Cosmic simulation reveals how black holes grow and evolve

A team of astrophysicists led by Caltech has for the first time successfully simulated the journey of primordial gas from the early universe to the stage where it is swept up in a disk of material that fuels a single supermassive black hole. The new computer simulation overturns ideas about such disks that astronomers have had since the 1970s, paving the way for new discoveries about how black holes and galaxies grow and evolve.

“Our new simulation is the culmination of years of work from two major collaborations that began here at Caltech,” said Phil Hopkins, the Ira S. Bowen Professor of Theoretical Astrophysics.

The first collaboration, nicknamed FIRE (Feedback in Realistic Environments), focused on the larger scales of the universe, studying questions such as how galaxies form and what happens when galaxies collide. The other, called STARFORGE, was designed to investigate much smaller scales, including how stars form in individual gas clouds.

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

To do that, the researchers had to build a simulation with a resolution more than 1,000 times greater than the best simulation in the field.

To the surprise of the team, as reported in The Open Journal of AstrophysicsThe simulation showed that magnetic fields play a much larger role than previously thought in the formation and shaping of the vast disks of material that orbit and feed supermassive black holes.

“Our theories told us that the disks should be flat, like pancakes,” Hopkins said. “But we knew that wasn’t right, because astronomical observations show that the disks are actually fluffy, more like angel food cake. Our simulation helped us understand that magnetic fields are holding the disk material up, making it fluffier.”

Visualizing the activity around supermassive black holes using ‘superzoom-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 voracious, mysterious bodies contain thousands to billions of times the mass of the sun, and so exert a huge effect on anything that comes near them.

Astronomers have known for decades that gas and dust aren’t immediately sucked in by the enormous gravity of these black holes. Instead, the material first forms a rapidly swirling disk called an accretion disk. And as the material is about to fall into it, it radiates a tremendous amount of energy, shining brightly like almost anything else in the universe. But much remains unknown 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 before (the Event Horizon Telescope imaged disks orbiting black holes at the heart of our own galaxy in 2022 and Messier 87 in 2019), these disks are much closer and tamer than the disks orbiting quasars.

To visualize what’s happening around these more active and distant black holes, astrophysicists use supercomputer simulations. They feed information about the physics at work in these galactic environments, from the basic equations that govern gravity to how dark matter and stars should be treated, into thousands of computer processors working in parallel.

This input includes many algorithms, or sets of instructions, that the computers must follow to recreate complex phenomena. For example, the computers know that when gas becomes dense enough, a star will form. But the process is not that simple.

“If you just say gravity pulls everything down and eventually the gas forms a star and then stars are born, you’re wrong,” Hopkins explains.

Stars do a lot of things that affect their environment. They emit radiation that can heat up or push away surrounding gas. They blow winds like the solar wind created by our own sun, which can sweep material away. They explode as supernovae, sometimes ejecting material from galaxies or changing the chemistry of their environment. So the computers also have to know all the ins and outs of this “stellar feedback,” because 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 is most important to include and the approximations that can be made differs from those at smaller scales. For example, at galactic scales, the intricate 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 region around a black hole, molecular chemistry can largely be ignored because the gas there is too hot for atoms and molecules to exist. Instead, what 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. Moreover, it required a code that could handle all the physics involved.

“There were some codes that had the physics you needed to solve the small-scale part of the problem, and other codes that had the physics you needed to solve the larger, cosmological part of the problem, but no code that had both,” Hopkins said.

The Caltech-led team used a code they call GIZMO for both the large-scale and small-scale simulation projects. Importantly, they built the FIRE project so that any physics they added to it would work with the STARFORGE project, and vice versa.

“We built it in a very modular way, so that you could turn on and off any piece of physics that you wanted to use for a particular problem, and yet they were all compatible with each other,” Hopkins said.

This allowed the scientists in the latest work to simulate a black hole about 10 million times more massive than our sun, starting in the early universe. The simulation then zooms in on that black hole at a point 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 to zoom in, resolving a finer region at each step as it follows the gas on its way to the hole.

Surprisingly soft, magnetic discs

“In our simulation, we see this accretion disk forming around the black hole,” Hopkins said. “We would have been very excited if we had actually seen that accretion disk, but what was very surprising is that the simulated disk does not look like what we have thought it should look like for decades.”

In two groundbreaking papers in the 1970s describing the accretion disks that fuel 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 enormous gravity they experience near the black hole. They recognized that magnetic fields might play a small role in helping to prop up the disks.

In contrast, the new simulation showed that the pressure from the magnetic fields of such disks was actually 10,000 times greater than the pressure created by the heat of the gas.

“So the disks are almost completely driven by the magnetic fields,” Hopkins said. “The magnetic fields have many functions, one of which is to hold the disks up and make the material spherical.”

This realization changes many predictions scientists can make about such accretion disks, such as their mass, how dense and thick they should be, how fast material from them should be able to move toward a black hole, and even their geometry (for example, whether the disks can be tilted).

Hopkins hopes that this new ability to bridge the gap in scales for cosmological simulations will open up many new avenues of research. For example, what happens in detail when two galaxies merge? What kinds of stars form in the dense regions of galaxies where conditions are different from those near our sun? What would the first generation of stars in the universe have looked like?

“There’s just so much to do,” he says.