Mid-infrared properties of OH megamaser host galaxies. I: Spitzer IRS low- and high-resolution spectroscopy

TL;DR

This study provides detailed mid-infrared spectra of 51 OH megamaser host galaxies, revealing their dust, molecular, and atomic emission features, and comparing their properties to non-masing galaxies to understand the conditions conducive to megamaser activity.

Contribution

First comprehensive mid-infrared spectral analysis of OH megamaser host galaxies, highlighting differences in dust absorption and molecular emission compared to non-masing galaxies.

Findings

OHM galaxies show stronger silicate absorption and steeper mid-IR continuum.

Most systems exhibit PAH emission and atomic/molecular lines, indicating active star formation.

OH column densities are similar in maser and non-masing galaxies, suggesting comparable molecular abundances.

Abstract

We present mid-infrared spectra and photometry from the Infrared Spectrograph on the Spitzer Space Telescope for 51 OH megamasers (OHMs), along with 15 galaxies confirmed to have no megamaser emission above L_OH = 10^2.3 L_sun. The majority of galaxies display moderate-to-deep 9.7 um amorphous silicate absorption, with OHM galaxies showing stronger average absorption and steeper 20-30 um continuum emission than non-masing galaxies. Emission from multiple polycyclic aromatic hydrocarbons (PAHs), especially at 6.2, 7.7, and 11.3 um, is detected in almost all systems. Fine-structure atomic emission (including [Ne II], [Ne III], [S III], and [S IV]) and multiple H2 rotational transitions are observed in more than 90% of the sample. A subset of galaxies show emission from rarer atomic lines, such as [Ne V], [O IV], and [Fe II]. 50% of the OHMs show absorption from water ice and hydrogenated amorphous carbon grains (HACs), while absorption features from CO2, HCN, C2H2, and crystalline silicates are also seen in several OHMs. Column densities of OH derived from 34.6 um OH absorption are similar to those derived from 1667 MHz OH absorption in non-masing galaxies, indicating that the abundance of masing molecules is similar for both samples. This data paper presents full mid-infrared spectra for each galaxy, along with measurements of line fluxes and equivalent widths, absorption feature depths, and spectral indices.