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A. M. Gilbert (U. C. Berkeley), M. Eracleous (Penn State U.), A. V. Filippenko (U. C. Berkeley), J. P. Halpern (Columbia U.)
Compelling dynamical evidence for the accretion disks thought to power active galactic nuclei comes from the subset of broad-line radio galaxies that emit double-peaked Balmer lines with FWHMs from 16,000 to 30,000 km s-1. We present double-peaked H\alpha spectra collected over a twenty year baseline for two of these objects, 3C 390.3 and 3C 332, and discuss trends in their dramatic profile variability on timescales of a few months to years. We review the reasons why the observed variability must be due to dynamical processes in the emission-line region, and not reveberation (light-echo) effects. We then investigate models of the long-term variability as the rotation of a spiral emissivity perturbation on a Keplerian accretion disk, and find that they can successfully reproduce the detailed profile variations of 3C 332, but only roughly reproduce the trends in profile variability of 3C 390.3. >From the best-fit models we derive rotation periods for the spiral patterns of about 20 years for 3C 332 and 20-30 years for 3C 390.3, but these times are considerably shorter than the predicted periods of one-armed normal modes of such disks in linear perturbation theory. However, the observed profile variations are of about 30-50%, so the applicability of linear theory may be questioned. Only emissivity perturbations that have the form of a one-armed spiral can provide the degree of nonaxisymmetry needed to cause the observed profile variations; higher order perturbations are too symmetric.
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