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A team of astrophysicists led by Caltech has for the first time been able to simulate the journey of primordial gas dating from the early universe to the stage where it is absorbed into a disk of material feeding a supermassive black hole. The new computer simulation overturns ideas about such discs that astronomers have held since the 1970s, and paves the way for 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,” said Phil Hopkins, the Ira S. Bowen Professor of Theoretical Astrophysics.

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

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

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

To the surprise of the team, as reported in The Open Journal of Astrophysicsthe simulation revealed that magnetic fields play a much larger role than previously thought in forming and shaping the huge disks of material that swirl around and power 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 revealed that the discs are actually fluffy – more like an angel cake. Our simulation helped us understand that the magnetic fields support the material in the disc, making it fluffier .”

Visualizing activity around supermassive black holes using ‘super magnifications’

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

Astronomers have known for decades that because gas and dust are pulled by the massive gravity of these black holes, they are not immediately sucked in. Instead, the material first forms a rapidly rotating disk called an accretion disk. And as the material is about to fall, it emits an enormous amount of energy, shining with a brilliance unmatched by almost anything in the universe. But not much is still known about these active supermassive black holes, called quasars, and how the disks that power them form and behave.

While discs around supermassive black holes have been imaged before – the Event Horizon telescope captured discs orbiting black holes at the heart of our own galaxy in 2022 and Messier 87 in 2019 – these discs are much closer and tamer of those revolving around quasars.

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

This input includes many algorithms, or series of instructions, for computers to 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 that easy.

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

After all, stars do a lot of things that affect their surroundings. They emit radiation that can heat or displace the surrounding gas. They blow winds like the solar wind created by our own sun, which can sweep away material. They explode as supernovae, sometimes shooting material out of galaxies or changing the chemistry of their surroundings. So computers also need to know all the ins and outs of this “stellar feedback,” as it regulates how many stars a galaxy can 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, on a galactic scale, 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 be mostly ignored because the gas there is too hot for atoms and molecules to exist. Instead, what exists there is a hot ionized plasma.

Creating a simulation that could cover all relevant scales down to the level of an 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 larger, cosmological part of the problem, but nothing, which had both,” Hopkins says.

The Caltech-led team uses code it calls GIZMO for both large and small simulation projects. The important thing is that they built the FIRE project so that all the physics they added to it could work with the STARFORGE project and vice versa.

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

This allowed scientists in the latest work to simulate a black hole about 10 million times the mass of our sun, starting from the early universe. The simulation then zooms in on this black hole at a time when a giant stream of material breaks off from 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 path 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 this accretion disk, but what was very surprising was that the simulated disk did not look the way we thought for decades it should.”

In two seminal papers from the 1970s that described the accretion disks feeding 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 influence of the enormous gravity they experience near the black hole. They acknowledged that magnetic fields may play a minor role in helping to strengthen the discs.

In contrast, the new simulation found 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 discs are almost completely controlled by the magnetic fields,” says Hopkins. “The magnetic fields perform many functions, one of which is to support the discs and make the material puffy.”

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 should be able to move from them into a black hole, and even their geometry (such as whether discs can be tilted).

Looking ahead, 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 might the first generation of stars in the universe have looked like?

“There’s so much to do,” he says.