Mitch Begelman

Trying to Solve a Key Black Hole Mystery: Simulating Magnetic Flows Around Black Holes

Steven Burrows — Tue, 18 Feb 2025 19:48:01 +0000

Trying to Solve a Key Black Hole Mystery: Simulating Magnetic Flows Around Black Holes Steven Burrows Tue, 02/18/2025 - 12:48

Kenna Hughes-Castleberry / JILA Science Communicator

Black holes have been fascinating subjects of study, not just because they are cosmic vacuum cleaners, but also as engines of immense power capable of extracting and redistributing energy on a staggering scale. These dark giants are often surrounded by swirling disks of gas and dust, known as accretion disks. When these disks are strongly magnetized, they can act like galactic power plants, extracting energy from the black hole’s spin in a process known as the Blandford-Znajek (BZ) effect.

While scientists have theorized that the BZ effect is the primary mechanism in the energy extraction process, many unknowns remain, like what determines how much energy is funneled into powerful jets—powerful streams of particles and energy ejected along the black hole's poles—or dissipated as heat.

To answer these questions, JILA postdoctoral researcher Prasun Dhang, and JILA Fellows and University of Colorado Boulder Astrophysical and Planetary Sciences professors Mitch Begelman and Jason Dexter, turned to advanced computer simulations. By modeling black holes surrounded by thin, highly magnetized accretion disks, they sought to uncover the underlying physics that drives these enigmatic systems. Their findings, published in The Astrophysical Journal, offer crucial insights into the complex physics around black holes and could redefine how we understand their role in shaping galaxies.

“It's long been known that infalling gas can extract spin energy from a black hole,” elaborates Dexter. “Usually, we assume this is important for powering jets. By making more precise measurements, Prasun has shown there's a lot more energy extracted than previously known. This energy could be radiated away as light, or it could cause gas to flow outwards. Either way, extracted spin energy could be an important energy source for lighting up the regions near the black hole event horizon.”

Comparing Black Hole to Black Hole

For decades, scientists have studied black holes and their interactions with surrounding gas and magnetic fields to understand how they power some of the universe’s most energetic phenomena. Early research focused primarily on low-luminosity black hole sources with quasi-spherical accretion flow as these systems are comparatively easier to simulate and align with many observed jets.

However, high-luminosity black holes with geometrically thinner, denser magnetized disks present a unique challenge. These systems are theoretically unstable due to imbalances in heating and cooling.

Artist render of a black hole surrounded by a highly magnetized thin disk. Image credit: Steven Burrows / JILA

However, previous studies, including those by Mitch Begelman, suggested that strong magnetic fields might stabilize these thin disks, but the details of their role in energy extraction and jet formation remained unclear in such conditions.

“We wanted to understand how energy extraction works in these highly magnetized environments,” Dhang explains.

Simulating Magnetized Flows Around Black Holes

The team used advanced computer simulations to explore this phenomenon, specifically, a special type of model called the 3D general relativistic magnetohydrodynamic (GRMHD) model. The GRMHD model works as a computational framework that simulates the behavior of magnetized plasma in the curved spacetime around black holes, combining the physics of magnetic fields, fluid dynamics, and Einstein's theory of general relativity to capture the complex interactions in these extreme environments. Using the framework, the researchers observed how magnetic fields interacted with black holes spinning at different speeds.

“The goal was to see how magnetic flux threading [permeating] the black hole impacts energy extraction and whether it leads to the formation of jets,” Dhang says.
The simulations modeled thin, magnetized accretion disks and examined how much energy the black hole transferred to its surroundings. By studying the efficiency of this energy extraction, the team identified various black hole spins and magnetic configurations with jets.

Manifestation of BZ power

From their simulations, the team found that depending on the black hole's spin, between 10% and 70% of the energy extracted through the BZ process was channeled into jets.

“The higher the spin, the more energy the black hole can release,” Dhang notes.

However, not all energy went into jets; some was absorbed back into the disk or dissipated as heat.

While the simulations couldn’t determine where the excess energy went, Dhang plans to study this further to better understand how jets form, as jets are often found in active galactic nuclei systems such as quasars.

