Magnetic Fields Form Fluffy Accretion Disks Around Black Holes, Reshaping Theories of Their Dynamics.

A Caltech astrophysics team has created a groundbreaking simulation tracing the path of primordial gas from the early universe into a supermassive black hole's accretion disk. This innovative work, which redefines decades-old theories dating back to the 1970s, offers fresh insights into the evolution of black holes and galaxies.

“This simulation is the result of years of collaboration between two major research teams at Caltech,” said Phil Hopkins, the Ira S. Bowen Professor of Theoretical Astrophysics.

Figure 1. Simulation of a Supermassive Black Hole Surrounded by an Accretion Disk

Closing the Scale Gap in Cosmic Simulations

The first project, FIRE (Feedback in Realistic Environments), examines large-scale cosmic processes like galaxy formation and collisions. The second, STARFORGE, delves into smaller-scale phenomena such as star formation within gas clouds. “But there was a significant gap between the two,” Hopkins notes. “Now, for the first time, we’ve bridged that gap.” This achievement required a simulation with over 1,000 times the resolution of previous attempts. Figure 1 shows Simulation of a Supermassive Black Hole Surrounded by an Accretion Disk.

Unexpectedly, the simulation, published in The Open Journal of Astrophysics, showed that magnetic fields play a far more significant role than previously thought in shaping the massive disks of material surrounding supermassive black holes. “Theories suggested these disks should be flat like crepes,” Hopkins explains. “But observations show they’re actually fluffy, more like angel food cake. Our simulation revealed that magnetic fields are supporting the disk material, creating this fluffier structure.”

Supercomputing Reveals Black Hole Accretion Disk Dynamics

In their latest simulation, researchers executed a "super zoom-in" on a single supermassive black hole—a colossal entity residing at the center of galaxies, including the Milky Way. These enigmatic giants, with masses ranging from thousands to billions of times that of the Sun, exert immense gravitational influence on their surroundings.

For decades, astronomers have understood that gas and dust captured by a black hole's gravity do not fall in immediately. Instead, they form a rapidly spinning structure known as an accretion disk. Just before being consumed, this material releases an extraordinary amount of energy, outshining almost everything else in the universe. Yet, the behavior and formation of these accretion disks, particularly around active black holes known as quasars, remain poorly understood.

While the Event Horizon Telescope has provided images of accretion disks around relatively calm black holes, such as those in the Milky Way (2022) and Messier 87 (2019), the highly energetic disks surrounding distant quasars remain out of reach for direct observation. To bridge this gap, astrophysicists rely on supercomputer simulations. These simulations integrate the fundamental laws of physics, including gravity, dark matter interactions, and star formation processes, into complex algorithms processed by thousands of computing cores running in parallel. These computational models allow researchers to recreate the intricate phenomena unfolding around these extreme cosmic environments.

“If you simply assume that gravity pulls everything together, forming stars as gas accumulates, your results will be completely off,” says Hopkins. Stars influence their surroundings in complex ways. They emit radiation that heats or pushes nearby gas, produce stellar winds like our Sun’s solar wind that sweep up material, and explode as supernovae, sometimes ejecting matter out of galaxies or altering local chemistry. Capturing these processes, collectively known as “stellar feedback,” is essential for accurate simulations, as this feedback governs how many stars a galaxy can realistically form.

Revealing New Perspectives on Black Hole Dynamics

The challenge of simulating black hole accretion lies in the need to incorporate both large-scale and small-scale physics. At the galactic scale, factors like atomic and molecular behaviors are crucial for accuracy, but when focusing on the region around a black hole, these details become less relevant. Instead, the gas turns into hot, ionized plasma, requiring different physics.

To address this, the Caltech team used their GIZMO code, which was adaptable for both large and small scales. By developing the FIRE and STARFORGE projects with modular physics components that could be turned on or off, they ensured compatibility between different simulation types. This innovative approach allowed them to simulate a black hole approximately 10 million times the mass of the Sun, starting in the early universe and zooming in on the black hole as material from a star-forming gas cloud begins to spiral into it.

Reevaluating the Role of Magnetic Fields in Accretion Disks

In the new simulation, Hopkins and his team observed the formation of an accretion disk around a supermassive black hole. Contrary to previous models from the 1970s, which emphasized thermal pressure as the dominant force stabilizing these disks, the simulation revealed that magnetic fields play a far greater role—10,000 times stronger than thermal pressure. Magnetic fields are crucial in preventing the disk from collapsing, making the material puffier rather than flat.

This finding shifts predictions regarding the mass, density, thickness, and movement of material within the disks, as well as their overall shape and stability. Looking ahead, Hopkins is eager to explore new research areas now made possible by bridging the scale gap in simulations, such as studying galaxy mergers, star formation in extreme environments, and the characteristics of the universe’s first stars.

The full study, “FORGE’d in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions,” is published in The Open Journal of Astrophysics.

Source: SciTechDaily

Cite this article:

Janani R (2024), Caltech Astrophysicists Challenge Black Hole Theories with Groundbreaking Simulations, AnaTechmaz, pp. 131