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.


    Access

    Check access

    Check availability in my library

    Order at Subito €


    Export, share and cite



    Title :

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


    Contributors:

    Published in:

    Publication date :

    1988-01-01


    Size :

    11 pages




    Type of media :

    Article (Journal)


    Type of material :

    Electronic Resource


    Language :

    English



    Massive Accretion Disks in Galactic Nuclei

    Scoville, N.Z. | Online Contents | 1999


    Massive Accretion Disks in Galactic Nuclei

    Scoville, N. Z. / InternationaL Council of Scientific Unions; Committee on Space Research; Scientific Commission E | British Library Conference Proceedings | 1999



    Is there Spectropolarimetric Evidence for Accretion Disks in Active Galactic Nuclei?

    Koratkar, A. / InternationaL Council of Scientific Unions; Committee on Space Research; Scientific Commission E | British Library Conference Proceedings | 1999