Mysteries Continue

From their models, the researchers found that the strong magnetic fields increased the disk's radiative efficiency, making it brighter. This extra luminosity may explain why some black holes appear far more luminous than theoretical models predict.
“The unused energy close to the black hole could heat the disk and contribute to a corona,” Dhang notes.

The corona, a region of hot gas surrounding the black hole that emits intense x-rays, is crucial for shaping the light we observe from these systems, but its exact formation process remains unclear.

The researchers hope to use further simulations to understand the dynamics of making a black hole corona.

This work was supported by the National Science Foundation, the NASA Astrophysics Theory Program, and the Alfred P. Sloan Fellowship.

JILA postdoctoral researcher Prasun Dhang, and JILA Fellows and University of Colorado Boulder Astrophysical and Planetary Sciences professors Mitch Begelman and Jason Dexter, turned to advanced computer simulations to model black holes surrounded by thin, highly magnetized accretion disks, to uncover the underlying physics that drives these enigmatic systems. Their findings, published in The Astrophysical Journal, offer crucial insights into the complex physics around black holes and could redefine how we understand their role in shaping galaxies.

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JILA Fellow and University of Colorado Boulder APS Distinguished Professor Mitch Begelman Inducted as 2025 AAS Fellow, Joining APS Professors James Green and J. Michael Shull in Prestigious Recognition

Steven Burrows — Mon, 13 Jan 2025 17:44:12 +0000

JILA Fellow and University of Colorado Boulder APS Distinguished Professor Mitch Begelman Inducted as 2025 AAS Fellow, Joining APS Professors James Green and J. Michael Shull in Prestigious Recognition Steven Burrows Mon, 01/13/2025 - 10:44

Kenna Hughes-Castleberry / JILA Science Communicator

Mitchell Begelman. Image credit: Glenn Asakawa, 2020.

JILA Fellow and the Department of Astrophysical and Planetary Sciences (APS) at the University of Colorado Boulder Distinguished Professor Mitch Begelman has been inducted as a 2025 American Astronomical Society (AAS) Fellow. Joining Professor Begelman in this recognition are APS Professors James Green and J. Michael Shull, now an Adjunct Professor of Physics and Astronomy at the University of North Carolina, Chapel Hill. Together, their contributions underscore ��Ҵ�ý�ƽ�� leadership in astrophysics and planetary sciences.

Professor Begelman was honored for his pioneering analytical and computational studies of high-energy astrophysical phenomena, including developing the “quasi-star theory” explaining the formation of supermassive black holes. His dedication to public engagement has further enriched the public’s understanding of black holes through two acclaimed books.

“I am deeply honored to be recognized by the AAS and to share this distinction with my esteemed colleagues,” said Begelman. “This recognition reflects the collaborative spirit of research at ��Ҵ�ý�ƽ�� and JILA, where groundbreaking ideas flourish.”

Green was commended for his exceptional contributions to ultraviolet space astronomy and his role in advancing spectrograph designs that have enabled groundbreaking discoveries.
Meanwhile, Shull was recognized for his theoretical modeling and observational studies that have provided transformative insights into intergalactic and interstellar gas.

“It’s wonderful to see that ��Ҵ�ý�ƽ�� APS department had three of the 24 AAS Fellows awarded this year,” Shull stated.

These accolades reflect the tireless dedication of all three researchers to advancing astrophysics and mentoring the next generation of scientists.

��Ҵ�ý�ƽ�� and JILA celebrate this recognition of Professors Begelman, Green, and Shull, whose achievements continue to inspire and elevate the global astronomy community.

Read the full AAS announcement here.

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JILA Fellow and Astrophysical and Planetary Sciences Professor Mitch Begelman is Elected to the National Academy of Sciences

Steven Burrows — Tue, 30 Apr 2024 17:28:12 +0000

JILA Fellow and Astrophysical and Planetary Sciences Professor Mitch Begelman is Elected to the National Academy of Sciences Steven Burrows Tue, 04/30/2024 - 11:28

Kenna Hughes-Castleberry / JILA Science Communicator

Mitchell Begelman. Image credit: Glenn Asakawa, 2020.

JILA is thrilled to announce that Dr. Mitch Begelman, a JILA Fellow and esteemed professor in the Department of Astrophysical and Planetary Sciences at the University of Colorado Boulder, has been elected as a member of the National Academy of Sciences. This prestigious honor is bestowed in recognition of his distinguished and ongoing contributions to original research in astrophysics.

