Does What a Black Hole Eats Affect Its Appearance?

When the supermassive black holes at the centers of galaxies siphon gas from their surroundings, the superheated gas radiates at wavelengths ranging from X-ray to radio. A recent research article explores whether the composition of the accreted gas affects the radiation we observe.

What Do Black Holes Eat?

visualization of the accretion disk around a supermassive black hole

A visualization of the accretion disk around a supermassive black hole with the main features of the gravitationally warped image labeled. Click to enlarge. [NASA’s Goddard Space Flight Center/Jeremy Schnittman]

In 2019 and 2022, the Event Horizon Telescope captured images of M87* and Sgr A*, the supermassive black holes at the centers of Messier 87 and the Milky Way, respectively. The glowing rings of radio emission surrounding these black holes are produced by electrons spiraling around magnetic field lines in an extremely hot gas — so hot that the atoms are split apart, forming a sea of bare nuclei and electrons.

The gas gobbled up by M87* and Sgr A* is likely a mix of hydrogen and helium with just a dash of heavier elements, but we don’t know its exact composition. In a new publication, George Wong (Institute for Advanced Study) and Charles Gammie (University of Illinois) pose a relevant question: does the composition of this gas affect the electromagnetic radiation emitted by a supermassive black hole’s accretion disk?

plots of how modeled paramters change with changing gas composition.

Effect of varying the amount of helium in the gas accreted my M87* (blue solid line) and Sgr A* (red dashed line) on model outputs. Θe is the electron temperature and τS, τQ, and τV are varying types of optical depth. Click to enlarge. [Wong & Gammie 2022]

Hydrogen vs. Helium

To approach this question, Wong and Gammie began with a simple model that allowed them to estimate some of the effects of tuning the gas composition from pure hydrogen to pure helium. In each case, the team used the Event Horizon Telescope observations of M87* and Sgr A* as a benchmark, adjusting the model parameters until the simulated flux equaled the observed flux.

These simulations suggest that as the amount of helium climbs, the electrons must be at a higher temperature, the plasma must be less dense, and the magnetic field must be weaker to produce the observed flux. In other words, changing the gas composition requires altering other physical properties of the system to get the same amount of radiation. These physical alterations may in turn affect other observational properties, such as the polarization, or orientation, of the emitted light waves.

Analyzing Accretion Options

In order to investigate changes in the polarization and other observational properties, Wong and Gammie used the results of these simple models to inform more complex relativistic fluid dynamics models and generate synthetic images. The authors considered two extreme cases — one in which the gas surrounding the supermassive black hole is pure hydrogen, and one in which it’s pure helium. The team also explored two proposed models for how gas is accreted — one in which the material forms an accretion disk that steadily feeds material to the black hole, and one in which the material is accreted in random bursts.

model results for polarized images of the Milky Way's supermassive black hole

Modeled polarized-light images for Sgr A* at three frequencies. For these models, which incorporate a smooth accretion flow, the composition of the accreted gas affects the observed polarization fraction. Click to enlarge. [Adapted from Wong & Gammie 2022]

The authors found that the gas composition affects the polarization that we observe, with the helium-only model having a more orderly polarization pattern. In addition, changing both the gas composition and the accretion method (steady or random) results in complex outcomes including changing where in the disk the emission is generated. These results show that the presence of helium can affect the electromagnetic radiation emitted by an accreting black hole, suggesting that future models should consider the composition of the accreted gas an important variable.


“Effects of Hydrogen versus Helium on Electromagnetic Black Hole Observables,” George N. Wong and Charles F. Gammie 2022 ApJ 937 60. doi:10.3847/1538-4357/ac854d