.

In the paper, we adopt the flat ΛCDM cosmology from Planck18 with H 0 = 67.4 km s −1 Mpc −1 and Ω m = 0.315 (ref. 43 ).

Emission line fitting

The emission lines were fitted using single or multiple Gaussian lines and a simple power law for continuum subtraction. The best-fit parameters for the continuum and Gaussian components were found using the MCMC (Markov chain Monte Carlo) algorithm to estimate the uncertainties. For the purposes of this paper, each line in the rest-frame UV was fitted independently, except for doublets or multiplets, whose line widths were forced to the same value (but see discussion below for the CIII] doublet) and the relative wavelength separation of the doublet and multiplet was forced to the nominal rest-frame wavelength. The absolute velocity of each line (or group of lines in the case of doublets and multiplets) was not constrained to the exact redshift given in ref. 9 , to allow for small wavelength calibration uncertainties associated with the positional uncertainties of the target within the shutter. (The uncertainties in the target acquisition may result in the target being offset by up to about 0.05″ relative to the nominal position, which is used by the pipeline for the wavelength solution. This unknown offset would introduce a wavelength offset by up to 0.5 spectral pixel, which corresponds to a different velocity offset (given the wavelength-dependent resolution) in different regions of the spectrum.) (Jakobsen, personal communication) and also to allow for small velocity shifts between different lines, which are common in AGN, and especially in the BLR. We restricted our fitting to the lines of interest for this paper.

Extended Data Table 1 provides a list of the fitted emission line widths and fluxes and Extended Data Fig. 1 shows the additional fitted lines not shown in the main text.

In the case of the NIII] multiplet, the 1,748.6 Å and 1,754.0 Å transitions come from the same upper level, hence their flux ratio is fixed by the associated Einstein coefficients, specifically F 1754 / F 1748 = 1.05. Similarly, 1,746.8 Å and 1,752.2 Å come from the same upper level and their flux ratio is fixed to F 1746 / F 1752 = 0.14. The inferred line widths are deconvolved from the line spread function as inferred for the GN-z11 light profile.

For the CIII]1906,1908 doublet, it is not possible to resolve the two components; attempting to fit it with two components makes the fit degenerate between the width and intensity of the two components. The additional caveat of this CIII] doublet is that it is also commonly seen in normal star-forming galaxies and in the NLR of AGN, so it can also have a contribution from the host galaxy, as for the [NeIII] emission. In ref. 44 , the authors use IFS spectroscopy to reveal that the CIII] emission is resolved on scales of several 100 pc. As a consequence, we do not include the (spectrally) unresolved CIII] in our analysis, as it does not provide constraints on either the BLR or the host galaxy. In Extended Data Table 1 , we report the total flux and width using a single Gaussian. However, for the sake of completeness, we report that by fitting two components with separate full-width at half maximum (FWHM), accounting for the NLR and BLR, gives C iii ] λ 1906/ λ 1908 of \({0.62}_{-0.37}^{+1.00}\) for the narrow components, with FWHMs of 314 ± 120 km s −1 (consistent with the [NeIII] width), and 560 ± 80 km s −1 for the C iii ] λ 1908 broad component (consistent with the NIV] width).

The [OII]3726,3729 doublet would potentially be an additional forbidden line, detected in the observed wavelength range, which could be used to constrain the velocity dispersion in the host galaxy. However, unfortunately, the doublet is unresolved. Attempting to fit it (by forcing the two components to have the same width) results in a FWHM of 365 ± 55 km s −1 and a flux ratio of \({0.62}_{-0.21}^{+0.31}\) .

CIV absorption and emission

In this section, we provide some additional details on the CIV absorption. As mentioned in the text, CIV P-Cygni profiles with a significant CIV blueshifted trough are seen associated with atmospheres of young, hot stars. Yet, the depth of this feature is a strong function of metallicity 21 , and the deep trough observed in GN-z11 would require stars with solar or even super-solar metallicities, completely inconsistent with the much lower metallicity inferred for GN-z11. To illustrate the inconsistency with the pure stellar origin, we have stacked 11 UV spectra from the CLASSY HST survey 17 , with metallicity around the value inferred for GN-z11 in ref. 9 ( Z = 0.1 Z ⊙ ), specifically 7.4 < 12 + log( O / H ) < 7.9. We were conservative by excluding galaxies with strong CIV emission. We also excluded one WR galaxy, as we discuss that the spectrum cannot be dominated by WR stars (see main text and section ‘ The WR scenario ’). The continuum of the spectra was normalized to one by using a simple linear fit in the spectral ranges 1,410–1,480 Å and 1,560–1,600 Å, consistent with the analysis of the spectrum of GN-z11 in the same spectral region (Fig. 1d ). The resulting stacked spectrum is shown with a dashed, orange line in Fig. 1d and illustrates inconsistency with the trough seen in GN-z11. To be conservative, in Fig. 1d , we also show the case of the most extreme spectrum among the 11 selected, the one with the deepest CIV absorption. Although the wings of the stellar winds can extend out to 2,000 km s −1 , the profile and depth observed trough at these metallicities is inconsistent with the observed trough in GN-z11 at 5 σ .

The blueshifted CIV trough (and redshifted emission), therefore, is not a P-Cygni feature associated with stellar (atmosphere) winds. Rather, it is tracing a galactic outflow, as observed in lower redshift starbursts 22 and in (mini-)BAL/NAL AGN 23 , 45 , 46 , 47 , 48 , 49 . The determination of the velocity requires knowledge of the exact wavelength of the redshifted, rest-frame CIV transition. Unfortunately, there are small wavelength uncertainties associated with each grating because of the uncertainties of the location of the sources within the shutter, as discussed above. In this specific case, we calibrate the velocity shift based on the NIV line, which is in the same gratings and has a similar ionization potential as CIV. The outflow velocity is also subject to different definitions. The centroid of the trough relative to the mean of the two CIV transitions gives a velocity of –790 km s −1 . If we consider the blue edge of the trough relative to the bluest of the two transitions (CIV λ 1548.19), then we obtain a velocity of –1,040 km s −1 . These velocities are significantly higher than those inferred from the CIV absorption in starburst-driven outflows 22 , 50 , but in the range of BAL quasars that can span from 500 km s −1 to several thousands km s −1 (refs. 23 , 45 , 46 , 47 , 48 , 49 , 51 , 52 ).

The classification boundary between mini-BAL and NAL AGN is not sharp, with different authors giving different definitions in terms of width and/or blueshift of the absorption 23 , 45 , 46 , 47 , 48 , 49 . Here we simply give a generic classification as mini-BAL/NAL without aiming at a more specific category.

It should be noted that some past works have reported some rare starburst galaxies showing outflows with high velocities, even in excess of 1,000 km s −1 (refs. 53 , 54 , 55 , 56 ). However, these outflows are traced by lower ionization transitions (MgII absorption and [OIII] emission). More importantly, a close inspection of those cases reveals that each of them shows some AGN signature ([NeV] emission and/or broad MgII emission and/or broad Hβ emission and/or X-ray emission and/or located in the AGN or composite region of diagnostic diagrams). Therefore, although the AGN contribution to the bolometric luminosity of these galaxies may be arguable (also taking into account the variable nature of AGN), it is likely that the high-velocity outflows seen in these rare cases are actually driven by the AGN that they host.

Finally, it should be noted that the CIV absorption trough goes nearly to zero (as in many BAL quasars), which implies the total covering factor of the emitting source by the outflowing ionized gas along our line of sight. However, the errors leave scope for a contribution of 30% of the emission potentially not covered by the CIV absorption, which can be associated with the extended host galaxy. Yet, if higher S/N data confirm the CIV trough going to zero, this would imply that the outflow has an extent covering also the host galaxy, that is, about 400 pc, which would be fully consistent with recent findings of BAL outflows extending on scales of up to several kpc (refs. 49 , 57 , 58 , 59 ).

Given that CIV is a resonant line, the observed redshifted emission is also tracing the CIV counterpart of the redshifted Lyα emission seen in ref. 9 —that is, the receding side of the outflow.

We finally note that the spectrum between Lyα and the NV doublet shows the tentative signature of an NV blueshifted trough (Extended Data Fig. 1d ), which would be associated with the highly ionized outflow, but it requires additional data to be confirmed.

Constraints from other emission lines and diagnostics

Although the paper focuses on a few lines discussed in the main text, in this section we also discuss other emission lines that have either lower S/N, more severe blending or whose upper (or lower) limits give line ratios that are fully consistent with the AGN scenario.

MgII and CIII]

The MgII2796,2804 doublet is well resolved with the grating and in principle a good tracer of gas density in the range between 10 9 cm −3 and 10 14 cm −3 . However, the observed ratio, \(1.3{6}_{-0.42}^{+0.67}\) , is so uncertain to be consistent both with the low-density regime (ratio of around 1) and the high-density regime (ratio of about 2). Moreover, even if additional data allows constraining the MgII doublet ratio more tightly, these are resonant transitions, which are, therefore, strongly sensitive to the optical depth and radiative transfer effects 60 .

