$Herschel$/PACS OH Spectroscopy of Seyfert, LINER, and Starburst Galaxies

TL;DR

This study uses Herschel/PACS spectroscopy to analyze OH doublets in 178 local galaxies, revealing how their emission and absorption features relate to galaxy type, AGN activity, and dust properties.

Contribution

It provides the first comprehensive analysis of multiple OH doublets across diverse galaxy types, linking spectral features to galaxy and dust characteristics.

Findings

OH79, OH119, and OH163 emission stronger in bright, unobscured AGN.

Obscured AGN and non-active galaxies show more absorption and weaker emission.

Dust extinction correlates with OH line profiles, affecting emission and absorption strengths.

Abstract

We investigated the 65$\mu$m, 71$\mu$m, 79$\mu$m, 84$\mu$m, 119$\mu$m, and 163$\mu$m OH doublets of 178 local (0 < $z$ < 0.35) galaxies. They were observed using the $Herschel$/PACS spectrometer, including Seyfert galaxies, LINERs, and star-forming galaxies. We observe these doublets exclusively in absorption (OH71), primarily in absorption (OH65, OH84), mostly in emission (OH79), only in emission (OH163) and an approximately even mix of the both (OH119). In 19 galaxies we find P-Cygni or reverse P-Cygni line profiles in the OH doublets. We use several galaxy observables to probe spectral classification, brightness of a central AGN/starburst component, and radiation field strength. We find that OH79, OH119, and OH163 are more likely to display strong emission for bright, unobscured AGN. For less luminous, obscured AGN and non-active galaxies, we find populations of strong absorption (OH119), weaker emission (OH163), and a mix of weak emission and weak absorption (OH79). For OH65, OH71 and OH84, we do not find significant correlations with the observables listed above. We do find relationships between OH79 and OH119 with both the 9.7$\mu$m silicate feature and Balmer decrement dust extinction tracers in which more dust leads to weaker emission / stronger absorption. The origin of emission for the observed OH doublets, whether from collisional excitation, or from radiative pumping by infrared photons, is discussed.