February 18, 2011 | 7
Black holes are the most massive compact objects in the universe—the supermassive variety are millions or even billions of times the mass of the sun. But new research may take these cosmic colossi down a peg or two.
According to an analysis in the February 17 issue of Nature, the masses of supermassive black holes at the cores of distant, luminous galaxies, known as active galactic nuclei (AGNs), have been overestimated by a factor of at least two. (Scientific American is part of Nature Publishing Group.)
Active galaxies give off bright radiation from their cores, where gas heats up as it is compressed into a swirling accretion disk encircling the black hole. The radiating region is too small to resolve telescopically at intergalactic distances, so researchers have turned to spectroscopy—splitting the radiation from an AGN into its component wavelengths—to get an idea of what goes on near an AGN’s black hole.
Spectral signals of individual atoms—say, the light emitted when a hydrogen atom is ionized—do not show up as discrete, narrow spikes at a specific wavelength. Rather, Doppler shifts spread them into wide lines surrounding that wavelength. The Doppler effect arises as the material swirls around the disk at thousands of kilometers per second. (The effect is analogous to standing at one end of a speedway—the pitch of a race car’s engine will rise and fall as it continuously approaches and then recedes from your vantage point.) Wolfram Kollatschny and Matthias Zetzl of the Institute for Astrophysics Göttingen in Germany used those smeared spectral lines from 37 AGNs to determine how much of the widening results from the disk’s rotation around the black hole, and how much results from turbulence and other phenomena.
Kollatschny and Zetzl’s analysis found that rotation is the primary driver of spectral line widening, allowing them to use the width of measured hydrogen lines to infer the velocity of the swirling gas and hence the mass of the object at the disk’s center. Their method indicates that the mass of distant AGN black holes as estimated by other spectral methods is two to 10 times too high.
That is not enough to cost supermassive black holes their title as heavyweight champs of the universe, but it may nonetheless affect studies of the formation and growth of black holes and the galaxies that feed them.
Artist’s conception of gas swirling around a black hole: NASA/Dana Berry, SkyWorks Digital