The C iii ] λλ 1907,1909 doublet would also be a good density tracer, as the ratio of its two components is primarily sensitive to the gas density and changes strongly between 10 4 cm −3 and 10 6 cm −3 (with the blue component λ 1907 going to zero at high densities), similar to the NIV] doublet. However, as discussed above, the two components are unresolved with the grating, and we cannot obtain reliable constraints on the gas density or on the line widths. More importantly, CIII] emission is commonly seen also in star-forming galaxies and in the NLR of AGN, so it may partially come also from the low-density ISM of the host galaxy, as is the case for [NeIII]. As already mentioned, recent IFS observations show CIII] to be resolved on scales of several 100 pc (ref. 44 ). It is interesting that when fitted with narrow and broad components, as discussed in the previous section, the narrow component gives widths formally consistent with the [NeIII], whereas the broad component is consistent with the NIV width.

Additional transitions from species requiring ionizing photon energy higher than about 60 eV, such as NV and NeV (in addition to NeIV seen in GN-z11), are often seen as evidence for the presence of an AGN. Yet, conversely, their absence should not be necessarily seen as evidence for the absence of an AGN, as often these lines are weak even in AGN and remain undetected if the S/N is not high enough 61 , 62 , 63 . Moreover, the intensity of these lines varies strongly from case to case.

With the prism it is not possible to assess the presence of NV because it is blended with Lyα and its damping wing. Regarding the gratings, the G140M band, in which NV is redshifted, is the least sensitive of the three medium-resolution spectra. Although there is a hint of the NV doublet (Extended Data Fig. 1a 2 σ integrated signal) we obviously do not quote it as a tentative detection. The inferred upper limit on the NV emission is not very constraining, but the important aspect in the context of this paper is that it is still fully consistent with the presence of an AGN. We demonstrate this in Extended Data Fig. 2 , in which the upper limits on the NV/CIV and NV/HeII ratios for GN-z11 are compared with a sample of the broad lines in type 1 AGN 61 and also with a sample of the NLR in type 2 AGN 62 , and illustrating that the non-detection of NV is fully consistent with the AGN scenario.

It is also interesting to compare NV with NIV, as this ratio is not dependent on the nitrogen abundance, although NIV is detected (or reported) less frequently in AGN. In the well-studied type 1.8 AGN at z = 5.5, GS-3073 (refs. 16 , 30 , 64 ), the NV is five times fainter than NIV, which would be totally undetected in our spectrum. In the type 1 quasars explored in ref. 65 , the NIV broad line is very strong, whereas NV is undetected, with an upper limit that is about 10 times lower than the NIV flux.

NeV is also not detected, neither in the grating nor in the prism spectrum. The upper limit on the NeV/NeIII ratio is about 0.2. However, in ref. 63 , the authors have shown that AGN models can have NeV/NeIII as low as 10 −2 −10 −4 . Hence the non-detection of NeV is also not constraining about the presence of an AGN.

Finally, we note that AGN accreting at super-Eddington rates have a lower energy cutoff, and hence are less likely to emit hard photons that can produce highly ionized species, such as NV and NeV.

HeII and CIV

HeII is detected in the prism and, more marginally, in the grating (Extended Data Fig. 1 ).

As already discussed, CIV is detected in the grating, but with a P-Cygni profile, hence its flux is a lower limit because of self-absorption.

The interpretation of these limits using photoionization models is very much model-dependent. We illustrate this in Extended Data Fig. 3a,b . Specifically, Extended Data Fig. 3a , as in ref. 9 , shows the location of GN-z11 on the CIII]/CIV versus HeII/CIII] diagram and in which the red-squared and blue-starred symbols show the location of models from refs. 10 , 66 for the NLR of AGNs and for star-forming galaxies, respectively, and in a range of about ±0.3 (see legend) dex of the metallicity inferred in ref. 9 for GN-z11. GN-z11 can be consistent with both AGN and star-forming models.

Extended Data Fig. 3b shows the same diagram in which we instead plot the models from ref. 67 , in the same (low) metallicity range for both AGN and SF galaxies. In this case, GN-z11 is much more consistent with the AGN models and inconsistent with the models for star-forming galaxies.

Yet, if the permitted and semi-forbidden lines of GN-z11 are coming from the BLR, as argued in this paper, then neither of the models above actually apply, as they are developed for the low-density environments of the NLR and HII regions. It is, therefore, more instructive to compare with the line ratios observed in the BLR of type 1 AGN. These are taken from the compilation of ref. 61 and shown with purple circles in Extended Data Fig. 3c . The line ratios observed in GN-z11 are fully consistent with the broad lines of type 1 AGN. For completeness, in the same panel we also plot the ratios observed for the NLR of type 2 AGN, compiled in ref. 62 (mostly overlapping with the ratios observed for the broad lines), and the star-forming galaxies from the CLASSY survey 18 .

The WR scenario

In this section, we discuss the scenario recently proposed that GN-z11 may be similar to local WR galaxies 24 .

The HeII marginal detection shows a potentially broad profile (about 10,00 km s −1 , although the broad wings are mostly in the noise), which may be associated with the inner BLR, but also may resemble the broad HeII profile characteristic of WR stars. Therefore, there might be a contribution from WR stars and, specifically, WN stars, given the strong nitrogen lines.

However, there are various spectral features that cannot be accounted for in the WN scenario.

WN stars are also characterized by very strong NIV λ 1718 resonant emission, stronger than the NIV λ 1486, and typically with a prominent P-Cygni profile 13 . In GN-z11, despite the very strong NIV λ 1486, there is no trace of the NIV λ 1718 line. Figure 1f shows the spectrum of GN-z11 at the expected location of NIV λ 1718 and in which the shaded red region shows the expected intensity of the line, based on the strength of the NIV λ 1486 line. The GN-z11 spectrum is totally inconsistent with the presence of the NIV λ 1718 WR signature.

Furthermore, neither [Ne iv ] λ 2424 nor CII* λ 1335 are ever seen associated with the WR population 13 .

Finally, even if WN show prominent NIII] emission, the strength of the λ 1754 component of the multiplet is much fainter in WR galaxies such as Mrk966 (ref. 17 ) and consistent with densities typical of the ISM.

In sum, although WR stars might be present in GN-z11, they are unlikely to dominate the excitation of most nebular lines.

Extended Data Table 2 summarizes more schematically the observational features consistent or inconsistent with the AGN scenario, the WR scenario and a compact starburst without WR stars.

Photoionization modelling

We used the C loudy photoionization code 68 to explore the effect of varying physical conditions on some emission line ratios constrained by JWST/NIRSpec. The primary goal is to explore the ratios of emission lines within a given doublet or multiplet, hence lines of the same ion (specifically NIII and NIV) that are effectively insensitive to the chemical abundance and ionization parameter, while sensitive to density and only with secondary dependence on temperature. For this reason, the details of the photoionization models are not as important as when exploring other line ratios. We considered a nebula of constant pressure in plane-parallel geometry. However, we have verified that other scenarios, such as a cloud with constant density, do not affect our findings. For completeness, we considered both AGN and stellar templates for the shape of the incident radiation field. Its normalization is set by the ionization parameter, defined as U ≡ Φ H /( n H c ), where Φ H is the surface flux of hydrogen-ionizing photons at the illuminated face of the nebula, n H is the number density of hydrogen and c is the speed of light. The hydrogen density and ionization parameter were varied in logarithmic steps of 1, respectively from n H = 1 cm −3 up to n H = 10 14 cm −3 , and starting at log 10 U = −3 and ending at log 10 U = −1 (refs. 10 , 66 , 67 ).

In the AGN scenario, we adopted the multi-component continuum template implemented in C loudy , consisting of a black body and a power law, varying the black body temperature ( T AGN = 10 6 K and 10 6 K) while fixing the power-law slope to α = −1.4 (note that this is the slope underlying the black body at energies above the Ly-edge) and leaving other optional parameters as default. For the AGN models, we considered gas-phase metallicities of Z neb = 0.1 Z ⊙ and Z neb = 1 Z ⊙ . By contrast, the star-forming models are restricted to Z neb = 0.1 Z ⊙ , as the hard ionizing spectra of metal-poor stars are essential to form sufficient triply ionized nitrogen (requiring 47.5 eV), whose presence in GN-z11 is evidenced by the strong NIV emission (EW NIV 1486 = 9.0 ± 1.1 Å; ref. 9 ), whereas metal-rich stars would not produce enough hard ionizing photons to make the NIV line visible. In the star-formation scenario, we used stellar population synthesis models, including binary stars generated by bpass v.2.1 (ref. 69 ) for a single burst of star formation (with varying ages, t * /Myr ∈ {1, 10, 100}), assuming the same metallicity as the gas (that is, Z * = Z neb = 0.1 Z ⊙ ) and an IMF 70 that ranges in stellar mass from 1 M ⊙ to 100 M ⊙ . Both in the AGN and star-formation cases, calculations are run until a neutral hydrogen column density of N HI = 10 21 cm −2 is reached to ensure that in all models the nebula is matter bounded; we note, however, that the highly ionized nitrogen lines are produced in the very inner part of the cloud, such that the boundary conditions do not significantly affect our results. In total, this results in a parameter grid of 15 different densities, 3 ionization parameters, 3 temperatures or stellar ages, 2 or 1 metallicities for the AGN and star-formation models, respectively, or a total of 15 × 3 × (3 × 2 + 3 × 1) = 405 possible model configurations.

The relevant nitrogen line ratios for all of these (except for eight cases in which C loudy reported a failure) are shown in Fig. 2 , from which we conclude that they are consistent between the AGN and star-formation scenario, and their density dependence is largely independent of ionization parameter, metallicity or the precise shape of the incident radiation field (that is, AGN or star formation and the corresponding parameter T AGN or t * ).

