Cosmic simulation reveals how black holes grow and evolve

A team of astrophysicists led by Caltech has succeeded for the first time in simulating the journey of primordial gas dating from the early universe to the stage in which it is hidden in a disk of material feeding a single supermassive black hole. The new computer simulation overturns ideas about such discs that astronomers have held since the 1970s and opens the way to new discoveries about how black holes and galaxies grow and evolve.

“Our new simulation marks the culmination of several years of work from two major 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), is focused on the largest scales in the universe, studying questions such as how galaxies form and what happens when galaxies collide. The other, called STARFORGE, was designed to examine much smaller scales, including how stars form in individual clouds of gas.

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

To do this, 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 Open Journal of Astrophysicsthe simulation revealed that magnetic fields play a much larger role than previously believed in the formation and shaping of large disks of material that spin and feed supermassive black holes.

“Our theories told us that discs should be flat as pancakes,” says Hopkins. “But we knew that wasn’t right because astronomical observations reveal that the discs are actually fluffy – more like an angel cake. Our simulation helped us understand that the magnetic fields are supporting the disc material, making it the happiest.”

Visualizing activity around supermassive black holes using ‘super zoom’

In the new simulation, the researchers performed what they call a “super zoom” on a single supermassive black hole, a monstrous object that sits at the heart of many galaxies, including our own Milky Way. These voracious and mysterious bodies contain anywhere from thousands to billions of times the mass of the sun, and thus exert a great effect on anything that approaches them.

Astronomers have known for decades that while gas and dust are pulled by the immense gravity of these black holes, they are not immediately absorbed. And as the material is about to fall in, it radiates an enormous amount of energy, shining with a brightness unmatched by almost anything else in the universe. But not much is still known about these 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 surrounding the black holes at the heart of our galaxy in 2022 and Messier 87 in 2019—these disks are much closer and more subdued than those that revolve around quasars.

To visualize what happens around these most active and distant black holes, astrophysicists turn to supercomputer simulations. They feed information about the physics at work in these galactic environments—everything from the basic equations that govern gravity to how to treat dark matter and stars—to thousands of computer processors working in parallel.

This entry includes many algorithms, or sets of instructions, that computers must follow to recreate complex phenomena. So, for example, computers know that once the gas becomes dense enough, a star forms. But the process is not so straightforward.

“If you just say that gravity pulls everything down and then the gas forms a star and stars just build, you’d be very wrong,” explains Hopkins.

After all, stars do a lot of things that affect their environment. They emit radiation that can heat or push the surrounding gas. They blow winds like the solar wind created by our sun itself, which can sweep away material. They explode as supernovae, sometimes ejecting material from outside galaxies or changing the chemical composition of their surroundings. So computers must also know all aspects of this “stellar feedback,” since it governs 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 what approximations can be made differ from those at smaller scales. For example, at the galactic scale, the intricate details of how atoms and molecules behave are extremely important and must be included in any simulation. However, scientists agree that when simulations focus on the closest region around a black hole, molecular chemistry can be largely 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 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 you needed to do the small-scale part of the problem, and some codes that had the physics you needed to do the large, cosmological part of the problem, but nothing that had both.” says Hopkins.

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

“We built it in a very modular way, so you could roll in and take out whatever parts of the physics you wanted for a particular problem, but they were all compatible,” says Hopkins.

This allowed scientists in the latest work to simulate a black hole that is about 10 million times the mass of our sun, starting in the early universe. The simulation then zooms in on that black hole at a moment when a giant stream of material breaks off from a cloud of star-forming gas and begins orbiting the supermassive black hole. The simulation can continue to zoom in, resolving a finer area at each step as it follows the gas on its way to the hole.

Surprisingly fluffy, magnetic discs

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

In two seminal papers from the 1970s describing the accretion discs that fueled supermassive black holes, scientists hypothesized that thermal pressure—the change in pressure caused by the changing temperature of the gas in the discs—played the dominant role in preventing such discs from collapsing. under the immense gravity they experience near the black hole. They acknowledged that magnetic fields may play a small role in helping harden the discs.

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 drives are almost completely controlled by magnetic fields,” says Hopkins. “The magnetic fields serve many functions, one of which is to support the discs and make the material swell.”

This realization changes a number 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 to a black hole, and even their geometry (such as whether the disks can be sideways).

Looking ahead, Hopkins hopes this new ability to bridge the scale gap for cosmological simulations will open up many new avenues of research. For example, what exactly happens when two galaxies merge? What types of stars form in the dense regions of galaxies where conditions are different from those in the neighborhood of our sun? What might the first generation of stars in the universe have looked like?

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