Double-peaked emission lines in radio-loud AGNs

Abstract

We report the completion of a large optical spectroscopic survey of radio-loud AGNs which had the goal of finding more objects with double-peaked Balmer lines. We have identified a small but distinct subclass of radio-loud AGNs whose Balmer line profiles are double-peaked and can be fitted with a model of emission from a relativistic circular disk. These objects stand out not just because of their disk-like lines but also because of a number of additional properties that they possess. We find accretion-disk emission to be the most successful interpretation because it can explain the double-peaked line profiles as well as all of the additional properties of disk-like emitters. The principal alternative suggestions (binary broad-line regions, bipolar outflows, anisotropically-illuminated spherical broad-line regions) are unsatisfactory because they cannot explain all of the unusual properties of disk-like emitters self-consistently, and because in some cases they do not represent long-lived physical systems. Thus, the double-peaked lines provide the most direct, dynamical evidence for the presence of (large) accretion disks in radio-loud AGNs and become tools for studying dynamical and thermal phenomena in these disks. To this end we have been monitoring the variability of many of the disk-like emitters with the aim of testing models of the disk structure including, for example, disk waves, disk warps, collisions of stars with the disk, a tidally induced eccentricity, or the formation of transient rings by the tidal disruption of stars by the central black hole.