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We present a comparison of Spitzer IRS data for 51 OH megamaser (OHM) hostsand 15 non-masing ULIRGs. 10-25% of OHMs show evidence for the presence of anAGN, significantly lower than the estimated AGN fraction from previous opticaland radio studies. Non-masing ULIRGs have a higher AGN fraction (50-95%) thanOHMs, although some galaxies in both samples show evidence of co-existingstarbursts and AGN. Radiative transfer models of the dust environment revealthat non-masing galaxies tend to have clumpy dust geometries commonlyassociated with AGN, while OHMs have deeper absorption consistent with asmooth, thick dust shell. Statistical analyses show that the major differencesbetween masing and non-masing ULIRGs in the mid-IR relate to the optical depthand dust temperature, which we measure using the 9.7 um silicate depth and30-20 um spectral slope from the IRS data. Dust temperatures of 40-80 K derivedfrom the IRS data are consistent with predictions of OH pumping models and witha minimum T_dust required for maser production. The best-fit dust opacities({\tau}_V ~ 100 - 400), however, are nearly an order of magnitude larger thanthose predicted for OH inversion, and suggest that modifications to the modelmay be required. These diagnostics offer the first detailed test of an OHMpumping model based only on the properties of its host galaxy and provideimportant restrictions on the physical conditions relevant to OHM production.

Mid-infrared properties of OH megamaser host galaxies. II: Analysis and modeling of the maser environment