The Hydroxyl-Water Megamaser Connection. I. Water Emission Toward OH Megamaser Hosts

Abstract: Questions surround the connection of luminous extragalactic masers to galactic processes. The observation that water and hydroxyl megamasers rarely coexist in the same galaxy has given rise to a hypothesis that the two species appear in different phases of nuclear activity. The detection of simultaneous hydroxyl and water megamaser emission toward IC694 has called this hypothesis into question but, because many megamasers have not been surveyed for emission in the other molecule, it remains unclear whether IC694 occupies a narrow phase of galaxy evolution or whether the relationship between megamaser species and galactic processes is more complicated than previously believed. In this paper, we present results of a systematic search for 22 GHz water maser emission among OH megamaser hosts to identify additional objects hosting both megamaser. Our work roughly doubles the number of galaxies searched for emission in both molecules which host at least one confirmed maser. We confirm with high degree of confidence ($> 8 \sigma$) the detection of water emission toward IIZw96, firmly establishing it as the second object to co-host both water and hydroxyl megamasers after IC694. We find high luminosity, narrow features in the water feature in IIZw96. All dual megamaser candidates appear in merging galaxy systems suggestive that megamaser coexistance may signal a brief phase along the merger sequence. A statistical analysis of the results of our observations provide possible evidence for an exclusion of H$_2$O kilomasers among OH megamaser hosts.