The National Academy of Sciences, which announced the election of 120 new members and 24 international members this year, selects individuals who have achieved significant accomplishments in their respective scientific fields.

Begelman's inclusion in this elite group brings the total number of active members to 2,617 and international members to 537, emphasizing the Academy's commitment to excellence and scientific advancement.

Begelman's research primarily explores the frontiers of theoretical and high-energy astrophysics, focusing on the dynamics of black holes and their energy outputs. His pioneering work has significantly advanced the understanding of how black holes influence their surrounding environments and contribute to the broader structure of the universe.

The election of Begelman to the National Academy of Sciences not only honors his individual achievements but also highlights JILA's and the University of Colorado Boulder's role as leading institutions in astrophysical research.

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New Findings From the JWST: How Black Holes Switched from Creating to Quenching Stars

Steven Burrows — Tue, 06 Feb 2024 18:27:01 +0000

New Findings From the JWST: How Black Holes Switched from Creating to Quenching Stars Steven Burrows Tue, 02/06/2024 - 11:27

Kenna Hughes-Castleberry / JILA Science Communicator

The transition in star formation rates and black hole growth as redshift decreases from regimes where positive feedback dominates to a later epoch when feedback is largely negative. Image Credit: Steven Burrows / JILA

This surprising discovery is not just a visual marvel, it's a clue that could unlock the secrets of how galaxies and their enigmatic black holes began their cosmic journey.
“The astonishing discovery from James Webb is that not only does the universe have these very compact and infrared bright objects, but they're probably regions where huge black holes already exist,” explains JILA Fellow and University of Colorado Boulder astrophysics professor Mitch Begelman. “That was thought to be impossible.”

Begelman and a team of other astronomers, including Joe Silk, a professor of astronomy at Johns Hopkins University, published their findings in The Astrophysical Journal Letters, suggesting that new theories of galactic creation are needed to explain the existence of these huge black holes.

“Something new is needed to reconcile the theory of galaxy formation with the new data,” elaborates Silk, the lead author of the potentially groundbreaking study.

The Traditional Tale of Galaxy Formation

Astronomers had previously posited a somewhat orderly evolution when thinking about how galaxies formed. Conventional theories held that galaxies form gradually, assembling over billions of years. In this slow cosmic evolution, stars were thought to emerge first, lighting up the primordial darkness.

“The idea was that you went from this early generation of stars to the galaxies really becoming mainly dominated by stars,” adds Begelman. “Then, towards the end of this process, you start building these black holes.”

Supermassive black holes, those enigmatic and powerful entities, were believed to appear after the first stars, growing quietly in the galactic core. They were seen as regulators, occasionally bursting into action to temper the formation of new stars, thereby maintaining a galactic balance.

Challenging Conventional Wisdom

Thanks to the observations of the “little red dots” by the JWST, the researchers found that the first galaxies in the universe were brighter than expected, as many showed stars coexisting with central black holes known as quasars.

“Quasars are the most luminous objects in the universe,” explains Silk. “They are the products of gas accretion onto massive black holes in galaxy nuclei that generate immense luminosities, outshining their host galaxies. They are like monsters in the cuckoo’s nest.”

Seeing the coexistence of stars with black holes, the researchers quickly realized that the conventional theories of galaxy formation had to be flawed. “[This new data] looks like [the process is] reversed, that these black holes formed along with the first stars, and then the rest of the galaxy followed,” says Begelman. “We're saying that the growth of the black hole, at first, promotes the stars. And only later, when conditions change, does it flip into a mode of turning off the stars.”

From this proposed new process, the researchers found that the relationship between star formation and black hole formation seemed closer than expected, as each initially amplified the growth of the other via a process known as positive feedback.

“Star formation accelerates massive black hole formation, and vice versa, in an inextricably connected interplay of violence, birth, and death that is the new beacon of galaxy formation,” says Silk.

Then, after almost a billion years, the nurturing giants became suppressive, depleting the gas reservoirs in their galaxies and quenching star formation. This “negative feedback” was due to energy-conserving outflows—powerful winds that drove gas out of the galaxies, starving them of the material needed to create new stars.

