A SURVEY FOR H2O MEGAMASERS IN ACTIVE GALACTIC NUCLEI .2. A COMPARISON OF DETECTED AND UNDETECTED GALAXIES

A survey for H2O megamaser emission from 354 active galaxies has resul ted in the detection of 10 new sources, making 16 known altogether. Th e galaxies surveyed include a distance-limited sample (covering Seyfer ts and LINERs with recession velocities less than 7000 km s(-1)) and a magnitude-limited sample (covering Seyferts and LINERs with m(B) less than or equal to 14.5). In order to determine whether the H2O-detecte d galaxies are ''typical'' active galactic nuclei (AGNs) or have speci al properties that facilitate the production of powerful masers, we ha ve accumulated a database of physical, morphological, and spectroscopi c properties of the observed galaxies. The most significant finding is that H2O megamasers are detected only in Seyfert 2 and LINER galaxies , not Seyfert 1's. This lack of detection in Seyfert 1's indicates eit her that they do not have molecular gas in their nuclei with physical conditions appropriate to produce 1.3 cm H2O masers or that the masers are beamed away from Earth, presumably in the plane of the putative m olecular torus that hides the Seyfert 1 nucleus in Seyfert 2's. LINERs are detected at a similar rate to Seyfert 2's, which constitutes a st rong argument that at least some nuclear LINERs are AGNs rather than s tarbursts, since starbursts have not been detected as H2O megamasers. We preferentially detect H2O emission from the nearer galaxies and fro m those that are apparently brighter at mid- and far-infrared and cent imeter radio wavelengths. There is also a possible trend for the H2O-d etected galaxies to be more intrinsically luminous in nuclear 6 cm rad io emission than the undetected ones, though these data are incomplete . We find evidence that Seyfert 2's with very high (N-H > 10(24) cm(-2 )) X-ray-absorbing columns of gas are more often detected as H2O maser emitters than Seyfert 2's with lower columns. It may be that the prob ability of detecting H2O maser emission in Seyfert galaxies increases with increasing column of cool gas to the nucleus, from Seyfert 1's th rough narrow-line X-ray galaxies to Seyfert 2's.