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New Simulation Shows How Galaxies’ Spiral Arms Feed Their Supermassive Black Holes

By improving resolution, the model allows scientists to follow the gas in flux across the galaxy in much greater detail.

A new computer simulation is powerful enough to show how black holes grow — at a resolution 1,000 times higher than previously thought possible.

“Other models can tell you a lot about what’s happening very close to the black hole, but they don’t contain information about what the rest of the galaxy is doing or even less about what the environment around the galaxy is doing,” said lead author Daniel Anglés-Alcázar.

“It turns out, it is very important to connect all these processes at the same time,” he said.

The new tool, from researchers at Northwestern University, accounts for the many factors that affect the growth of black holes. It details how gas flows across the universe, before being scooped up by a galaxy’s spiral arms, which are made of young stars, to feed the supermassive black hole at its center.

Anglés-Alcázar said major events like a supernova — the massive explosion at the end of a star’s life — releases a lot of energy and influences the evolution of a galaxy.

That’s important because supermassive black holes contain the mass of millions or billions of suns and can consume 10 times the mass of our sun each year. But not every supermassive black hole has a constant supply of gas to consume.  Some can lie dormant for millions of years before a sudden influx of gas reawakens them.

“So we need to incorporate all of these details and physical processes to capture an accurate picture,” he said.

Researchers developed their model to include various factors, such as the expansion of the universe, feedback from massive stars, and gravity gas hydrodynamics to get a better picture of the galactic environment around a black hole.

Following the work of the Feedback In Realistic Environments (FIRE) project, the new simulation improves model resolution, allowing it to follow the gas in flux across the galaxy in much greater detail.

A still image from the simulation showing a large region containing tens of galaxies, 6 million light-years across. (Anglés-Alcázar et al. 2021, ApJ, 917, 53.)

“The very existence of supermassive black holes is quite amazing, yet there is no consensus on how they formed,” co-author Claude-André Faucher-Giguère said. “The reason supermassive black holes are so difficult to explain is that forming them requires cramming a huge amount of matter into a tiny space. How does the universe manage to do that?

“Until now, theorists developed explanations relying on patching together different ideas for how matter in galaxies gets crammed into the innermost one millionth of a galaxy’s size,” he said.

The study, published Aug. 17 in the Astrophysical Journal, also provides insight into the nature of quasars — incredibly bright, fast-growing black holes.

“The light we observe from distant quasars is powered as gas falls into supermassive black holes and gets heated up in the process,” said Faucher-Giguère.

“Our simulations show that galaxy structures, such as spiral arms, use gravitational forces to ‘put the brakes on’ gas that would otherwise orbit galaxy centers forever,” he said. “This braking mechanism enables the gas to instead fall into black holes and the gravitational brakes, or torques, are strong enough to explain the quasars that we observe.”

Researchers can now model how the interconnected processes work, which can help them to understand the origin of the supermassive black hole at the center of our own Milky Way galaxy. The model would also allow them to understand the supermassive black hole at the center of the Messier 87 galaxy, which was seen in an image captured by the Event Horizon Telescope in 2019.

Next, the authors want to study large statistical populations of galaxies and their central black holes to understand how they form and grow.

Edited by Kristen Butler and Fern Siegel

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