A New Galactic Timeline

Armed with the revelation of the black holes’ nurturing behavior, the researchers proposed a new timeline for the shift from positive to negative feedback in early galaxy formation. By looking at the different light spectra and chemical signatures emitted from these “little red dots,” the researchers suggested that this shift occurred around 13 billion years ago, one billion years after the Big Bang, a period astronomers classify as “z ≈6.”

Identifying this transition epoch helps astronomers target specific periods in the universe's history for observation. It can guide future observational strategies using telescopes like JWST and others to study the early universe more effectively. Additionally, by understanding when this shift occurred, astronomers can better contextualize the characteristics of modern galaxies, including size, shape, star composition, and activity level.

Validating A Novel Process

To validate this new theory of collaborative galactic formation between the stars and black holes, and provide further insight into the processes involved, computer simulations are needed.

“This will take some time,” Begelman says. “The current computer simulations are rather primitive, and you need high resolution to understand everything. It takes a lot of computing power and is expensive.”

Until then, there are other steps the astronomy community can take to review and validate this new theory.

“The next steps will come from improved observations,” Silk adds. “The full power of JWST to study the spectra of the most distant galaxies will be unleashed over the next years.”

Both Begelman and Silk are optimistic about the rest of their field adopting their proposed idea.

“As far as I know, we're the first to go in quite this extreme direction,” adds Begelman. “I was kind of pushing the envelope over the years with my collaborators working on this black hole formation problem. But JWST shows us that we didn't think outside the box enough.”

Astronomers have long sought to understand the early universe, and thanks to the James Webb Space Telescope (JWST), a critical piece of the puzzle has emerged. The telescope's infrared detecting “eyes” have spotted an array of small, red dots, identified as some of the earliest galaxies formed in the universe.

This surprising discovery is not just a visual marvel, it's a clue that could unlock the secrets of how galaxies and their enigmatic black holes began their cosmic journey.

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A surging glow in a distant galaxy could change the way we look at black holes

Steven Burrows — Mon, 09 May 2022 17:00:32 +0000

A surging glow in a distant galaxy could change the way we look at black holes Steven Burrows Mon, 05/09/2022 - 11:00

Daniel Strain / ��Ҵ�ý�ƽ�� Strategic Communications

JILA Fellow Mitch Begelman and his team have worked with NASA to research more into black holes. Image credit: Pixabay, Open Commons Images

An international team of astrophysicists, including scientists from ��Ҵ�ý�ƽ��, may have pinpointed the cause of that shift. The magnetic field lines threading through the black hole appear to have flipped upside down, causing a rapid but short-lived change in the object’s properties. It was as if compasses on Earth suddenly started pointing south instead of north.

The findings, published May 5 in The Astrophysical Journal, could change how scientists look at supermassive black holes, said study coauthor Nicolas Scepi.

“Normally, we would expect black holes to evolve over millions of years,” said Scepi, a postdoctoral researcher at JILA, a joint research institute between ��Ҵ�ý�ƽ�� and the National Institute of Standards and Technology (NIST). “But these objects, which we call changing-look AGNs, evolve over very short time scales. Their magnetic fields may be key to understanding this rapid evolution.”

Scepi, alongside JILA Fellows Mitchell Begelman and Jason Dexter, first theorized that such a magnetic flip-flop could be possible in 2021.

The new study supports the idea. In it, a team led by Sibasish Laha of NASA’s Goddard Space Flight Center collected the most comprehensive data yet on this far-away object. The group drew on observations from seven telescope arrays on the ground and in space, tracing the flow of radiation from 1ES 1927+654 as the AGN blazed bright then dimmed back down.

Read the full article here.

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Mitch Begelman named CU Distinguished Professor

Steven Burrows — Wed, 09 Dec 2020 21:58:13 +0000

Mitch Begelman named CU Distinguished Professor Steven Burrows Wed, 12/09/2020 - 14:58

Steven Burrows / JILA Science Communications Manager

Mitchell Begelman. Image credit: Glenn Asakawa, 2020.

JILA fellow Mitch Begelman has been named CU Distinguished Professor by the CU Board of Regents. Begelman is the 5^th JILA fellow of only 118 CU professors who have been awarded this honor since 1977. He joins JILA fellows Dick McCray, Carl Lineberger, Carl Wieman, and Margaret Murnane with this distinction.

