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

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 similar to 15% of the galaxies in our control sample are disturbed in H I, whereas the remaining similar to 85% show no disturbances whatsoever in H I. Even at a spatial resolution of similar to 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 similar to 28% of the 18 Seyfert galaxies in that sample are visibly disturbed in optical starlight. By contrast, similar to 94% of the same Seyfert galaxies are disturbed spatially and usually also kinematically in H I gas on galactic scales of greater than or similar to 20 kpc. In at least similar to 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 similar to implicates tidal interactions in initiating events that lead to luminous Seyfert activity in a large fraction of local disk galaxies.