The geometry of nuclear black hole accretion disks
Supermassive black holes with millions or even billions of solar-masses of material are found at the nuclei of most galaxies, including our Milky Way. A torus of dust and gas orbits around the black hole (at least according to most theories) and radiates in ultraviolet light when material falling toward the black hole heats the disk to millions of degrees. The accretion process can also power the ejection of jets of rapidly moving charged particles. Such actively accreting supermassive black holes in galaxies are called active galactic nuclei (AGN).
Astronomers who model the physical processes in one of these huge dynamos start with the gas motions and geometry of the region. The gas motions can be measured straightforwardly from emission lines in the gas, typically optical lines of hydrogen that are excited by the uv radiation. As for geometry, simple calculations estimate that the radius of line-emitting gas should be a few thousand astronomical units (one AU is the average distance of the Earth from the Sun). Because most AGN are too far away to be able to measure dimensions this small, astronomers have come to rely on the technique of "reverberation mapping." Radiation from the accretion disk is highly variable. Since it takes time for the uv to travel from the accretion disk near the black hole out to the line-emitting gas, there is a delay between an event seen in the continuum and then in the hydrogen lines
Source
Credit: NASA, Andrew S. Wilson U.Maryland; Patrick L. Shopbell CIT; Chris Simpson Subaru; Thaisa Storchi-Bergmann and F. K. B. Barbosa, UFRGS, Brazil; and Martin J. Ward, U. Leicester
A Hubble image of the Circinus galaxy with its active galactic nucleus (AGN). Astronomers have measured the sizes of the accreting regions around the supermassive black holes in four distant AGN using techniques of reverberation mapping.
Source / Image Courtesy
The disk is based on a non-relativistic Smoothed-Particle-Hydrodynamics (SPH) simulation.
The apparent distortion of the disk is due to the bending of light within the Schwarzschild spacetime. Besides the primary image of the disk, there are also higher order images (lower ring and upper thin ring) visible.
The lower ring shows the rear bottom side of the disk. The color encodes the apparent temperature where blue is hot and red is cold. The left part of the disk approaches the observer and, thus, appears hotter due to the Doppler effect. You also see some bright spots which smear out already after one revolution around the black hole.