“This is a special honor, since CU is such an amazing community of scholars. Being part of JILA, in particular, has enabled me to work with some of the best faculty colleagues, students and postdocs in the world,” Begelman says of the award.

Begelman’s research has been recognized through numerous awards. He was awarded a Guggenheim Fellowship, the Helen B. Warner Prize of the American Astronomical Society, an Alfred P. Sloan Foundation Research Fellowship, and a Presidential Young Investigator Award. In 2018, Begelman was named a Professor of Distinction by the College of Arts and Sciences.

His group at JILA investigates high-energy and theoretical astrophysics. His seminal research on the growth of black holes and how they interact with their environments has shed new light on their key role in the Universe. He has also made highly notable contributions to understanding energetic astrophysical plasmas, including those in cosmic jets and accretion disks.

Begelman has fostered the beginning careers of dozens of Ph.D. students and postdoctoral scholars. His leadership has enabled them to go on to form their own research groups throughout the world. Begelman developed the highly popular Black Holes course, of which he says, “Where else would I have had the opportunity to develop and teach one of the first courses anywhere on the astrophysics of black holes, aimed at non-science majors?” In 1996 he was awarded the American Institute of Physics Science Writing Award for “Gravity’s Fatal Attraction: Black Holes in the Universe”; a book he co-authored with Sir Martin Rees, which is used for this course.

The title of Distinguished Professor is well deserved of such a prestigious, prolific JILA fellow.

Congratulations Mitch Begelman!

Mitch Begelman becomes the 5th JILA Fellow to be named a Distinguished Professor named CU Distinguished Professor.

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The Fast and the Furious

Steven Burrows — Fri, 03 Mar 2017 19:09:16 +0000

The Fast and the Furious Steven Burrows Fri, 03/03/2017 - 12:09

Julie Phillips / Science Communicator

The Crab Nebula (shown here) was created by a supernova and its spinning-neutron star remnant known as a pulsar. Pulsar wind nebulae have a high content of charged particles traveling at near the speed of light. Acceleration of these particles is likely caused by turbulence in the plasmas. Hubble Space Telescope mosaic image assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000 (NASA).

The lovely Crab Nebula was created by a supernova and its spinning-neutron-star remnant known as a pulsar. Pulsar wind nebulae, such as the Crab, shine because they contain plasmas of charged particles, such as electrons and positrons, traveling at near the speed of light. A key question in astrophysics has long been: What process accelerates some of the charged particles in plasmas to energies much higher than the average particle energy, giving them near light speeds? It can’t be collisions because the particles are simply too far apart to run into each other. It can’t just be high temperature, because cold plasma can also contain a small population of particles that move at a significant fraction of the speed of light. But, it must be something fairly ordinary, because relativistic plasmas are not uncommon.

Research associate Vladimir Zhdankin, Fellow Mitch Begelman and their University of Colorado Boulder colleagues believe they have found the answer.

“The physical mechanism responsible for accelerating these particles may have been hiding in plain sight,” Zhdankin explained. “The turbulence that inevitably forms in astrophysical plasmas can be an efficient particle accelerator.”

To test this idea, the researchers performed a massive supercomputer-based simulation of the effects of turbulence on relativistic plasmas in which the particles do not collide, but rather interact through their mutual electric and magnetic fields. The simulation showed how the energy of large-scale motions and magnetic fields starts a process that leads to a fraction of the particles gaining a massive amount of energy. As the turbulent motions approach relativistic speeds, particle acceleration becomes more efficient––sufficiently efficient to explain the acceleration of the charged particles in the plasma to near light speeds.

The researchers also found evidence that particle acceleration may also be the result of the random scattering of particles between turbulent magnetic structures, a mechanism originally proposed by Enrico Fermi in 1949. This mechanism suggests that turbulence may produce energetic particles in a variety of high-energy astrophysical systems.

Either way, Zhdankin says that turbulence has a big advantage over other proposed mechanisms, such as shockwaves and magnetic reconnection, as the main mechanism for accelerating charged particles to relativistic speeds in plasmas. Turbulence is likely to be inevitable in astrophysical plasmas. In contrast, the alternatives mechanisms require specific large-scale configurations that may not be always present.

