A Survey for H 2 O Megamasers in Active Galactic Nuclei. II. A Comparison of Detected and Undetected Galaxies

Abstract

A survey for H₂O megamaser emission from 354 active galaxies has resulted in the detection of 10 new sources, making 16 known altogether. The galaxies surveyed include a distance-limited sample (covering Seyferts and LINERs with recession velocities less than 7000 km s^-1) and a magnitude-limited sample (covering Seyferts and LINERs with m_B ≤ 14.5). In order to determine whether the H₂O-detected galaxies are "typical" active galactic nuclei (AGNs) or have special properties that facilitate the production of powerful masers, we have accumulated a database of physical, morphological, and spectroscopic 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 either 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 molecular 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 strong argument that at least some nuclear LINERs are AGNs rather than starbursts, since starbursts have not been detected as H₂O megamasers. We preferentially detect H₂O emission from the nearer galaxies and from those that are apparently brighter at mid- and far-infrared and centimeter radio wavelengths. There is also a possible trend for the H₂O-detected galaxies to be more intrinsically luminous in nuclear 6 cm radio emission than the undetected ones, though these data are incomplete. We find evidence that Seyfert 2's with very high (N_H > 10²⁴ 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 probability of detecting H₂O maser emission in Seyfert galaxies increases with increasing column of cool gas to the nucleus, from Seyfert 1's through narrow-line X-ray galaxies to Seyfert 2's.