H2O megamaser emission in active galactic nuclei

We report an extensive search for 22 GHz H{dollar}sb2{dollar}O maser emission from bright (m{dollar}sb{lcub}B{rcub} le{dollar} 14) and nearby (cz {dollar}<{dollar} 7000 km s{dollar}sp{lcub}-1{rcub}{dollar}) active galaxies, as well as a number of active galaxies at higher redshift. In all, 354 galaxies have been surveyed. Ten new H{dollar}sb2{dollar}O megamaser sources have been detected, making 16 galaxies currently known to contain H{dollar}sb2{dollar}O masers with isotropic luminosities {dollar}>{dollar} 20 {dollar}Lsb{lcub}odot{rcub}{dollar}. The maser detection rate of active galaxies with cz {dollar}<{dollar} 7000 km s{dollar}sp{lcub}-1{rcub}{dollar} is 5.4%, and with cz {dollar}<{dollar} 2000 km s{dollar}sp{lcub}-1{rcub}{dollar} is 11%. VLA observations of NGC 1052, NGC 1386, NGC 5506, and IC 2560 show that the maser emission in these galaxies is unresolved (1{dollar}sp{lcub}primeprime{rcub}{dollar}-6{dollar}sp{lcub}primeprime{rcub}{dollar} beams) and is coincident with the nuclear radio continuum peak in each galaxy.; We have compiled a database of physical, morphological, and spectroscopic properties of the observed galaxies. In comparing the H{dollar}sb2{dollar}O-detected and undetected galaxies, the most significant finding is that megamasers are found only in Seyfert 2 and LINER galaxies, not Seyfert 1s. Hence Seyfert 1s either do not have nuclear molecular gas with conditions appropriate to produce 1.3 cm H{dollar}sb2{dollar}O masers or the masers are beamed away from Earth, presumably in the plane of the putative "obscuring torus". LINERs are detected at a similar rate to Seyfert 2s, constituting a strong argument that at least some LINERs are AGN rather than starbursts. We preferentially detect H{dollar}sb2{dollar}O emission from the nearer and apparently brighter galaxies. There is also a possible trend for the H{dollar}sb2{dollar}O-detected galaxies to be more luminous in nuclear radio continuum emission than the undetected ones. In addition, we find evidence that Seyfert 2s with very high (N{dollar}sb{lcub}H{rcub} >{dollar} 10{dollar}sp{lcub}24{rcub}{dollar} cm{dollar}sp{lcub}-2{rcub}{dollar}) X-ray absorbing columns of gas are more often detected as H{dollar}sb2{dollar}O maser emitters than Seyfert 2s with lower columns.; We have periodically monitored the H{dollar}sb2{dollar}O sources in order to probe the dynamics of the nuclear gas. The maser line in NGC 2639 is found to have a systematic redward velocity drift of 6.6 {dollar}pm{dollar} 0.4 km s{dollar}sp{lcub}-1{rcub}{dollar} yr{dollar}sp{lcub}-1{rcub}{dollar}. The similarities of the intrinsic maser spectra and the velocity drifts in NGC 2639 and NGC 4258 suggest that the masers in NGC 2639 exist in a thin edge-on disk. No systematic velocity drifts are found for the other galaxies with narrow-line masers. However, we also report a drift of {dollar}sim{dollar}30 km s{dollar}sp{lcub}-1{rcub}{dollar} yr{dollar}sp{lcub}-1{rcub}{dollar} in the unusual broad H{dollar}sb2{dollar}O emission feature in NGC 1052.