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Cosmic simulation reveals how black holes grow and evolve
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This still from the simulation shows a supermassive black hole, or
quasar, surrounded by a swirling disk of material called an accretion
disk. Credit: Caltech/Phil Hopkins group
A team of astrophysicists led by Caltech has managed for the first
time to simulate the journey of primordial gas dating from the early
universe to the stage at which it becomes swept up in a disk of
material fueling a single supermassive black hole. The new computer
simulation upends ideas about such disks that astronomers have held
since the 1970s and paves the way for new discoveries about how black
holes and galaxies grow and evolve.
"Our new simulation marks the culmination of several years of work
from two large collaborations started here at Caltech," says Phil
Hopkins, the Ira S. Bowen Professor of Theoretical Astrophysics.
The first collaboration, nicknamed FIRE (Feedback in Realistic
Environments), has focused on the larger scales in the universe,
studying questions such as how galaxies form and what happens when
galaxies collide. The other, dubbed STARFORGE, was designed to examine
much smaller scales, including how stars form in individual clouds of
gas.
"But there was this big gap between the two," Hopkins explains. "Now,
for the first time, we have bridged that gap."
To do that, the researchers had to build a simulation with a
resolution that is more than 1,000 times greater than the previous
best in the field.
To the team's surprise, as reported in _The Open Journal of
Astrophysics_ , the simulation revealed that magnetic fields play a
much larger role than previously believed in forming and shaping the
huge disks of material that swirl around and feed the supermassive
black holes.
"Our theories told us the disks should be flat like crepes," Hopkins
says. "But we knew this wasn't right because astronomical observations
reveal that the disks are actually fluffy—more like an angel cake. Our
simulation helped us understand that magnetic fields are propping up
the disk material, making it fluffier."
VIDEO
Credit: California Institute of Technology
**Visualizing the activity around supermassive black holes using
'super zoom-ins'**
In the new simulation, the researchers performed what they call a
"super zoom-in" on a single supermassive black hole, a monstrous
object that lies at the heart of many galaxies, including our own
Milky Way. These ravenous, mysterious bodies contain anywhere from
thousands to billions of times the mass of the sun, and thus exert a
huge effect on anything that comes near.
Astronomers have known for decades that as gas and dust are pulled in
by the tremendous gravity of these black holes, they are not
immediately sucked in. Instead, the material first forms a rapidly
swirling disk called an accretion disk. And as the material is just
about to fall in, it radiates a huge amount of energy, shining with a
brilliance unmatched by just about anything in the universe. But much
is still not known about these active supermassive black holes, called
quasars, and how the disks that feed them form and behave.
While disks around supermassive black holes have been imaged
previously—the Event Horizon Telescope imaged disks circling black
holes at the heart of our own galaxy in 2022 and Messier 87 in
2019—these disks are much closer and more tame than the ones that
churn around quasars.
To visualize what happens around these more active and distant black
holes, astrophysicists turn to supercomputer simulations. They feed
information about the physics at work in these galactic
settings—everything from the basic equations that govern gravity to
how to treat dark matter and stars—into thousands of computing
processors that work in parallel.
This input includes many algorithms, or series of instructions, for
the computers to follow to recreate complicated phenomena. So, for
example, the computers know that once gas becomes dense enough, a star
forms. But the process is not that straightforward.
"If you just say gravity pulls everything down and then eventually the
gas forms a star and stars just build up, you'll get everything wildly
wrong," Hopkins explains.
After all, stars do many things that affect their surroundings. They
shine radiation that can heat up or push surrounding gas. They blow
winds like the solar wind created by our own sun, which can sweep up
material. They explode as supernovae, sometimes launching material
clear out of galaxies or changing the chemistry of their surroundings.
So, the computers must know all the ins and outs of this "stellar
feedback" as well, as it regulates how many stars a galaxy can
actually form.
## Building a simulation that spans multiple scales
But at these larger scales, the set of physics that are most important
to include and what approximations can be made differ from those at
smaller scales. For example, on the galactic scale, the complicated
details of how atoms and molecules behave are extremely important and
must be built into any simulation. However, scientists agree that when
simulations focus on the more immediate area around a black hole,
molecular chemistry can be mostly ignored because the gas there is too
hot for atoms and molecules to exist. Instead, what is exists there is
hot ionized plasma.
Creating a simulation that could cover all the relevant scales down to
the level of a single accretion disk around a supermassive black hole
was a huge computational challenge—one that also required a code that
could handle all the physics.
"There were some codes that had the physics that you needed to do the
small-scale part of the problem and some codes that had the physics
that you needed to do the larger, cosmological part of the problem,
but nothing that had both," Hopkins says.
An earlier still from the simulation shows a tangle of merging
galaxies. Credit: Caltech/Phil Hopkins group
The Caltech-led team used a code they call GIZMO for both the large-
and small-scale simulation projects. Importantly, they built the FIRE
project so that all the physics they added to it could work with the
STARFORGE project, and vice versa.
"We built it in a very modular way, so that you could flip on and off
any of the pieces of physics that you wanted for a given problem, but
they were all cross compatible," Hopkins says.
This allowed the scientists in the latest work to simulate a black
hole that is about 10 million times the mass of our sun, beginning in
the early universe. The simulation then zooms in on that black hole at
a moment when a giant stream of material is torn off a cloud of star-
forming gas and begins to swirl around the supermassive black hole.
The simulation can continue zooming in, resolving a finer area at each
step as it follows the gas on its way toward the hole.
## Surprisingly fluffy, magnetic disks
"In our simulation, we see this accretion disk form around the black
hole," Hopkins says. "We would have been very excited if we had just
seen that accretion disk, but what was very surprising was that the
simulated disk doesn't look like what we've thought for decades it
should look like."
In two seminal papers from the 1970s that described the accretion
disks fueling supermassive black holes, scientists assumed that
thermal pressure—the change in pressure caused by the changing
temperature of the gas in the disks—played the dominant role in
preventing such disks from collapsing under the tremendous gravity
they experience close to the black hole. They acknowledged that
magnetic fields might play a minor role in helping to shore up the
disks.
In contrast, the new simulation found that the pressure from the
magnetic fields of such disks was actually 10,000 times greater than
the pressure from the heat of the gas.
"So, the disks are almost completely controlled by the magnetic
fields," Hopkins says. "The magnetic fields serve many functions, one
of which is to prop up the disks and make the material puffy."
This realization changes a host of predictions scientists can make
about such accretion disks, such as their mass, how dense and thick
they should be, how fast material should be able to move from them
into a black hole, and even their geometry (such as whether the disks
can be lopsided).
Looking forward, Hopkins hopes this new ability to bridge the gap in
scales for cosmological simulations will open many new avenues of
research. For example, what happens in detail when two galaxies merge?
What types of stars form in the dense regions of galaxies where
conditions are unlike those in our sun's neighborhood? What might the
first generation of stars in the universe have looked like?
"There's just so much to do," he says.
**More information:** Philip F. Hopkins et al, FORGE'd in FIRE:
Resolving the End of Star Formation and Structure of AGN Accretion
Disks from Cosmological Initial Conditions, _The Open Journal of
Astrophysics_ (2024). DOI: 10.21105/astro.2309.13115
**Citation** : Cosmic simulation reveals how black holes grow and
evolve (2024, July 2) retrieved 3 July 2024 from
https://phys.org/news/2024-07-cosmic-simulation-reveals-black-
holes.html
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