Modeling a Conversation Between a Black Hole and Its Galaxy

A new modeling method allows black holes and the gas that surrounds them to “talk” back and forth, painting a more realistic picture of how black holes collect material and churn out energy.

A Problem of Scale

Centaurus A

Composite image of Centaurus A, a galaxy whose appearance is dominated by the large-scale jets powered by the supermassive black hole at its center. [ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray); CC BY 4.0]

Black holes at the centers of galaxies across the universe consume gas, dust, and even stars from their surroundings. In exchange for this feast, accreting black holes emit powerful jets and radiation that disrupt and heat nearby gas. This process, known as feedback, cements the link between a black hole and its home galaxy.

Supermassive black holes, though enormous, are tiny compared to their host galaxies — the Milky Way’s central black hole’s event horizon stretches roughly 15 million miles, just a minuscule fraction of our galaxy’s half-quintillion-mile diameter. Despite this size mismatch, supermassive black holes are so powerful that they can influence entire galaxies, leaving researchers with the enormous challenge of modeling the complex processes of accretion and feedback across a wide range of spatial scales.

Building a Two-Way Radio

Typically, models of hungry black holes handle the spatial scale issue by nesting simulations spanning different scales within each other and running them in sequence, starting far from the black hole and spiraling in toward it. This strategy helps the model communicate to the black hole what’s going on around it — how much gas there is to snack on, for example — but it needs to let the black hole talk back, too. That’s where a new technique from a team led by Hyerin Cho (조혜린), Center for Astrophysics | Harvard & Smithsonian and the Black Hole Initiative, comes in.

This new technique uses general relativistic magnetohydrodynamics to model black hole accretion and feedback across seven orders of magnitude in spatial scale. The key advance is that the model spirals from large scales down to small scales — and back — hundreds of times, allowing the black hole to chat freely with its surroundings.

Focusing on Feedback

To demonstrate the new method’s capabilities, Cho and collaborators first showed that they could reproduce the standard analytical solution for a black hole accreting unmagnetized gas. Then, they moved on to a more realistic system that includes magnetic fields. Unlike the unmagnetized case, where gas swirls toward the black hole in a smooth and orderly way, the magnetized case is chaotic: random, turbulent movements as the gas is pulled toward the black hole make the accretion rate vary wildly.

simulation results showing the plasma beta and the plasma density across eight orders of magnitude of spatial scale

Maps of plasma beta (β; the ratio of thermal pressure to magnetic pressure within a plasma) and plasma density (ρ) across eight orders of magnitude in spatial scale. Click to enlarge. [Adapted from Cho et al. 2023]

Where does the turbulence come from? Cho’s team found that magnetic field lines close to the black hole are constantly rearranging, relaxing into new configurations that convert pent-up magnetic energy into kinetic energy. In other words, the reconfiguring of the magnetic field heats and accelerates the surrounding gas, prompting large-scale motions that transport energy away from the black hole — and this outward transport of energy signals that black hole feedback is actually taking place!

Importantly, Cho’s team’s results mesh with what researchers have seen for the black holes they’ve observed closely, especially the central supermassive black holes of the Milky Way and the massive elliptical galaxy Messier 87. While this two-way communication represents a huge advance in the modeling of black hole accretion and feedback, there’s more work to be done; future investigations will tackle spinning black holes surrounded by rotating gas.


“Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD,” Hyerin Cho et al 2023 ApJL 959 L22. doi:10.3847/2041-8213/ad1048