At high densities, the NIV λ 1483/NIV λ 1486 ratio approaches zero ( n H ≳ 10 6 cm −3 ), whereas N iii ] λ 1754 plateaus at a fractional contribution to the multiplet of about 0.23 at higher densities still ( n H ≳ 10 10 cm −3 ), both pointing towards the presence of a broad line region in GN-z11 given the observed values.

Finally, to increase the readability of Fig. 2 , we have separated the AGN and star-forming models in two separate panels in Extended Data Fig. 4 .

Continuum shape

If GN-z11 is a type 1 AGN, then we should be directly seeing the light from the accretion disc. In the case that the accretion disc dominates, the UV-to-optical continuum should follow a simple power law of the form F λ ∝ λ β with β = −7/3 ≈ −2.33 (ref. 71 ), as observed in type 1 AGN, and NLSy1 72 , 73 , modulo the UV turnover whose wavelength increases with black hole mass and also modulo effects of dust reddening, which often makes the spectrum redder.

In the case of GN-z11, the spectrum is contributed to also by the underlying galaxy identified in ref. 8 in the NIRCam images. This component is significantly fainter than the nuclear point-like component. It is difficult to quantitatively establish its contribution to the spectrum, because part of the light may fall outside the shutter, and in a different fraction in the four dither or pointing positions, and not easy to reconstruct because of the slight positional uncertainties discussed above. In Extended Data Fig. 5a , we show the contribution from the galactic component (dotted-orange line) to the spectrum, assuming that the entire light of the galaxy is captured by the spectrum, corresponding to about one-third of the flux and using the spectral template inferred in ref. 8 for the extended component.

The additional component to take into account is the nebular continuum associated with the BLR (as well as any other ionized gas in the host galaxy). The BLR typically has a low covering factor 74 , therefore the nebular continuum is not expected to be strong, but its contribution must be quantified. In most physical conditions typical of the ionized gas in the BLR, NLR or HII regions, the nebular continuum is linked to the intensity of the Balmer lines. We have estimated the nebular emission using a Cloudy model with a metallicity of 0.1 Z ⊙ and a density of 10 6 cm −3 (between the BLR and ISM origin scenarios) and normalized to have the same Hγ flux as observed in the spectrum of GN-z11. The nebular spectrum does not change drastically as a function of density, except obviously for the emission of the forbidden and semi-forbidden lines; however, our focus is on the nebular continuum, so we ignore the mismatch of the emission lines, as a detailed photoionization modelling of their flux is beyond the scope of this paper. We note that the nebular continuum is also included in the model spectrum fit to the extended component in ref. 8 ; therefore, not to include it twice, we have measured the Hγ flux in the ref. 8 spectrum and normalized the Cloudy nebular spectrum only to the Hγ flux obtained by the difference between the observed value and the flux in the ref. 8 model spectrum. The resulting nebular spectrum is shown with a dashed purple line in Extended Data Fig. 5a . Again, the mismatch of the emission lines should be disregarded, as the goal is not to reproduce them with the Cloudy model.

Extended Data Fig. 5b shows again the observed spectrum, in log–log scale, in which the galactic and nebular components have been subtracted. Although the noise is large, especially at long wavelengths, also as a consequence of the model subtraction procedure, the resulting spectrum is well fitted by a simple power law, in the parts not affected by the emission lines. The best-fitting slope is –2.26 ± 0.10, hence consistent with the continuum expected from an accretion disc. Note that this is not evidence in support of the presence of an AGN, as also young galaxies may have power-law shapes, it is only meant to show consistency with the AGN scenario.

We finally note that, although with a large scatter, the UV spectrum of AGN often shows a FeII hump between about 2,300 Å and about 3,100 Å (refs. 75 , 76 , 77 , 78 , 79 ). The prism spectrum of GN-z11 does not show an obvious FeII bump, although a more detailed analysis and modelling is required to assess the presence or absence of such a bump, which is deferred to a separate paper. However, we note that at such early epochs there is little time for the ISM to be enriched with iron through the SNIa channel 80 , so a weak or absent FeII bump would not be unexpected.

Variability

The luminosity of AGN can be variable, from a few per cent to a factor of a few, on short (days) and long (years) timescales. We have investigated the possible presence of variability. Before the recent NIRCam images obtained in February 2023 (ref. 8 ), deep photometric observations were obtained with HST about 10 years earlier 81 , 82 , corresponding to about 1 year in the rest frame of GN-z11. Most of the HST photometric data points have error bars that are too large to be useful for constraining variability. However, the photometric point reported in ref. 81 with the F160W filter has a relatively well constrained value: 150 ± 10 nJy, within an aperture of 0.35″. NIRCam does not have the same filter, however, the photometry obtained in the F150W filter can be used and transposed to the F160W filter by using the NIRSpec prism spectrum. After extracting photometry from a 0.35″ aperture (as in ref. 81 ), and extrapolating with the NIRSpec spectrum, we obtain a F160W equivalent photometry of 141 ± 2 nJy, which is consistent with the HST previous photometry within 1 σ . If we consider that about 30% of the flux is diluted by the host galaxy, the comparison of the photometry between the two epochs would indicate a variability of 10% at only 1 σ . This is certainly not a detection of variability, but it is consistent with the range of variability amplitudes observed in NLSy1 and, more broadly, in type 1 AGN 83 .

X-ray emission

GN-z11 is not detected in X-rays. Flux limits are obtained from the Chandra Deep Field North, which was a 2 Ms observation performed in 2002 (see 84 for final results). Their sensitivity map gives a point source limit in the soft (0.5–2 keV), hard (2–7 keV) and full (0.5–7 keV) bands of 1.54 × 10 −17 , 7.9 × 10 −17 erg cm −2 s −1 and 4.9 × 10 −17 erg cm −2 s −1 . Source detection requires a no-source probability P < 0.004. The tightest limit in the soft band translates to a rest frame 5.8–23.2 keV luminosity limit at z = 10.6 of 2.2 × 10 43 erg s −1 . Assuming a typical NLS1 photon index of 2.3 means that L X (2–10 keV) is less than 3 × 10 43 erg s −1 .

The bolometric correction for NLS1 in the 2–10 keV band, BC X , is about 100 (ref. 85 ). There is a significant systematic uncertainty here due to the unseen flux in the FUV, in which the emission is expected to peak (see fig. 3 in ref. 86 ). Moreover, the 2–10 keV flux entirely originates from the corona, the early development of which and possible dependence on black hole spin are unknown (ref. 87 cautions against using his X-ray BC values for NLS1). Proceeding with BC X = 100 means that the Chandra upper limit is almost three times above the luminosity inferred from the JWST flux at 1,400 Å. We predict a conservative SB flux of 5 × 10 −18 erg cm −2 s −1 . This would be detectable in about 1 Ms with the candidate NASA Probe mission AXIS. The coronal emission from local NLS1s is highly variable and the above BC represents a mean value (note that the intrinsic disc flux seen in the UV is much less variable 86 ).

Black hole mass estimate

For the vast majority of high redshift AGN, the black hole masses are inferred using single-epoch measurements and the so-called virial relations, that is, relations between the black hole mass, the width of the lines of the BLR and the continuum or line luminosity 37 , 88 , 89 , 90 , 91 , 92 , 93 , 94 . These relations are calibrated on nearby AGN, using either reverberation mapping techniques and/or direct dynamical measurements of the black hole. The black hole mass scales about as the square power of the width of the BLR lines and about as the square root power of the luminosity, with a proportionality constant that depends on the specific waveband (or line) for the luminosity estimation.

The most accurate virial relations would be those using Hα and Hβ. In our case, Hγ could be used as a proxy. However, as discussed, the Balmer lines are probably contributed to by the star formation in the host galaxy, hence not reliable to trace the black hole mass.

The CIII] doublet is also sometimes used to infer the black hole mass. However, this is not well resolved and, as for the case of the Balmer lines, this is probably contaminated by the ISM and star formation in the host galaxy.

MgII is often used. In our case, the MgII doublet is clearly detected, but the S/N is fairly low for the measurement of the width (Extended Data Fig. 1 ). If we take the width resulting from the fit and the relation provided in ref. 95 :

then we get a black hole mass of 1.4 × 10 6 M ⊙ . However, given the low S/N on the MgII doublet, we prefer to use as representative width of the BLR lines the profile of the high S/N and isolated NIV line. If we adopt this width into the equation above, we obtain a black hole mass of 1.6 × 10 6 M ⊙ . The uncertainty is totally dominated by the scatter in the virial scaling relation, which is about 0.3 dex (ref. 96 ).

