Ammonia Thermometry of Star Forming Galaxies

Abstract

With a goal toward deriving the physical conditions in external galaxies, we present a study of the ammonia (NH$_3$) emission and absorption in a sample of star forming systems. Using the unique sensitivities to kinetic temperature afforded by the excitation characteristics of several inversion transitions of NH$_3$, we have continued our characterization of the dense gas in star forming galaxies by measuring the kinetic temperature in a sample of 23 galaxies and one galaxy offset position selected for their high infrared luminosity. We derive kinetic temperatures toward 13 galaxies, 9 of which possess multiple kinetic temperature and/or velocity components. Eight of these galaxies exhibit kinetic temperatures $>100$ K, which are in many cases at least a factor of two larger than kinetic temperatures derived previously. Furthermore, the derived kinetic temperatures in our galaxy sample, which are in many cases at least a factor of two larger than derived dust temperatures, point to a problem with the common assumption that dust and gas kinetic temperatures are equivalent. As previously suggested, the use of dust emission at wavelengths greater than 160 $\mu$m to derive dust temperatures, or dust heating from older stellar populations, may be skewing derived dust temperatures in these galaxies to lower values. We confirm the detection of high-excitation OH $^2\Pi_{3/2}$ J=9/2 absorption toward Arp220 (Ott et. al. 2011). We also report the first detections of non-metastable NH$_3$ inversion transitions toward external galaxies in the (2,1) (NGC253, NGC660, IC342, and IC860), (3,1), (3,2), (4,3), (5,4) (all in NGC660) and (10,9) (Arp220) transitions.