Optical classification of megamaser galaxies

We have obtained spectroscopic observations of the nuclear regions of 42 ga laxies known to harbor strong OH masers. These megamaser galaxies represent a subsample of FIR (ultra)luminous galaxies, which typically have FIR lumi nosities in excess of 10(11) L.. The primary goal of this study is to inves tigate the nuclear activity sources of OH megamaser galaxies. We are able t o classify the nuclear emission-line spectra of all but one of our sample, and we find that this class of galaxies is dominated by active galactic nuc lei (AGNs), although starburst galaxies do make up an appreciable fraction of the megamasers as well. Fully 45% of the megamasers exhibit Seyfert or L INER spectra, predominantly of Seyfert 2 type, although two galaxies with b road lines are observed. This observation is consistent with the current un ification models for the two types of Seyfert activity, since the assumed g eometry necessary to detect a megamaser places the Seyfert nucleus behind a high column density of molecular gas (i.e., the molecular torus). Starburs t-nucleus galaxies comprise 32.5% of our sample, while 22.5% are classified as "composite nuclear spectra" (CSN) sources, showing evidence of both AGN and starburst activity. A number of objects show unusual emission-line rat ios, not surprising for a group of galaxies that are known a priori to poss ess substantial absorbing material along the line of sight to their nuclei. Our results are compared to previous studies of FIR-selected galaxy sample s, as well as to radio-continuum observations of these galaxies. The activi ty classification obtained from the radio data disagrees with the optical c lassifications in roughly 25% of the sources; we discuss possible explanati ons for these discrepancies.