Moreover, there are various other systematic uncertainties and caveats that can affect the black hole mass estimate. To begin with, it is not obvious that the local virial relations apply at high redshift. The main issue is whether the dependence of the BLR radius on luminosity evolves with redshift or not. The most plausible scenario is that the square root dependence of the BLR radius from luminosity is primarily set by the dust sublimation radius. In ref. 97 , the authors argue that, given the extremely high densities in the nuclear region of AGN (hence high optical thickness even at very low dust-to-gas ratios), unless the nuclear region is totally devoid of dust, the same R BL – L relation is unlikely to evolve with redshift. Assessing whether the virial relations depend on the accretion rate or not is more problematic. On the one hand, in ref. 98 , the authors argue that the effect of radiation pressure is to reduce the effective gravitational force on the clouds of the BLR; the net result is that the standard virial relations applied to BHs accreting close to the Eddington rate could underestimate the black hole mass by a factor of several. On the other hand, reverberation mapping of AGN accreting at super-Eddington has revealed that in these cases the size of the BLR is a factor of several, and up to an order of magnitude, smaller than expected from the R BL – L relation for sub-Eddington AGN (ref. 99 and references therein), which would imply that the standard virial relations overestimate, by a factor of several, the black hole masses in AGN accreting at super-Eddington. Overall, it is possible that the radiation pressure effect and the offset from the R BLR – L relation might cancel each other out. However, currently it is not really possible to provide an accurate assessment on how much AGN accreting at or beyond the Eddington rate might deviate from the standard virial relations.

Finally, the black hole masses from other JWST studies at z ≈ 4–8 (refs. 30 , 93 , 97 , 5 with CEERS. Astrophys. J. Lett. 954, L4 (2023)." href="/articles/s41586-024-07052-5#ref-CR100" id="ref-link-section-d48084151e3923">100 , 101 ) are shown in Fig. 4 . These are based on the Hα or Hβ width and flux. We clarify that these are re-estimated by using the same calibrations used in ref. 36 for local galaxies.

AGN bolometric luminosity estimate

We derive the bolometric luminosity of the AGN by using the continuum luminosity at λ rest = 1,400 Å and the luminosity-dependent bolometric correction given in ref. 87 :

We also assume, as discussed in the previous sections, that 30% of the continuum flux at this wavelength is because of the underlying galactic component 8 and that, therefore, the AGN continuum luminosity at this wavelength is 0.7 of the observed value. We infer a bolometric luminosity of 1.08 × 10 45 erg s −1 . The resulting ratio between bolometric and Eddington luminosity is 5.5, also affected by an uncertainty of a factor of at least 2, coming from the uncertainty on the black hole mass.

Comparison with cosmological and hydrodynamical simulations

There is a vast literature discussing the formation of early black holes and on how they evolve in the first thousand million years, by using hydrodynamical and cosmological simulations, as well as semi-analytical models. The production and elaboration of models in this area have recently seen surge with the goal of specifically interpreting the results from JWST. It is beyond the scope of this paper to provide an exhaustive description of the assumptions and results of the several models and simulations. However, in this section, we briefly discuss that many of them can explain the properties of GN-z11 and provide some possible constraints on the seeding scenarios.

We start by considering the results obtained in ref. 7 from the FABLE hydrodynamical, cosmological simulation, in which they focused on the largest halo at z = 6 (with a virial mass M 200 = 6.9 × 10 12 M ⊙ of the Millennium box). The latter may appear an extreme choice; however, we note that GN-z11 does live in an overdense region and probably at the core of a protocluster 8 , 102 . In the FABLE simulation, the black hole seed has a mass of 10 5 M ⊙ at z = 13. The accretion rate is capped to Eddington and uses the Bondi–Hoyle–Littleton-based formalism; however, as small scale, non-isotropic accretion is unresolved in the simulation, FABLE, like Illustris, uses a Bondi–Hoyle–Littleton rate boosted by a factor of 100. Feedback energy in FABLE scales as 10% of the available accretion energy, \(\dot{E}={\epsilon }\dot{M}{c}^{2}\) , where ϵ = 0.1 is the radiative efficiency of the accretion flow. At high redshifts, this is primarily injected as thermal energy in the vicinity of the black hole, with a duty cycle of 25 Myr. We overplot the fiducial model in ref. 7 in Fig. 3 (orange solid line, labelled as B23), illustrating that this can easily reproduce the mass of the black hole in GN-z11 at z = 10.6.

To explain the most massive BHs observed at z ≈ 6–7, the same study as above 7 also explores the scenario of earlier seeding ( z = 18) and allows the black hole to accrete at up to two times the Eddington limit; in this case, the model could explain a black hole nearly five times more massive than GN-z11 at z = 10.6.

In ref. 35 , the authors explored the early evolution of black holes using the TRINITY cosmological empirical model 103 , which is based on halo statistics from N-body simulations and incorporating empirical galactic scaling relations. The authors specifically explore the case of GN-z11. They illustrate that its mass and black hole to stellar mass ratio can be explained by their model starting with an intermediate mass seed of a few times 10 3 seeded at z = 15, accreting on average at sub-Eddington rates, but intermittently also at super-Eddington. Their track is shown with a solid-teal line in Fig. 3 (labelled as Z23).

Recently, in ref. 25 , the authors have explored the properties of GN-z11 within the context of the semi-analytical model CAT. They find that the black hole mass of GN-z11 and its location on the M BH – M star diagram can be interpreted both in terms of light seeds (at z = 20–23) that can have super-Eddington accretion phases, or Eddington-limited heavy seeds formed at z = 14–16. Out of their various tracks, Fig. 3 shows only two samples of their tracks, in the case of a light (red-solid) and a heavy seed (red-dashed), which can both reproduce the mass of GN-z11 at z = 10.6 (labelled as S23). In both cases, the semi-analytical model can also reproduce the black hole to stellar mass observed in GN-z11.

In ref. 9 JWST galaxies hosted massive black holes? Mon. Not. R. Astron. Soc. 521, 241–250 (2023)." href="/articles/s41586-024-07052-5#ref-CR3" id="ref-link-section-d48084151e4372">3 , the authors suggested that the detectability of accreting BHs at high redshift by JWST implies that these are probably originating from heavy seeds. Specifically, their models can reproduce the mass of GN-z11 at z = 10.6 but only with seeds that are several times 10 5 M ⊙ , already in place before z = 14. GN-z11 would fall in this category, and the tracks obtained in ref. 9 JWST galaxies hosted massive black holes? Mon. Not. R. Astron. Soc. 521, 241–250 (2023)." href="/articles/s41586-024-07052-5#ref-CR3" id="ref-link-section-d48084151e4388">3 would also explain the black hole to stellar mass ratio observed in GN-z11.

Other studies have proposed other scenarios, visualizing different seeding mechanisms, at different redshifts, and with different assumptions about the accretion and merging rates, and which are capable of reproducing the black hole mass of GN-z11 by z = 10.6, and generally also its black hole to stellar mass ratio 4 , 26 , 27 , 104 .

In sum, the properties of the black hole in GN-z11 can be explained using different assumptions, which can be broadly grouped in heavy seeds accreting at sub-Eddington rates, or intermediate–light seeds experiencing super-Eddington phases and/or modelled with a boosted Bondi accretion.

More statistics on objects such GN-z11 are required to discriminate between different scenarios. For the time being, to our knowledge, GN-z11 remains the most luminous object at z > 10 in all HST Deep fields (including CANDLES and Frontier Fields). It is hoped that JWST observations on larger areas (for example, in Cosmos-WEB) will find more AGN at z > 10 similar to GN-z11. For the time being, as discussed in the text, it is interesting to note that models and simulations were expecting a few accreting black holes with masses in the range 10 6 –10 7 M ⊙ at 10 < z < 11 in the JADES Medium-Deep survey in the GOODS fields 9 JWST galaxies hosted massive black holes? Mon. Not. R. Astron. Soc. 521, 241–250 (2023)." href="/articles/s41586-024-07052-5#ref-CR3" id="ref-link-section-d48084151e4434">3 , 105 . Therefore, the discovery of a 2 × 10 6 M ⊙ black hole in GN-z11 is not unexpected, and a few more might be present (probably accreting at a lower rate) in the GOODS fields.

GN-z11 and its large-scale environment

We have shown that the high nitrogen enrichment of GN-z11 is probably restricted to the BLR, whose small mass and compact size has probably undergone very rapid chemical enrichment, requiring only a few SNe.

We note that the high chemical enrichment of GN-z11 is not in contrast with the recent claim of pristine gas in the halo of GN-z11 44 . These claims are on totally different scales, with the pristine gas found several kpc away from GN-z11, whereas the high chemical enrichment is estimated to be in the nucleus of GN-z11. Regarding the claim of pristine gas in the halo of GN-z11, models expect that high- z massive galaxies may host pockets of pristine gas in their haloes, even down to z ≈ 3 (refs. 106 , 107 ).

Data availability

The electronic version of the processed data used to produce the figures (including the 1D and 2D spectra of GN-z11) is available at the JADAES web site ( jades-survey.github.io/ ). The NIRSpec raw data can be accessed at the JWST archive ( archive.stsci.edu ).

Change history

17 may 2024.

