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

TL;DR

This study compares mid-infrared properties of OH megamaser host galaxies and non-masing ULIRGs, revealing differences in dust geometry, optical depth, and temperature that influence maser activity and challenge existing pumping models.

Contribution

It provides the first detailed mid-IR analysis linking host galaxy dust properties to OH megamaser activity, testing and constraining maser pumping models.

Findings

OHMs show lower AGN fraction than previous estimates.

Non-masing ULIRGs have clumpy dust geometries associated with AGN.

Dust temperatures align with maser pumping models, but opacities are higher than predicted.

Abstract

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.