Cosmic simulation reveals how black holes grow and evolve

For the first time, a team of astrophysicists led by Caltech has succeeded in simulating the journey of primordial gas from the early universe to the stage where it is engulfed in the disk of material that powers a single supermassive black hole. The new computer simulation adds to the notions of such discs that astronomers have held 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 several years of work by two major collaborations started here at Caltech,” says Phil Hopkins, the Ira S. Bowen Professor of Theoretical Astrophysics.

Dubbed FIRE (Feedback in Realistic Environments), the first collaboration focused on larger scales in 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 clouds of gas.

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

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

To the surprise of the team, as stated in Open Access Journal of AstrophysicsThe 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 supermassive black holes.

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

Visualizing activity around supermassive black holes using ‘super zoom’

In a new simulation, scientists took what they call a “super zoom” on a single supermassive black hole, a monstrous object that lies at the heart of many galaxies, including our own Milky Way. These hungry, mysterious bodies contain thousands to billions of times the mass of the Sun, and therefore have a huge effect on anything that approaches.

Astronomers have known for decades that as gas and dust are pulled in by the enormous gravity of these black holes, they are not immediately sucked in. Instead, the material first forms a rapidly spinning disk called an accretion disk. And as the material is about to fall in, it emits enormous amounts of energy and shines with a brilliance unmatched by almost anything in the universe. However, not much is still 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 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 those orbiting quasars . .

To imagine what happens around these more active and more distant black holes, astrophysicists use 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 handle dark matter and stars—to thousands of computing processors that work in parallel.

This input includes many algorithms, or series of instructions, for computers to follow to recreate complicated phenomena. So, for example, computers know that once the gas gets dense enough, a star will form. But the process is not so straightforward.

“If you just say that gravity pulls everything down and then eventually the gas forms a star and the stars just pile up, you’re going to get it all very wrong,” Hopkins explains.

Stars do many things that affect their surroundings. They emit radiation that can heat or pressurize the surrounding gas. They blow winds like the solar wind created by our own sun that can sweep away material. They explode as supernovae, sometimes ejecting material from galaxies or changing the chemistry of their surroundings. So computers must also know all the details of this “stellar feedback,” because it regulates how many stars a galaxy can actually make.

Creating 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 are different from those at smaller scales. At the galactic scale, for example, the intricate details of how atoms and molecules behave are extremely important and must be built into every simulation. But scientists agree that when simulations focus on the more immediate region around a black hole, molecular chemistry can be mostly ignored because the gas is too hot for atoms and molecules to exist. Instead, what exists is a 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 code to handle all the physics.

“There were some codes that had the physics you needed to do the small part of the problem, and some codes that had the physics you needed to do the larger, cosmological part of the problem, but nothing that had both.” says Hopkins.

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

“We built it in a very modular way, so you could turn any part of the physics on and off that you wanted for a given problem, but they were all compatible with each other,” says Hopkins.

This allowed the scientists, in the latest work, to simulate a black hole about 10 million times the size of our Sun, dating back to the early universe. The simulation then zooms in on this black hole at the moment when the giant stream of material breaks away from the cloud of star-forming gas and starts swirling around the supermassive black hole. The simulation can continue to zoom in, distinguishing a finer region at each step as it follows the gas on its way to the hole.

Surprisingly fluffy, magnetic disks

“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 the 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 that described the accretion discs powering supermassive black holes, scientists hypothesized that thermal pressure—the change in pressure caused by the changing temperature of the gas in the discs—plays a dominant role in preventing such discs from collapsing underwater. the enormous gravity they experience near a black hole. They acknowledged that magnetic fields may play a minor role in helping to support 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 magnetic fields,” says Hopkins. “Magnetic fields serve many functions, one of which is to support the discs and inflate the material.”

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 move from them into the black hole, and even their geometry (such as whether discs may be bent).

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

“There’s so much to do,” he says.