Mid-infrared Properties of OH Megamaser Host Galaxies. II. Analysis and Modeling of the Maser Environment

We present a comparison of Spitzer Infrared Spectrograph (IRS) data for 51 OH megamaser (OHM) hosts and 15 non-masing ultraluminous infrared galaxies (ULIRGs). 10%-25% of the OHMs show evidence for the presence of an active galactic nucleus (AGN), significantly lower than the estimated AGN fraction from previous optical and radio studies. Non-masing ULIRGs have a higher AGN fraction (50%-95%) than OHMs, although some galaxies in both samples show evidence of co-existing starbursts and AGNs. Radiative transfer models of the dust environment reveal that non-masing galaxies tend to have clumpy dust geometries commonly associated with AGN, while OHMs have deeper absorption consistent with a smooth, thick dust shell. Statistical analyses show that the major differences between masing and non-masing ULIRGs in the mid-IR relate to the optical depth and dust temperature, which we measure using the 9.7 μm silicate depth and 30-20 μm spectral slope from the IRS data. Dust temperatures of 40-80 K derived from the IRS data are consistent with predictions of OH pumping models and with a minimum T_(dust) required for maser production. The best-fit dust opacities (τ_V ~ 100-400), however, are nearly an order of magnitude larger than those predicted for OH inversion, and suggest that modifications to the model may be required. These diagnostics offer the first detailed test of an OHM pumping model based only on the properties of its host galaxy and provide important restrictions on the physical conditions relevant to OHM production