  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
twenty-three active galaxies in atomic hydrogen (HI) 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 [OIII] luminosities within two orders of magnitude of Quasi-Stellar
Objects. Only ~28% of these Seyfert galaxies are visibly disturbed in optical
DSS2 images. By contrast, ~94% of the same galaxies are disturbed in HI, in
nearly all cases not just spatially but also kinematically on galactic (>~20
kpc) scales. In at least ~67% and perhaps up to ~94% of cases, the observed HI
disturbances can be traced to tidal interactions with neighboring galaxies
detected also in HI. The majority of these neighboring galaxies have projected
separations of <~ 100 kpc and differ in radial velocities by <~100 km/s 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
HI disturbances. Our results suggest that: i) most Seyfert galaxies (with high
nuclear luminosities) have experienced tidal interactions in the recent past;
ii) in most cases, these tidal interactions are responsible for initiating
events that lead to their nuclear activity.
