Optical Spectral Classification of Major Mergers: OH Megamaser Hosts versus Nonmasing (Ultra)Luminous Infrared Galaxies

Abstract

We investigate the types of nuclear environments that produce OH megamasers (OHMs) with a study of the optical spectra of 40 OHM host galaxies and a control sample of 30 nonmasing (ultra)luminous infrared galaxies ([U]LIRGs). The optical spectrophotometry provides spatially resolved spectra of multiple nuclei in a few of these merging systems. Of the 40 OHMs classified, 33% are starbursts, 42% are LINERs, and 25% are Seyfert 2 galaxies; the classification of the nonmasing control sample is similar. The OHM fraction in mergers does not depend on nuclear classification, and there are no distinguishing spectral properties to differentiate OHMs from nonmasing mergers; OHM galaxies appear to represent the same optical population as the (U)LIRGs. There are likewise few trends between OHM host-galaxy properties and the properties of the OHMs they produce. OHMs do appear to be somewhat blueshifted with respect to their hosts, particularly LINERs, and Seyfert and LINER hosts tend to produce broader OH emission-line profiles than pure starburst hosts. This suggests that OHMs are in some cases associated with outflows and that the phase space available for masing in LINER and Seyfert hosts is larger than in pure starbursts. The paucity of significant correlations between OHMs and the optical properties of their hosts is presumably due to the severe dust extinction found in (U)LIRGs, especially at the high-density sites of molecular masing.