Mid-infrared properties of OH megamaser host galaxies. II: Analysis and
  modeling of the maser environment
  We present a comparison of Spitzer IRS data for 51 OH megamaser (OHM) hosts
and 15 non-masing ULIRGs. 10-25% of OHMs show evidence for the presence of an
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 AGN. 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 um silicate depth and
30-20 um 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
({\tau}_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.
