Prevalence of Tidal Interactions among Local Seyfert Galaxies

Abstract

No mechanisms have hitherto been conclusively demonstrated to be responsible for initiating optically luminous nuclear (Seyfert) activity in local disk galaxies. Only a small minority of such galaxies are visibly disturbed in optical starlight, with the observed disturbances being at best marginally stronger than those found in matched samples of inactive galaxies. Here we report the first systematic study of an optically selected sample of 23 active galaxies in atomic hydrogen (H i) gas, which is the most sensitive and enduring tracer known of tidal interactions. Eighteen of these galaxies are (generally) classified as Seyferts, with over half (and perhaps all) having [O iii] luminosities within 2 orders of magnitude of QSOs. Only $28% of these Seyfert galaxies are visibly disturbed in optical DSS2 images. By contrast, $94% of the same galaxies are disturbed in H i, in nearly all cases not just spatially but also kinematically on galactic (k20 kpc) scales. In at least $67% and perhaps up to $94% of cases, the observed H i disturbances can be traced to tidal interactions with neighboring galaxies detected also in H i. The majority of these neighboring galaxies have projected separations of P100 kpc and differ in radial velocities by P100 km s À1 from their respective Seyfert galaxies, and many have optical luminosities ranging from the Small to Large Magellanic Clouds. In a companion paper we show that only $15% of a matched control sample of inactive galaxies display comparable H i disturbances. Our results suggest that (1) most Seyfert galaxies (with high nuclear luminosities) have experienced tidal interactions in the recent past and (2) in most cases these tidal interactions are responsible for initiating events that lead to their nuclear activity.