A team of astrophysicists led by the California Institute of Technology has for the first time simulated the journey of primordial gas from the early universe to the stage when it coalesced into a disk of material that fed a single supermassive black hole. The new computer simulations have been improving astronomers’ understanding of such disks since the 1970s and paving the way for new discoveries about how black holes and galaxies grow and evolve.

“Our new simulations mark the culmination of several years of work on two large collaborative projects initiated at Caltech,” says Phil Hopkins, the Ira S. Bowen Professor of Theoretical Astrophysics.

The first collaboration, called FIRE (Feedback in Realistic Environments), focused on larger scales of the universe, examining questions such as galaxy formation and what happens when galaxies collide. Another, called STARFORGE, is designed to study much smaller scales, including how stars form in individual clouds of gas.

“But there was a huge gap between them,” Hopkins explains. “Now for the first time we’ve closed that gap.”

To do this, the researchers needed to create a simulation with a resolution 1,000 times higher than the previous best results in this field.

As reported, to the surprise of the team, Open Astrophysics JournalThe simulation showed that magnetic fields play a much larger role than previously thought in shaping and molding the massive disks of material that orbit and feed supermassive black holes.

“Our theories told us that the disks should be flat like pancakes,” Hopkins says. “But we knew that was wrong because astronomical observations showed that the disks were actually puffy, more like angel food cake. Our simulations helped us understand that magnetic fields support the material of the disk, making it soft.”

Visualizing activity around supermassive black holes using “super magnifications”

In the new simulation, the researchers performed what they call a “supermagnification” of a single supermassive black hole, the massive object that lies at the heart of many galaxies, including our own Milky Way. These voracious, mysterious objects have a mass ranging from thousands to billions of times the mass of the Sun, and therefore have a huge impact on anything that comes near them.

Astronomers have known for decades that when gas and dust are pulled in by the immense gravity of these black holes, they are not immediately sucked in. Instead, the material first forms a rapidly spinning disk called an accretion disk. As the material falls, it emits enormous amounts of energy and shines with a brightness unmatched by anything else in the universe. But much remains unknown about how these active supermassive black holes, called quasars, and 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 the black holes at the heart of our own galaxy in 2022, and Messier 87 in 2019—these disks are much closer and more docile than those orbiting quasars.

To visualize what’s happening around these more active and distant black holes, astrophysicists are turning to supercomputer simulations, feeding information about the physics at work in these galactic environments—from the fundamental equations that govern gravity to how to deal with dark matter and stars—to thousands of computer processors working in parallel.

These inputs contain many algorithms, or sets of instructions, that computers must follow to recreate complex events. For example, computers know that a star forms when gas becomes dense enough. But the process is not that simple.

“If you say gravity pulls everything down and then gas forms a star and stars accrete, you’re getting it very wrong,” Hopkins explains.

After all, stars do many things that affect their environment. They emit radiation that can heat or push gas around. They blow out winds that can sweep away material, like the solar wind generated by our own sun. They explode as supernovae, sometimes throwing material out of galaxies or changing the chemical composition of their surroundings. So computers also need to know all the details of this “stellar feedback,” because it governs how many stars can actually form in a galaxy.

Create simulations spanning multiple scales

But at these larger scales, the set of physical elements that are most important to include and what predictions can be made are different from those at smaller scales. For example, the intricate details of how atoms and molecules behave on a galactic scale are extremely important and must be included in every simulation. However, scientists agree that when simulations focus on the near-term region of a black hole, molecular chemistry can be largely ignored because the gas there is too hot for atoms and molecules to exist. Instead, there 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 massive computational task that also required code that could handle all the physics.

“There was some code that contained the physics needed to solve the small-scale part of the problem, and there was some code that contained the physics needed to solve the larger, cosmological part of the problem, but there was nothing that had both,” Hopkins says.

The Caltech-led team used a codename they called GIZMO for both large and small simulation projects. The important thing is that they made Project FIRE so that all the physics they added would work with Project STARFORGE, and vice versa.

“We made it very modular, so you could plug in and remove the physical parts you needed for a particular task, but they were all cross-compatible,” Hopkins says.

This allowed the scientists to model a black hole in their latest study, roughly 10 million times the mass of our Sun, dating back to the early universe. The simulation then zooms in on the black hole as the giant flow of material breaks away from the star-forming gas cloud and begins orbiting the supermassive black hole. The simulation can continue to scale up, resolving a thinner region with each step as it follows the gas toward the hole.

Surprisingly soft magnetic disks

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

In two seminal papers from the 1970s describing accretion disks that feed supermassive black holes, scientists proposed that thermal pressure (the pressure change caused by changes in the temperature of the gas in the disks) played a dominant role in preventing such disks from collapsing under the influence of the enormous gravity they experience near a black hole. They noted that magnetic fields may play a minor role in strengthening the disks.

In contrast, new simulations showed that the pressure from the magnetic fields of such disks is actually 10,000 times greater than the pressure from the heat of the gas.

“So the disks are almost entirely controlled by magnetic fields,” Hopkins says. “The magnetic fields have several functions, one of which is to support the disks and to loosen the material.”

This realization changes many of the predictions that scientists can make about such accretion disks, such as their mass, how dense and thick they should be, how fast material should travel from them to the black hole, and even their geometry (for example, whether the disks will be one-sided).

Going forward, Hopkins hopes that this new capability to bridge the cosmological simulation scale gap 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 might the first generation of stars in the universe look like?

“There is so much to do,” he says.