THE QUEST FOR THE DYNAMICAL SIGNATURE OF AC- CRETION DISKS IN ACTIVE GALAXIES

The most direct, dynamical evidence for the presence of accretion disks in the central engines of active galaxies comes from observations of very broad, double-peaked emission lines. These \exotic" line proles are found preferentially in radio-loud AGNs; they occur in about 10% of such objects. Their hosts share a number of common spectral properties which can be understood consistently in the context of a physical model for line-emitting disks. Very recent UV spectroscopy of double-peaked emitters with the HST provides very strong additional evidence for an accretion disk origin of the double-peaked Balmer lines. Variability of the line proles is an independent and complementary approach for assessing the applicability of disk models to double-peaked emission lines. Moreover, once an accretion disk origin is established, the variability of the line proles will provide a wealth of information on the structure and dynamics of the disk (much like in cataclysmic variables). The dynamical time in the line-emitting part of the disk is expected to be about six months, which is much longer than the light crossing time. Thus the slow variations of the line proles over time scales of a few years can be used to test models for the dynamical behaviour of the disk. These models include spiral waves in the disk, elliptical disks, supermassive binary black holes, and interactions of the disk with stars or gas clouds in its vicinity. Tests can be carried out using spectra of double-peaked emission lines with high signal-to-noise ratio, taken every few months, and spanning a baseline of several years.