Optical and ultraviolet spectra of active galactic nuclei: Searching for accretion disks

Abstract

The most important characteristics of the optical, ultraviolet (UV), and (to a lesser extent) near-infrared spectra of active galactic nuclei are described. These include the broad and narrow emission lines, a nonstellar continuum having approximately power-law shape from near-infrared to red wavelengths, and a large excess of radiation over the power law at visual and UV wavelengths. Aspects of the spectra that may indicate the existence and properties of accretion disks are examined in greatest detail. Specifically, the “big blue bump” is probably thermal radiation from an accretion disk. Its presence may contribute to the observed inverse correlation between UV continuum luminosity and equivalent width of UV emission lines (the “Baldwin effect”). The excess UV radiation should also affect some of the emission-line intensity ratios. Various inconsistencies in current photoionization models, such as that between the observed and inferred shapes of the ionizing continuum, can be eliminated if the UV photons are emitted anisotropically whereas the X-rays are isotropic. Variability of the intensity ratios may reveal changes in the accretion rate. If the disk produces emission lines, their profiles should be double-peaked, although emission originating from the standard broad-line clouds may mask this effect. At high velocities within the line profiles, however, the relative strengths of lines should nevertheless differ from those produced by the standard clouds. Finally, some evidence for disks and thick tori is provided by certain correlations between optical and radio properties of quasars, as well as by spectropolarimetric studies of type 2 Seyfert galaxies.