A Correction to this paper has been published: https://doi.org/10.1038/s41586-024-07494-x

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Acknowledgements

We acknowledge the suggestions and discussions we had with several colleagues, in particular: G. Risaliti, A. Marconi, J. Bennett, S. Koudmani, D. Sijacki, R. Schneider, M. Volonteri, R. Valiante, A. Trinca, D. Berg, R. Ellis, J. Dunlop, M. Pettini, H. Katz, W. N. Brandt and R. Tripodi. F.D., J.S., L.S., R.M. and T.J.L. acknowledge support from the Science and Technology Facilities Council (STFC), from the ERC through advanced grant 695671 ‘QUENCH’, and from the UKRI Frontier Research grant RISEandFALL. R.M. also acknowledges funding from a research professorship from the Royal Society. A.B., G.C.J. and J.C. acknowledge funding from the ‘FirstGalaxies’ Advanced Grant from the European Research Council (ERC) under the Horizon 2020 research and innovation programme of the European Union (grant agreement No. 789056). B.R. acknowledges support from the NIRCam Science Team contract to the University of Arizona, NAS5-02015. B.R.D.P., M.P. and S.A. acknowledge support from grant PID2021-127718NB-I00 funded by the Spanish Ministry of Science and Innovation/State Agency of Research (MICIN/AEI/10.13039/501100011033). M.P. also acknowledges support from the Programa Atraccion de Talento de la Comunidad de Madrid by grant 2018-T2/TIC-11715. C.N.A.W., E.E. and F.S. acknowledge a JWST/NIRCam contract to the University of Arizona NAS5-02015. D.J.E. is supported as a Simons Investigator and by JWST/NIRCam contract to the University of Arizona, NAS5-02015. E.C.-L. acknowledges support of an STFC Webb Fellowship (ST/W001438/1). H.Ü. acknowledges support from the Isaac Newton Trust and from the Kavli Foundation through a Newton–Kavli Junior Fellowship. J.W. acknowledges support from the ERC advanced grant 695671, ‘QUENCH’, and the Foundation MERAC. S.Carniani acknowledges support by HE ERC Starting grant no. 101040227 - WINGS of the European Union. The research of C.C.W. is supported by NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This research is supported in part by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project no. CE170100013.

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Roberto Maiolino, Jan Scholtz, Joris Witstok, Francesco D’Eugenio, Hannah Übler, Sandro Tacchella, Tobias J. Looser, William M. Baker, Nicolas Laporte & Lester Sandles

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Contributions

R.M., J.S., J.W., S.C., F.D., A.d.G., H.Ü. and S.T. contributed to the writing of the paper, methods and creation of figures. All authors contributed to the interpretation of the results. N.K., B.R.d.P. contributed to the design, construction and commissioning of NIRSpec. S.A., S. Carniani, M.C., J.W., M.P. and B.R.d.P. contributed to the NIRSpec data reduction and to the development of the NIRSpec pipeline. S.A. contributed to the design and optimization of the MSA configurations. A.B., C.N.A.W., E.C.-L., K.B. and H.Ü. contributed to the selection, prioritization and visual inspection of the targets. S. Charlot, J.C., E.C.-L., R.M., J.W., F.D., T.J.L., M.C., A.d.G. and L.S. contributed to the analysis of the spectroscopic data, including redshift determination and spectral modelling. F.D., T.J.L., M.C., B.R.d.P., R.M., S.A. and J.S. contributed to the development of the tools for the spectroscopic data analysis, visualization and fitting. C.W. contributed to the design of the spectroscopic observations and MSA configurations. C.N.A.W., C.J.W., D.J.E., R.M. and S.A. contributed to the design of the JADES survey. E.E., K.N.H. and C.C.W. contributed to the design, construction and commissioning of NIRCam. B.R., D.J.E., I.S., S.T., C.N.A.W. and Z.J. contributed to the JADES imaging data reduction. B.R. contributed to the JADES imaging data visualization.

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Extended data figures and tables

Extended data fig. 1 zoom in on the additional emission lines fitted..

a ) MgII λ 2796,2804 doublet; b ) HeII λ 1640; c ) Ly α , NV λ 1238,1242 doublet (undetected) and SiII λ 1260,1264 (undetected), corrected for the Ly α damping wing; d ) C iii ] λ 1906,1908 doublet. As the doublet is unresolved, the fit turns out degenerate between line width and fluxes of the two components; moreover it is also contributed to by star formation in the host galaxy (see text for details); e ) [Ne iii ] λ 3869 profile compared with the Balmer lines H δ and H γ . In all panels, the continuum is subtracted. The black dotted lines indicate the 1sigma noise level.

Extended Data Fig. 2 NV/CIV versus NV/HeII flux ratio diagram.

GN-z11 (golden circle) is compared with the ratios observed for the broad lines of type 1 AGN (purple stars 61 ), and for the NLR of type 2 AGN (red squares 62 ), illustrating that the non-detection of NV for GN-z11 is not constraining and consistent with the AGN scenario.

Extended Data Fig. 3 CIII]/CIV versus CIII]/HeII flux ratio diagrams.

GN-z11 (golden circle) is compared with: a) the AGN-NLR models (red squares) by 10 and SF galaxies (blue stars) by 66 (left) and with b) the AGN-NLR models (red squares) and SF models (blue triangles) by 67 (centre). All models have been chosen in a low metallicity range, around the value inferred by 9 for GN-z11. c) Comparison of GN-z11 with the ratios observed for the broad lines of type 1 AGN (purple circles 61 ), narrow lines of type 2 AGN (red squares 62 ), and starburst galaxies (blue stars 18 ).

Extended Data Fig. 4 Flux ratios of density-sensitive nitrogen lines as a function of hydrogen gas density, n H .

Same as Fig. 2 but where we have separated the photoionization models for AGN (left) and Star Forming galaxies (right).

Extended Data Fig. 5 Low resolution (prism) spectrum of GN-z11.

Left: Observed prism spectrum (black solid) compared with the (maximum) contribution from the host galaxy of the AGN as inferred by 8 (orange dotted), and the nebular emission inferred from a simple Cloudy model (purple dashed) normalized to the H γ flux not included in the galaxy model. Right: Spectrum subtracted of the galactic and nebular continua, in a log-log scale, whose regions not affected by emission lines (solid black) have been fitted with a simple powerlaw (red-dashed) resulting into a slope of −2.26 ± 0.10, consistent with the slope expected for an accretion disc (−2.33, dotted green line).

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Maiolino, R., Scholtz, J., Witstok, J. et al. A small and vigorous black hole in the early Universe. Nature 627 , 59–63 (2024). https://doi.org/10.1038/s41586-024-07052-5

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By Jeremy Bernstein

G reat science sometimes produces a legacy that outstrips not only the imagination of its practitioners but also their intentions. A case in point is the early development of the theory of black holes and, above all, the role played in it by Albert Einstein. In 1939 Einstein published a paper in the journal Annals of Mathematics with the daunting title On a Stationary System with Spherical Symmetry Consisting of Many Gravitating Masses. With it, Einstein sought to prove that black holes--celestial objects so dense that their gravity prevents even light from escaping--were impossible.

The irony is that, to make his case, he used his own general theory of relativity and gravitation, published in 1916--the very theory that is now used to argue that black holes are not only possible but, for many astronomical objects, inevitable. Indeed, a few months after Einstein's rejection of black holes appeared--and with no reference to it--J. Robert Oppenheimer and his student Hartland S. Snyder published a paper entitled On Continued Gravitational Contraction. That work used Einstein's general theory of relativity to show, for the first time in the context of modern physics, how black holes could form.

Perhaps even more ironically, the modern study of black holes, and more generally that of collapsing stars, builds on a completely different aspect of Einstein's legacy--namely, his invention of quantum-statistical mechanics. Without the effects predicted by quantum statistics, every astronomical object would eventually collapse into a black hole, yielding a universe that would bear no resemblance to the one we actually live in.

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Bose, Einstein and Statistics

EINSTEIN'S CREATION of quantum statistics was inspired by a letter he received in June 1924 from a then unknown young Indian physicist named Satyendra Nath Bose. Along with Bose's letter came a manuscript that had already been rejected by one British scientific publication. After reading the manuscript, Einstein translated it himself into German and arranged to have it published in the prestigious journal Zeitschrift fr Physik .

Why did Einstein think that this manuscript was so important? For two decades he had been struggling with the nature of electromagnetic radiation--especially the radiation trapped inside a heated container that attains the same temperature as its walls. At the start of the 20th century German physicist Max Planck had discovered the mathematical function that describes how the various wavelengths, or colors, of this black body radiation vary in intensity. It turns out that the form of this spectrum does not depend on the material of the container walls. Only the temperature of the radiation matters. (A striking example of black-body radiation is the photons left over from the big bang, in which case the entire universe is the container. The temperature of these photons has been measured at 2.726 0.002 kelvins.)

Somewhat serendipitously, Bose had worked out the statistical mechanics of black-body radiation--that is, he derived the Planck law from a mathematical, quantum-mechanical perspective. That outcome caught Einstein's attention. But being Einstein, he took the matter a step further. He used the same methods to examine the statistical mechanics of a gas of massive molecules obeying the same kinds of rules that Bose had used for the photons. He derived the analogue of the Planck law for this case and noticed something absolutely remarkable. If one cools the gas of particles obeying so-called Bose-Einstein statistics, then at a certain critical temperature all the molecules suddenly collect themselves into a degenerate, or single, state. That state is now known as Bose-Einstein condensation (although Bose had nothing to do with it).

An interesting example is a gas made up of the common isotope helium 4, whose nucleus consists of two protons and two neutrons. At a temperature of 2.18 kelvins, this gas turns into a liquid that has the most uncanny properties one can imagine, including frictionless ow (that is, superuidity). More than a decade ago U.S. researchers accomplished the difficult task of cooling other kinds of atoms to several billionths of a kelvin to achieve a Bose-Einstein condensate.

Not all the particles in nature, however, show this condensation. In 1925, just after Einstein published his papers on the condensation, Austrian-born physicist Wolfgang Pauli identified a second class of particles, which includes the electron, proton and neutron, that obey different properties. He found that no two such identical particles--two electrons, for example--can ever be in exactly the same quantum-mechanical state, a property that has since become known as the Pauli exclusion principle. In 1926 Enrico Fermi and P.A.M. Dirac invented the quantum statistics of these particles, making them the analogue of the Bose-Einstein statistics.