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Dancing with the Stars

Steven Burrows — Tue, 22 Nov 2016 19:23:21 +0000

Dancing with the Stars Steven Burrows Tue, 11/22/2016 - 12:23

Julie Phillips / Science Communicator

Computer simulation of a tidal disruption event involving a pair of supermassive black holes in the center of a recently merged galaxy. The flow patterns create a distinctive signal of the presence of a pair of closely orbiting black holes. Image credit: Eric R. Coughlin, JILA

Galaxy mergers routinely occur in our Universe. And, when they take place, it takes years for the supermassive black holes at their centers to merge into a new, bigger supermassive black hole. However, a very interesting thing can happen when two black holes get close enough to orbit each other every 3–4 months, something that happens just before the two black holes begin their final desperate plunge into each other. And, according to former JILA graduate student Eric Coughlin and his colleagues, if one of the black holes happens to tidally disrupt an errant star, the process will send out a signal that will allow Earthlings to “see” which galaxies contain these pairs of black holes.

“If two black holes happen to be that close together, and a star gets disrupted by one of the black holes, there’s a reasonable probability that the debris stream will actually miss the black hole that disrupted it and hit the second black hole,” said Mitch Begelman, Coughlin’s thesis advisor at JILA. “You get this kind of dance between the two black holes, and of course you get fantastic flow patterns that are just neat.” Begelman added that these flow patterns create a distinctive signal that there are two black holes involved in the tidal disruption of a single star.

Right now, existing space-based telescopes could detect one of these events every few years. However, in 2019 or 2020, the huge Large Synoptic Survey Telescope (LSST) will come online. And, thanks to Coughlin’s new study that tells astronomers what to look for, the LSST should be able to see a handful of the binary black-hole mergers every year among the many galaxies in our Universe.

“It is a notoriously difficult thing to discern the presence of one black hole, and this is a way to find two,” Coughlin explained. “We think binary black-hole systems should be common, considering how we think our own galaxy evolved via multiple galactic collisions.”

Coughlin said that astronomers now have a new probe in tidal disruption events, which are well understood, to learn something about the evolution of galaxies. As part of his research into tidal disruption events and how they can be used to identify pairs of black holes in the center of merging galaxies, Coughlin has created a stunning animation of the process in action.[2]

The researchers responsible for this work include recently minted JILA Ph.D. Coughlin, recent visitor and former research associate Chris Nixon, and Fellows Phil Armitage and Mitch Begelman.

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Interstellar Spaghetti, with Meatballs Inside

Steven Burrows — Tue, 16 Aug 2016 19:19:13 +0000

Interstellar Spaghetti, with Meatballs Inside Steven Burrows Tue, 08/16/2016 - 13:19

Julie Phillips / Science Communicator

The tidal disruption of a star by a supermassive black hole creates a stream of star stuff that looks like a strand of spaghetti stretching from the star to the black hole. The star‚Äôs self-gravity causes lumps of star stuff to form inside the stream. Image credit: Steven Burrows / JILA

When an ordinary star like our Sun wanders very close to a supermassive black hole, it’s very bad news for the star. The immense gravitational pull of the black hole (i.e., tidal forces) overcomes the forces of gravity holding the star together and literally pulls the star apart. Over time, the black hole swallows half of the star stuff, while the other half escapes into the interstellar medium. This destructive encounter between a supermassive black hole and a star is known as a tidal disruption event.

The idea of doing a detailed simulation of a tidal disruption event recently captured the imaginations of graduate student Eric Coughlin and research associate Chris Nixon (Begelman group). Others had done this kind of work, but either with such low resolution that it wasn’t possible to see what was happening in the star or with unphysical parameters, such as modeling a supermassive black hole as having the mass of 1,000 suns when realistically it would have a mass of around 1,000,000 suns.

“We wanted to see exactly what happens if you have a star that’s wandering around and finds itself on a path that’s going to bring it very close to a supermassive black hole like the one at the center of our Galaxy,” Coughlin said, adding that he and Nixon wanted to see what was happening to all the parts of the star during a tidal-disruption process. To do this, the researchers had to create a realistic simulation that used as many real physical parameters as possible. They also had to make sure their wandering star didn’t get close enough to their simulated supermassive giant black hole to be swallowed whole.

