Black Holes Dominate Large Regions of Space, But They’re Mysterious

In the beginning, the Universe was all primordial gas. Somehow, some of it was swept up into supermassive black holes (SMBHs), the gargantuan singularities that reside at the heart of galaxies. The details of how that happened and how SMBHs accumulate mass are some of astrophysics’ biggest questions.

Black hole science took a big step in 2019 when the Event Horizon Telescope captured the first image of a black hole. That SMBH was in Messier 87, a supergiant elliptical galaxy over 50 million light-years from Earth. As fascinating an accomplishment as that was, it didn’t answer our longstanding questions about how these objects become so massive.

Scientists know that two main processes govern SMBH growth: They accrete cold gas from their host galaxy, and they merge during galaxy collisions.

But there are some mysterious, unanswered questions. One concerns their origins. We can see SMBHs accreting matter, but the speed at which they acquire mass can’t really explain their size. Some of them are billions of times more massive than the Sun. Did SMBHs have some type of growth spurt in the Universe’s early ages?

What about intermediate-mass black holes (IMBHs.) Are these elusive objects, which may reside in the center of globular clusters, stepping stones to SMBHs?

Black hole jets are also mysterious. These jets are extremely powerful and accelerate matter to extreme speeds. Astrophysicists understand the basics of how SMBHs create these jets. But these jets can reach relativistic speeds and how they do that is unclear.

Since SMBHs are so difficult to observe in detail, scientists rely on theories to explain them. Over time, they try to refine their theories. But sometimes, as our observing power increases, our theories don’t match our observations. This is true of the accretion disks around SMBHs. While theory says these disks should be flat like pancakes, observations show that they’re puffy.

This is where simulations come in.

Detailed simulations are one of astrophysicists’ best tools for understanding SMBHs. New research published in The Open Journal of Astrophysics examines the accretion disks around SMBHs with simulations. These disks are the reservoirs of gas that feed SMBH growth. The research is “FORGE’d in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions.” The lead author is Philip Hopkins, a professor of Theoretical Astrophysics at Caltech.

“Our new simulation marks the culmination of several years of work from two large collaborations started here at Caltech,” said lead author Hopkins in a press release.

Hopkins is talking about FIRE (Feedback in Realistic Environments) and STARFORGE (Star Formation in Gaseous Environments.) STARFORGE is a small-scale simulator that focuses on how individual stars form in clouds of gas called molecular clouds. FIRE focuses on galaxy formation, including things like black hole feedback and quenching.

FIRE and STARFORGE are on opposite ends of a scale, and the new work fills in the gap between the two.

“But there was this big gap between the two,” Hopkins explains. “Now, for the first time, we have bridged that gap.”

“It has recently become possible to zoom in from cosmological to sub-pc scales in galaxy simulations to follow accretion onto supermassive black holes (SMBHs),” the authors write in their research. “However, at some point, the approximations used on ISM <interstellar medium> scales (e.g. optically-thin cooling and stellar-population-integrated star formation [SF] and feedback [FB]) break down.”

The physics driving small-scale accretion is different from the physics driving large-scale accretion. “It is by no means clear what physically occurs when the different physics most relevant on different scales intersect,” the researchers write.

Large-scale simulations are based on things like the collective effects of entire star populations and the initial mass function. Small-scale simulations are based on things like the formation of individual protostars and stellar winds from individual stars. At an even smaller scale, simulations focus on individual aspects of accretion disks around SMBHs.

“As a result, there have not been simulations that can span all three of these regimes simultaneously and self-consistently,” Hopkins and his co-authors explain.

Bridging the gap wasn’t a simple matter. Hopkins and his fellow researchers needed a simulation with much higher resolution. The resolution had to be over 1,000 times greater than the previous best simulator.

“This allows us to span scales from ~100 Mpc down to <100 au (~300 Schwarzschild radii) around an SMBH at a time where it accretes as a bright quasar in a single simulation,” the researchers explain in their paper.

Their simulations had a surprise in store. They show that magnetic forces play a larger role in SMBH accretion disks than thought.

Theory shows that the rotating accretion disks around SMBHs should be flat like pancakes. This is due to the conservation of angular momentum and viscous forces in the disk that distribute momentum, keeping the disk flat. But our theories don’t line up with observations.

“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.”

Supermassive black holes have different activity levels. When they’re actively accreting lots of material, they’re extremely luminous and emit light across the electromagnetic spectrum. In this case, they’re called quasars, and their light output can exceed that of an entire galaxy as large as the Milky Way.

Quasars are enormously powerful, and astrophysicists are keen to understand how the disks around these SMBHs work. These researchers used their simulations to do what they call a “super zoom-in.” For that to work across multiple scales, the simulations must include all kinds of formulae that govern things from simple gravity to dark matter. These things must be computed in parallel, and they feed into each other.

“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. Stars are complex objects. They have stellar winds. They can heat up nearby gas. Some are small and dim and last for trillions of years. Some are massive and hot and explode as supernovae at the end of their short lives. Nature is extraordinarily complex, as most people interested in astronomy understand.

Building a simulation that could take all of the details across multiple scales into account is an enormously complex task.

“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.

The team’s work led to a simulation of an SMBH in the early Universe with ten million solar masses. It zooms in as a giant stream of star-forming gas is torn away from its cloud into the accretion disk swirling around the black hole. It keeps zooming in as the gas is drawn closer to the hole.

“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.”

Black hole theory, dating back to the 1970s, shows that thermal pressure is a dominant force in supermassive black hole accretion disks. These theories show that thermal pressure prevents the disks from collapsing under the extreme gravity exerted by the SMBH. Magnetic fields played a lesser role.

But these simulations show otherwise. They show that the magnetic pressure on the disk is about 10,000 times stronger than the thermal pressure from 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 result changes a lot.

“We show that magnetic fields are critical for a wide range of effects on sub-pc scales within the accretion disk, ranging from maintaining efficient torques and high inflow rates, explaining the scale heights and vertical profiles of the disk structure, the outer size/boundary of the accretion disk, and perhaps most importantly the suppression of star formation at sub-pc scales,” the authors write.

A disk can still form without a magnetic field, but things are drastically different. The disk will be a magnitude or more smaller than a disk with a field. The accretion rate onto the disk can be more than 100 times lower, and the disk can fragment and form stars.

This is just the beginning of the team’s simulations. They intend to publish two additional papers in a series. In those papers, they’ll focus on more details, like star formation and the initial mass function in the inner region around quasars accretion disks.