Because of the Pauli principle, the last thing in the world these particles want to do at low temperatures is to condense. In fact, they exhibit just the opposite tendency. If you compress, say, a gas of electrons, cooling it to very low temperatures and shrinking its volume, the electrons are forced to begin invading one another's space. But Pauli's principle forbids this, so they dart away from one another at speeds that can approach that of light. For electrons and the other Pauli particles, the pressure created by these eeing particles--the degeneracy pressure--persists even if the gas is cooled to absolute zero. It has nothing to do with the fact that the electrons repel one another electrically. Neutrons, which have no charge, do the same thing. It is pure quantum physics.

Quantum Statistics and White Dwarfs

BUT WHAT HAS quantum statistics got to do with the stars? Before the turn of the century, astronomers had begun to identify a class of peculiar stars that are small and dim: white dwarfs. The one that accompanies Sirius, the brightest star in the heavens, has the mass of the sun but emits about 1/360 the light. Given their mass and size, white dwarfs must be humongously dense. Sirius's companion is some 61,000 times denser than water. What are these bizarre objects? Enter Sir Arthur Eddington.

When I began studying physics in the late 1940s, Eddington was a hero of mine but for the wrong reasons. I knew nothing about his great work in astronomy. I admired his popular books (which, since I have learned more about physics, now seem rather silly to me). Eddington, who died in 1944, was a neo-Kantian who believed that everything of significance about the universe could be learned by examining what went on inside one's head. But starting in the late 1910s, when Eddington led one of the two expeditions that confirmed Einstein's prediction that the sun bends starlight, until the late 1930s, when Eddington really started going off the deep end, he was truly one of the giants of 20th-century science. He practically created the discipline that led to the first understanding of the internal constitution of stars, the title of his classic 1926 book. To him, white dwarfs were an affront, at least from an aesthetic point of view. But he studied them nonetheless and came up with a liberating idea.

In 1924 Eddington proposed that the gravitational pressure that was squeezing a dwarf might strip some of the electrons off protons. The atoms would then lose their boundaries and might be squeezed together into a small, dense package. The dwarf would eventually stop collapsing because of the Fermi-Dirac degeneracy pressure--that is, when the Pauli exclusion principle forced the electrons to recoil from one another.

The understanding of white dwarfs took another step forward in July 1930, when Subrahmanyan Chandrasekhar, who was 19, was onboard a ship sailing from Madras to Southampton. He had been accepted by British physicist R. H. Fowler to study with him at the University of Cambridge (where Eddington was, too). Having read Eddington's book on the stars and Fowler's book on quantum-statistical mechanics, Chandrasekhar had become fascinated by white dwarfs. To pass the time during the voyage, Chandrasekhar asked himself: Is there any upper limit to how massive a white dwarf can be before it collapses under the force of its own gravitation? His answer set off a revolution.

A white dwarf as a whole is electrically neutral, so all the electrons must have a corresponding proton, which is some 2,000 times more massive. Consequently, protons must supply the bulk of the gravitational compression. If the dwarf is not collapsing, the degeneracy pressure of the electrons and the gravitational collapse of the protons must just balance. This balance, it turns out, limits the number of protons and hence the mass of the dwarf. This maximum is known as the Chandrasekhar limit and equals about 1.4 times the mass of the sun. Any dwarf more massive than this number cannot be stable.

Chandrasekhar's result deeply disturbed Eddington. What happens if the mass is more than 1.4 times that of the sun? He was not pleased with the answer. Unless some mechanism could be found for limiting the mass of any star that was eventually going to compress itself into a dwarf, or unless Chandrasekhar's result was wrong, massive stars were fated to collapse gravitationally into oblivion.

Eddington found this intolerable and proceeded to attack Chandrasekhar's use of quantum statistics--both publicly and privately. The criticism devastated Chandrasekhar. But he held his ground, bolstered by people such as Danish physicist Niels Bohr, who assured him that Eddington was simply wrong and should be ignored.

A Singular Sensation

AS RESEARCHERS explored quantum statistics and white dwarfs, others tackled Einstein's work on gravitation, his general theory of relativity. As far as I know, Einstein never spent a great deal of time looking for exact solutions to his gravitational equations. The part that described gravity around matter was extremely complicated, because gravity distorts the geometry of space and time, causing a particle to move from point to point along a curved path. More important to Einstein, the source of gravity--matter--could not be described by the gravitational equations alone. It had to be put in by hand, leaving Einstein to feel the equations were incomplete. Still, approximate solutions could describe with sufficient accuracy phenomena such as the bending of starlight. Nevertheless, he was impressed when, in 1916, German astronomer Karl Schwarzschild came up with an exact solution for a realistic situation--in particular, the case of a planet orbiting a star.

In the process, Schwarzschild found something disturbing. There is a distance from the center of the star at which the mathematics goes berserk. At this distance, now called the Schwarzschild radius, time vanishes, and space becomes infinite. The equation becomes what mathematicians call singular. The Schwarzschild radius is usually much smaller than the radius of the object. For the sun, for example, it is three kilometers, whereas for a one-gram marble it is 10 28 centimeter.

Schwarzschild was, of course, aware that his formula went crazy at this radius, but he decided that it did not matter. He constructed a simplified model of a star and showed that it would take an infinite gradient of pressure to compress it to his radius. The finding, he argued, served no practical interest.

But his analysis did not appease everybody. It bothered Einstein, because Schwarzschild's model star did not satisfy certain technical requirements of relativity theory. Various people, however, showed that one could rewrite Schwarzschild's solutions so that they avoided the singularity. But was the result really nonsingular? It would be incorrect to say that a debate raged, because most physicists had little regard for these matters--at least until 1939.

To make his point, Einstein focused on a collection of small particles moving in circular orbits under the inuence of one another's gravitation--in effect, a system resembling a spherical star cluster. He then asked whether such a configuration could collapse under its own gravity into a stable star with a radius equal to its Schwarzschild radius. He concluded that it could not, because at a somewhat larger radius the stars in the cluster would have to move faster than light in order to keep the configuration stable. Although Einstein's reasoning is correct, his point is irrelevant: it does not matter that a collapsing star at the Schwarzschild radius is unstable, because the star collapses past that radius anyway. I was much taken by the fact that the then 60-year-old Einstein presents in this paper tables of numerical results, which he must have gotten by using a slide rule. But the paper, like the slide rule, is now a historical artifact.

From Neutrons to Black Holes

WHILE EINSTEIN was doing this research, an entirely different enterprise was unfolding in California. Oppenheimer and his students were creating the modern theory of black holes. The curious thing about the black hole research is that it was inspired by an idea that turned out to be entirely wrong. In 1932 British experimental physicist James Chadwick found the neutron, the neutral component of the atomic nucleus. Soon thereafter speculation began--most notably by Fritz Zwicky of the California Institute of Technology and independently by the brilliant Soviet theoretical physicist Lev D. Landau--that neutrons could lead to an alternative to white dwarfs.

When the gravitational pressure got large enough, they argued, an electron in a star could react with a proton to produce a neutron. (Zwicky even conjectured that this process would happen in supernova explosions; he was right, and these neutron stars we now identify as pulsars.) At the time of this work, the actual mechanism for generating the energy in ordinary stars was not known. One solution placed a neutron star at the center of ordinary stars, in somewhat the same spirit that many astrophysicists now conjecture that black holes power quasars.

The question then arose: What was the equivalent of the Chandrasekhar mass limit for these stars? Determining this answer is much harder than finding the limit for white dwarfs. The reason is that the neutrons interact with one another with a strong force whose specifics we still do not fully understand. Gravity will eventually overcome this force, but the precise limiting mass is sensitive to the details. Oppenheimer published two papers on this subject with his students Robert Serber and George M. Volkoff and concluded that the mass limit here is comparable to the Chandrasekhar limit for white dwarfs. The first of these papers was published in 1938 and the second in 1939. (The real source of stellar energy--fusion--was discovered in 1938 by Hans Bethe and Carl Friedrich von Weizscker, but it took a few years to be accepted, and so astrophysicists continued to pursue alternative theories.)

To simplify matters, Oppenheimer told Snyder to make certain assumptions and to neglect technical considerations such as the degeneracy pressure or the possible rotation of the star. Oppenheimer's intuition told him that these factors would not change anything essential. (These assumptions were challenged many years later by a new generation of researchers using sophisticated high-speed computers--poor Snyder had an old-fashioned mechanical desk calculator--but Oppenheimer was right. Nothing essential changes.) With the simplified assumptions, Snyder found out that what happens to a collapsing star depends dramatically on the vantage point of the observer.

Two Views of a Collapse

LET US START with an observer at rest a safe distance from the star. Let us also suppose that there is another observer attached to the surface of the star--co-moving with its collapse--who can send light signals back to his stationary colleague. The stationary observer will see the signals from his moving counterpart gradually shift to the red end of the electromagnetic spectrum. If the frequency of the signals is thought of as a clock, the stationary observer will say that the moving observer's clock is gradually slowing down.

Indeed, at the Schwarzschild radius the clock will slow down to zero. The stationary observer will argue that it took an infinite amount of time for the star to collapse to its Schwarzschild radius. What happens after that we cannot say, because, according to the stationary observer, there is no after. As far as this observer is concerned, the star is frozen at its Schwarzschild radius.

