The Masses of Supermassive Black Holes
The galaxy NGC 5548 has a supermassive black hole at its nucleus (a supermassive black hole has a mass exceeding a million solar-masses). Astronomers had reported that the masses of nuclear black holes seem to correlate with the stellar masses of their host galaxies, suggesting that they somehow evolve together, but these studies were based primarily on galaxy averages. A new analysis of the X-ray emission of individual galaxies concludes that that nuclear black holes and stellar masses do indeed evolve together, and that some earlier conclusions were biased because of selection effects.
Most galaxies are thought to host at its nucleus a supermassive black hole (SMBH), an object with a mass exceeding a million solar-masses. Our Milky Way, for example, has a four million solar-mass black hole at its center, and the most extreme examples are estimated to have as much as ten billion solar-masses. Both active galaxies and inactive galaxies have SMBHs, but the former are actively accreting material and radiating from the hot environment. The masses of these monsters are usually measured directly from the kinematics of gas or stars moving under the strong gravitational presence of the nucleus. Indirect measurements can also be made from ancillary correlations that have been found; for example, the masses of SMBHs appear to be tightly correlated with galaxies' stellar masses and with the spread of motions ("velocity dispersion") seen in the galactic hosts. Since black holes usually grow over time, these correlations suggest that there is some kind of co-evolution with the galaxy but what that is and how it evolves is not understood. Inactive galaxies, for example, sometimes show a different correlation than active ones; perhaps this is due to selection biases. Some scientists have argued that the correlation with stellar mass is just a by-product of the more fundamental connections with velocity dispersion.
CfA astronomer Francesca Civano is a member of a team of astronomers that used data from the Chandra X-Ray Observatory and other X-ray missions to probe the key issue of whether observational selection effects result in the appearance of a correlation. The limited capabilities of modern telescopes, for example, inevitably favor galaxies whose gas and stars have the largest motions, and computer simulations have shown that this effect alone could account for the appearance of a correlation. The scientists instead looked at the X-ray luminosity of a sample of galaxies, a measure of the accretion onto their SMBHs, which in turn is a measure of their masses and their efficiency of producing radiation. The astronmers' technique uses X-ray results from individual galaxies to obtain masses and is more reliable than similar, older attempts that used combined X-ray averages.
The astronomers find that the stellar masses of galaxies and their nuclear SMBHs do appear to grow together, and that this relation is nearly independent of galaxy epoch as far back as about ten billion years. This result provides independent evidence that earlier correlations were biased by selection effects, at least for those derived from kinematics. The team reports one surprise: the efficiency of accretion radiation is apparently about fifteen percent, nearly ten times higher than expected from theory, and implies that the black holes are spinning rapidly since spining black holes should be more efficent radiators.
"Probing Black Hole Accretion Tracks, Scaling Relations, and Radiative Efficiencies from Stacked X-ray Active Galactic Nuclei," Francesco Shankar et al. MNRAS, 493, 1500, 2020.