Prevalence of tidal interactions among local Seyfert galaxies: The control experiment

Abstract

We test whether there is a relation between the observed tidal interactions and Seyfert activity by imaging in atomic hydrogen (H I 27) inactive galaxies at the same spatial resolution and detection threshold as the Seyfert sample. This control sample of inactive galaxies was closely matched in Hubble type, range in size, and inclination and has roughly comparable galaxy optical luminosity to the Seyfert galaxies. We find that only ∼15% of the galaxies in our control sample are disturbed in H I, whereas the remaining ∼85% show no disturbances whatsoever in H I. Even at a spatial resolution of ∼10 kpc, none of the latter galaxies show appreciable H I disturbances reminiscent of tidal features. In a companion paper we report results from the first systematic imaging survey of Seyfert galaxies in H I gas. We find that only ∼28% of the 18 Seyfert galaxies in that sample are visibly disturbed in optical starlight. By contrast, ∼94% of the same Seyfert galaxies are disturbed spatially and usually also kinematically in H I gas on galactic scales of ≳20 kpc. In at least ∼67% and up to perhaps ∼94% of cases, the observed disturbances can be traced to tidal interactions with neighboring galaxies detected also in H I. The dramatic contrast between the observed prevalence of H I disturbances in the Seyfert and control samples implicates tidal interactions in initiating events that lead to luminous Seyfert activity in a large fraction of local disk galaxies. © 2008. The American Astronomical Society. All rights reserved.