Indeed, until December 1967, when physicist John A. Wheeler of Princeton University coined the name black hole in a lecture he presented, these objects were often referred to in the literature as frozen stars. This frozen state is the real significance of the singularity in the Schwarzschild geometry. As Oppenheimer and Snyder observed in their paper, the collapsing star tends to close itself off from any communication with a distant observer; only its gravitational field persists. In other words, a black hole has been formed.

But what about observers riding with collapsing stars? These observers, Oppenheimer and Snyder pointed out, have a completely different sense of things. To them, the Schwarzschild radius has no special significance. They pass right through it and on to the center in a matter of hours, as measured by their watches. They would, however, be subject to monstrous tidal gravitational forces that would tear them to pieces.

The year was 1939, and the world itself was about to be torn to pieces. Oppenheimer was soon to go off to war to build the most destructive weapon ever devised by humans. He never worked on the subject of black holes again. As far as I know, Einstein never did, either. In peacetime, in 1947, Oppenheimer became the director of the Institute for Advanced Study in Princeton, N.J., where Einstein was a professor. From time to time they talked. There is no record of their ever having discussed black holes. Further progress would have to wait until the 1960s, when discoveries of quasars, pulsars and compact x-ray sources reinvigorated thinking about the mysterious fate of stars.

JEREMY BERNSTEIN is professor emeritus of physics at the Stevens Institute of Technology in Hoboken, N.J. He was a staff writer for the New Yorker from 1961 to 1995 and is the recipient of many science writing awards. He is a former adjunct professor at the Rockefeller University and a vice president of the board of trustees of the Aspen Center for Physics, of which he is now an honorary trustee. Bernstein has written 12 books on popular science and mountain travel. This article is adapted from his collection of essays, A Theory for Everything , published by Copernicus Books in 1996.

Physical Review Journals

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The Work of Stephen Hawking in Physical Review

To mark the passing of Stephen Hawking, we gathered together his 55 papers in Physical Review D and Physical Review Letters . They probe the edges of space and time, from "Black holes and thermodynamics” to "Wave function of the Universe."

90 citations

Occurrence of singularities in open universes, s. w. hawking, phys. rev. lett. 15 , 689 (1965) – published 25 october 1965, 97 citations, singularities in the universe, phys. rev. lett. 17 , 444 (1966) – published 22 august 1966, 738 citations, gravitational radiation from colliding black holes, phys. rev. lett. 26 , 1344 (1971) – published 24 may 1971, 50 citations, theory of the detection of short bursts of gravitational radiation, g. w. gibbons and s. w. hawking, phys. rev. d 4 , 2191 (1971) – published 15 october 1971, 958 citations, black holes and thermodynamics, phys. rev. d 13 , 191 (1976) – published 15 january 1976, 757 citations, path-integral derivation of black-hole radiance, j. b. hartle and s. w. hawking, phys. rev. d 13 , 2188 (1976) – published 15 april 1976, 1,507 citations, breakdown of predictability in gravitational collapse, phys. rev. d 14 , 2460 (1976) – published 15 november 1976, 2,179 citations, cosmological event horizons, thermodynamics, and particle creation, phys. rev. d 15 , 2738 (1977) – published 15 may 1977, 2,185 citations, action integrals and partition functions in quantum gravity, phys. rev. d 15 , 2752 (1977) – published 15 may 1977, 110 citations, quantum gravity and path integrals, phys. rev. d 18 , 1747 (1978) – published 15 september 1978, 394 citations, bubble collisions in the very early universe, s. w. hawking, i. g. moss, and j. m. stewart, phys. rev. d 26 , 2681 (1982) – published 15 november 1982, milestone 1,974 citations, wave function of the universe, phys. rev. d 28 , 2960 (1983) – published 15 december 1983, 468 citations, origin of structure in the universe, j. j. halliwell and s. w. hawking, phys. rev. d 31 , 1777 (1985) – published 15 april 1985, 134 citations, arrow of time in cosmology, phys. rev. d 32 , 2489 (1985) – published 15 november 1985, 300 citations, wormholes in spacetime, phys. rev. d 37 , 904 (1988) – published 15 february 1988, 113 citations, spectrum of wormholes, s. w. hawking and don n. page, phys. rev. d 42 , 2655 (1990) – published 15 october 1990, 16 citations, wormholes in string theory, alex lyons and s. w. hawking, phys. rev. d 44 , 3802 (1991) – published 15 december 1991, 506 citations, chronology protection conjecture, phys. rev. d 46 , 603 (1992) – published 15 july 1992, 89 citations, evaporation of two-dimensional black holes, phys. rev. lett. 69 , 406 (1992) – published 20 july 1992, 36 citations, kinks and topology change, phys. rev. lett. 69 , 1719 (1992) – published 21 september 1992, 51 citations, origin of time asymmetry, s. w. hawking, r. laflamme, and g. w. lyons, phys. rev. d 47 , 5342 (1993) – published 15 june 1993, 7 citations, quantum coherence in two dimensions, s. w. hawking and j. d. hayward, phys. rev. d 49 , 5252 (1994) – published 15 may 1994, 5 citations, superscattering matrix for two-dimensional black holes, phys. rev. d 50 , 3982 (1994) – published 15 september 1994, 305 citations, entropy, area, and black hole pairs, s. w. hawking, gary t. horowitz, and simon f. ross, phys. rev. d 51 , 4302 (1995) – published 15 april 1995, 71 citations, pair production of black holes on cosmic strings, s. w. hawking and simon f. ross, phys. rev. lett. 75 , 3382 (1995) – published 6 november 1995, 69 citations, probability for primordial black holes, r. bousso and s. w. hawking, phys. rev. d 52 , 5659 (1995) – published 15 november 1995, 39 citations, quantum coherence and closed timelike curves, phys. rev. d 52 , 5681 (1995) – published 15 november 1995, 157 citations, duality between electric and magnetic black holes, phys. rev. d 52 , 5865 (1995) – published 15 november 1995, 74 citations, virtual black holes, phys. rev. d 53 , 3099 (1996) – published 15 march 1996, 176 citations, pair creation of black holes during inflation, raphael bousso and stephen w. hawking, phys. rev. d 54 , 6312 (1996) – published 15 november 1996, 17 citations, evolution of near-extremal black holes, s. w. hawking and m. m. taylor-robinson, phys. rev. d 55 , 7680 (1997) – published 15 june 1997, 26 citations, loss of quantum coherence through scattering off virtual black holes, phys. rev. d 56 , 6403 (1997) – published 15 november 1997, 59 citations, trace anomaly of dilaton-coupled scalars in two dimensions, raphael bousso and stephen hawking, phys. rev. d 56 , 7788 (1997) – published 15 december 1997, 25 citations, models for chronology selection, m. j. cassidy and s. w. hawking, phys. rev. d 57 , 2372 (1998) – published 15 february 1998, 136 citations, (anti-)evaporation of schwarzschild–de sitter black holes, phys. rev. d 57 , 2436 (1998) – published 15 february 1998, 18 citations, bulk charges in eleven dimensions, phys. rev. d 58 , 025006 (1998) – published 12 june 1998, 15 citations, inflation, singular instantons, and eleven dimensional cosmology, s. w. hawking and harvey s. reall, phys. rev. d 59 , 023502 (1998) – published 7 december 1998, 114 citations, gravitational entropy and global structure, s. w. hawking and c. j. hunter, phys. rev. d 59 , 044025 (1999) – published 26 january 1999, 164 citations, nut charge, anti–de sitter space, and entropy, s. w. hawking, c. j. hunter, and don n. page, phys. rev. d 59 , 044033 (1999) – published 28 january 1999, 416 citations, rotation and the ads-cft correspondence, s. w. hawking, c. j. hunter, and m. m. taylor-robinson, phys. rev. d 59 , 064005 (1999) – published 1 february 1999, 23 citations, lorentzian condition in quantum gravity, phys. rev. d 59 , 103501 (1999) – published 29 march 1999, 166 citations, charged and rotating ads black holes and their cft duals, s. w. hawking and h. s. reall, phys. rev. d 61 , 024014 (1999) – published 20 december 1999, 357 citations, brane-world black holes, a. chamblin, s. w. hawking, and h. s. reall, phys. rev. d 61 , 065007 (2000) – published 25 february 2000, 197 citations, brane new world, s. w. hawking, t. hertog, and h. s. reall, phys. rev. d 62 , 043501 (2000) – published 29 june 2000, 53 citations, gravitational waves in open de sitter space, s. w. hawking, thomas hertog, and neil turok, phys. rev. d 62 , 063502 (2000) – published 31 july 2000, 130 citations, trace anomaly driven inflation, phys. rev. d 63 , 083504 (2001) – published 5 march 2001, 202 citations, living with ghosts, s. w. hawking and thomas hertog, phys. rev. d 65 , 103515 (2002) – published 9 may 2002, 28 citations, why does inflation start at the top of the hill, phys. rev. d 66 , 123509 (2002) – published 20 december 2002, 275 citations, information loss in black holes, phys. rev. d 72 , 084013 (2005) – published 18 october 2005, 46 citations, populating the landscape: a top-down approach, phys. rev. d 73 , 123527 (2006) – published 23 june 2006, no-boundary measure of the universe, james b. hartle, s. w. hawking, and thomas hertog, phys. rev. lett. 100 , 201301 (2008) – published 23 may 2008, 120 citations, classical universes of the no-boundary quantum state, phys. rev. d 77 , 123537 (2008) – published 25 june 2008, 33 citations, no-boundary measure in the regime of eternal inflation, james hartle, s. w. hawking, and thomas hertog, phys. rev. d 82 , 063510 (2010) – published 8 september 2010, 35 citations, local observation in eternal inflation, phys. rev. lett. 106 , 141302 (2011) – published 8 april 2011, featured in physics editors' suggestion 511 citations, soft hair on black holes, stephen w. hawking, malcolm j. perry, and andrew strominger, phys. rev. lett. 116 , 231301 (2016) – published 6 june 2016.