Coughlin and Nixon's simulation followed the tidal disruption encounter for the equivalent of 10 years. They already knew tidal disruption was a gradual process, and they wanted to learn more about how a spherical star gets turned into a stream of star stuff rather like a strand of cooked spaghetti.

“What we found was really surprising,” Coughlin said. “��Ҵ�ý�ƽ�� a month after the disruption, portions of the stream started to fragment. It sort of broke up into individual fragments all along it and formed localized gravitationally bound clumps.”

In other words, meatball-like globs of star stuff appeared inside (rather than on top of) the "spaghetti" strand formed by the tidal disruption. The self-gravity of the stream (left over from the star’s self-gravity) was creating these “meatballs.” Inside the stream, the star particles were both attracted to each other as well as to the black hole that was pulling the whole stream into it. The discovery of the lumps of star stuff has complicated things considerably.

Intriguingly, about an hour after the star's disruption in the simulation, the researchers observed the front and back of the star start to compress and attempt to switch places. This squeezing, caused by the gravitational field of the black hole, increases the stellar stream's self-gravity and causes the stream to fragment—cooking the meatballs for the black hole to eat. This weird result led the researchers to perform more complex simulations (in collaboration with Fellows Mitch Begelman and Phil Armitage) to test whether the compression is real or just an artifact of the original simulation. Early results suggest there’s something really interesting happening. Stay tuned.

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Black Holes Can Have Their Stars and Eat Them Too

Steven Burrows — Thu, 11 Aug 2016 18:44:12 +0000

Black Holes Can Have Their Stars and Eat Them Too Steven Burrows Thu, 08/11/2016 - 12:44

Julie Phillips / Science Communicator

An accretion disk forms stars around a black hole's equator at the same time the black hole is feasting on vast amounts of matter from the thicker parts of the disk above and below the equator. Image credit: Steven Burrows / JILA

Fellow Mitch Begelman’s new theory says it’s possible to form stars while a supermassive black hole consumes massive amounts of stellar debris and other interstellar matter. What’s more, there’s evidence that this is exactly what happened around the black hole at the center of the Milky Way some 4–6 million years ago, according to Associate Fellow Ann-Marie Madigan.

Relatively recently on the cosmic scale of things, the sleeping giant at the center of our Galaxy roared to life as an active galactic nucleus (AGN),¹ swallowing enough matter to increase its size by more than 10% and creating a necklace of new stars around its equator.

“You can form some stars along the equator of the black hole’s thick inner disk, but most of the matter flows into the black hole from above and below the equator,” Begelman explained. “It works this way because we now know that magnetic fields can expand vertically and puff up the disk.² This process allows the matter above and below the inner disk to feed the black hole.”

In this scenario, the upper and lower parts of the “accretion disk” look like wedges that expand vertically from the inner disk at angles of approximately 50 degrees. The whole structure looks like a doughnut without an outer edge that extends outwards a million times the radius of the black hole. The huge amount of gas in these regions flows into the black hole at the same time stars are forming around the equator.

“What we didn’t understand was that a black hole that was forming stars could also accrete,” Begelman said. “Now we understand for the first time that you can have your stars and eat them, too.” Begelman clarified that black holes don’t actually eat the stars, but rather the gas left over from making stars.

Interestingly, the black hole at the center of our Milky Way Galaxy may have been an AGN 4–6 million years ago. The smoking gun is a necklace of stars around our own, now rather sedate black hole. The disk of stars is located exactly where Begelman’s new model predicted it would be.

“With our new model, we can say how bright that AGN would have been,” Begelman said. “Right now the center of the Milky Way has a luminosity of about 100 Suns, but a few million years ago when it was an AGN, it would have been as bright as a billion Suns.” Begelman added that about 1% of the mass of the central black hole was turned into stars during that episode. At the same time, the black hole grew about 10%. In other words, the beautiful necklace of stars we see today is just the leftover crumbs from the most recent feeding frenzy of our Galaxy’s central black hole.

Begelman collaborated on this work with his colleague Joseph Silk of the Institut d’Astrophysique de Paris, Université Pierre et Marie Curie.

¹AGN’s are some of the most luminous sources of electromagnetic radiation in the Universe. The most luminous and energetic of the AGNs are the quasars.

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