Triggering AGNs — Interactions or Bars?

Abstract

Studies in optical starlight have failed to defintively address whether galaxy interactions or galactic bars play an important role in triggering luminous active galactic nuclei (AGNs). Here, we present the first systematic imaging study of Seyfert (disk) galaxies in the 21-cm line of neutral atomic hydrogen (HI) gas. HI is the most sensitive and enduring tracer of galaxy interactions, and can reveal tidal features not otherwise visible in optical starlight. Our sample, selected from the Véron-Cetty & Véron (1998) catalog, comprises a volume-limited set of Quasi-Stellar Objects (QSOs) with absolute B-band nuclear magnitudes M _B ≤ −23 at redshifts z < 0.07 visible from the VLA, and all known galaxies with nuclear magnitudes −19 ≥ M _B > −23 (including also LINERS and HII galaxies) at 0.015 ≤ z ≤ 0.017 in the northern hemisphere. We have detected most of the QSO hosts so far studied, and nearly all show strong HI distortions that can usually be traced to tidal interactions with a companion galaxy. Similarly, we have detected nearly all the AGN/LINER/HII galaxies observed, and find a much higher incidence of tidal interactions among the AGN hosts by comparison with our matched control sample. Those AGN hosts with uncertain or no clear tidal features show disturbed HI morphologies and/or kinematics, as well as HI companion galaxies, much more frequently than the control sample. Our study demonstrates that the undisturbed optical appearence of active galaxies can be deceptive, and implicates galaxy-galaxy interactions as a major factor in triggering luminous AGNs and low-luminosity QSOs. The majority of the active galaxies studied here appear to be at a relatively early stage of an encounter rather than late in a merger.