A black hole may carry “soft hair,” low-energy quantum excitations that release information when the black hole evaporates.

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There Are 40 Billion Billions Of Black Holes In The Universe

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Lumen Boco, co-author of the paper, comments: “Our work provides a robust theory for the generation of light seeds for (super)massive black holes at high redshift, and can constitute a starting point to investigate the origin of ‘heavy seeds’, that we will pursue in a forthcoming paper.

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Can Black Holes Be Created From Pure Light? New Paper Challenges Theory.

Squeeze enough stuff into one spot, space-time itself will pucker up in a sweet cosmic kiss known as a black hole .

As far as Einstein's sums are concerned, that 'stuff' includes the massless glow of electromagnetic radiation. Given E = mc 2 , which describes the equivalence between mass and energy, the energy of light itself should – in theory – be capable of creating a black hole if enough of it is concentrated in one spot.

Before you crack out the big-gun lasers and punch some holes into the Universe's floorboards, there's one thing researchers from the Complutense University of Madrid in Spain and the University of Waterloo in Canada want you to know.

Something called the Schwinger effect could make the whole thing impossible before you even get started.

Einstein's general theory of relativity is a description of space and time distorting in relation to the presence of energy, such as that contained by a mass. Put enough mass in one spot and the distortion will become so extreme, nothing – not even light – will escape.

Back in the mid-1950s , American theoretical physicist John Wheeler discovered there was nothing in Einstein's theory to rule out the possibility that the energy within a sufficient concentration of gravitational or electromagnetic waves could warp space-time enough to keep those same waves trapped in place.

He called this exotic object a geon , and considered it a kind of hypothetical, highly unstable particle.

Today, geons are a relic of an age of scientific musings that also gave us wormholes and white holes ; theoretical toys that tell us more about the limits of mathematical models than they do about physical reality.

Yet a form of geon that Wheeler referred to as a " kugelblitz " pops up every now and then in science fiction as a fantastic power source. German for 'ball lightning ', these itty-bitty proton-sized black holes were proposed to form in the intense focus of incredibly energetic beams of light, such as a futuristic high-powered laser.

While general relativity gives the green light to kugelblitze, quantum physics has its doubts. So theoretical physicist Álvaro Álvarez-Domínguez from the Complutense University of Madrid and his team ran the numbers on the behavior of electromagnetic fields as their energy rises to extreme levels.

The quantum landscape is like a casino where waves of possibility ripple constantly like non-stop roulette wheels. Small bets rarely pay, but pile enough cash on any one table, you're almost guaranteed a win.

Similarly, a strong electromagnetic field in an otherwise empty space almost guarantees pairs of electrons and positrons will emerge from the quantum flurry of endless possibilities.

In a paper that is yet to be peer-reviewed, Álvarez-Domínguez and his team showed this phenomenon known as the Schwinger effect would prevent the formation of kugelblitze ranging in size from nearly twice the size of Jupiter down to a fraction of the size of a proton.

In effect, piling all of that light in one spot would provide the necessary energy for pairs of charged particles to pop into existence and fly off close to the speed of light, preventing the growing dimple in space-time from ever developing a black hole-defining event horizon.

"Our analysis strongly suggests that the formation of black holes solely from electromagnetic radiation is impossible, either by concentrating light in a hypothetical laboratory setting or in naturally occurring astrophysical phenomena," the team writes in their analysis.

That's not to rule out the possibility completely. The researchers admit things might have been different in the " exceptionally extreme conditions " of the early Universe.

Other forms of geon, such as those based on gravitational waves , remain a curiosity that could have also existed in the nascent cosmos billions of years in the past.

Those who are banking on a kugelblitz-powered spacecraft to jet them to the stars now might have to go back to the drawing board, however.

This paper is available on the preprint server arXiv .

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The surprising behavior of black holes in an expanding universe

by David Appell , Phys.org

event horizon of a black hole -- make a realistic image of space in the background, and a black circle in the middle, which represents a black hole, with slight light shining around the black circle

A physicist investigating black holes has found that, in an expanding universe, Einstein's equations require that the rate of the universe's expansion at the event horizon of every black hole must be a constant, the same for all black holes. In turn this means that the only energy at the event horizon is dark energy, the so-called cosmological constant. The study is published on the arXiv preprint server.

"Otherwise," said Nikodem Popławski, a Distinguished Lecturer at the University of New Haven, "the pressure of matter and curvature of spacetime would have to be infinite at a horizon, but that is unphysical."

Black holes are a fascinating topic because they are about the simplest things in the universe: their only properties are mass, electric charge and angular momentum (spin). Yet their simplicity gives rise to a fantastical property—they have an event horizon at a critical distance from the black hole, a nonphysical surface around it, spherical in the simplest cases. Anything closer to the black hole, that is, inside the event horizon, can never escape the black hole.

Black holes were predicted in 1916 by Karl Schwarzschild while serving as a German soldier at the Russian front, while he was suffering from the painful autoimmune skin disease pemphigus .

Using Einstein's equations of general relativity , he assumed a massive, nonrotating, perfectly round object in an otherwise empty and unchanging universe and discovered the event horizon. The radius of the event horizon is proportional to a black hole's mass. Inside the horizon, not even light, the fastest object in the universe, can escape the hole.

Schwarzschild also found an apparent singularity at the black hole's center, a place of infinite density where Einstein's laws of gravity apparently breakdown.

Astronomers have since found that most galaxies appear to have a supermassive black hole at their center; for the Milky Way it is Sagittarius A*, with a mass over four million times that of the sun. A black hole was directly imaged only in 2019, a black spot with a halo of light around it, located in the center of the galaxy Messier 87, 55 million light-years from Earth.

Going beyond Schwarzschild, Popławski assumed a massive, centrally symmetric object in an expanding universe . In this case, the solution to Einstein's equations for the structure of spacetime around the mass was first obtained in 1933 by the British mathematician and cosmologist George McVittie.

McVittie found that near the mass, spacetime is like that of Schwarzschild's, with an event horizon, but far from the mass the universe is expanding like our universe is today. The Hubble parameter, also called the Hubble constant , specifies the rate of expansion of the universe.

Popławski used McVittie's solution to find that the rate of the expansion of space at the event horizon must be a constant, related only to the cosmological constant (which can be interpreted as the energy density of the vacuum of spacetime). Today we know this as the density of dark energy . That is, the only energy at the horizon is dark energy. The consequence, he said, is that different parts of the universe expand at different rates.

In fact, something similar has been found with the so-called " Hubble tension ," a statistically significant discrepancy between two different measured values of the Hubble parameter, depending on whether "late universe" measurements are used or "early universe" techniques based on measurements of the cosmic microwave background. In his work, Popławski said this discrepancy "is a natural consequence of a correct analysis of the spacetime of a black hole in an expanding universe within Einstein's general theory of relativity."

Furthermore, his equations show that a consequence of the universe expanding at different rates is that the cosmological constant—and hence the value of dark energy—must be positive. Otherwise, without that constant, Popławski said, "a closed universe would be oscillatory and could not create cosmic voids."

"It is the simplest explanation of the observed current acceleration of the universe."

For a star, say, the universe is also expanding at its surface boundary, but the body does not expand because it is gravitationally and electromagnetically bound.

An event horizon, though, is a mathematically-abstract thing, not anything made of matter or energy but made simply of points of space, so a constant expansion rate of space there is not surprising. The event horizon itself (and thus a black hole) is not expanding; points of space outside the horizon are moving away from it.

Real black holes rotate, but if the rotation is typically slow, Popławski's conclusions should apply to them as well to a good approximation. But measuring the Hubble parameter at an event horizon is currently impossible, unless new techniques are developed.

An observer at the event horizon could in principle measure the Hubble parameter there but would be forever unable to communicate his value to the rest of the universe as he is falling past the event horizon, and no information can possibly be sent back across it.

This ties in, Popławski said, with a hypothesis he published in 2010 : that every black hole is actually a wormhole (an Einstein-Rosen bridge ) to a new universe on the other side of its event horizon.

"The event horizon is a doorway from one universe to another," he said. "This doorway does not grow with the expansion of the universe ... If this occurs for the event horizon of the black hole forming a universe, it should also work for the event horizons of other black holes in that universe."

Journal information: arXiv

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Extended data figures and tables

Extended Data Fig. 2 NV/CIV versus NV/HeII flux ratio diagram.

Extended Data Fig. 3 CIII]/CIV versus CIII]/HeII flux ratio diagrams.

Extended Data Fig. 4 Flux ratios of density-sensitive nitrogen lines as a function of hydrogen gas density, n H .

Extended Data Fig. 5 Low resolution (prism) spectrum of GN-z11.

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