AMMONIA THERMOMETRY OF STAR-FORMING GALAXIES

Abstract

With a goal toward deriving the physical conditions in external galaxies, we\npresent a study of the ammonia (NH$_3$) emission and absorption in a sample of\nstar forming systems. Using the unique sensitivities to kinetic temperature\nafforded by the excitation characteristics of several inversion transitions of\nNH$_3$, we have continued our characterization of the dense gas in star forming\ngalaxies by measuring the kinetic temperature in a sample of 23 galaxies and\none galaxy offset position selected for their high infrared luminosity. We\nderive kinetic temperatures toward 13 galaxies, 9 of which possess multiple\nkinetic temperature and/or velocity components. Eight of these galaxies exhibit\nkinetic temperatures $>100$ K, which are in many cases at least a factor of two\nlarger than kinetic temperatures derived previously. Furthermore, the derived\nkinetic temperatures in our galaxy sample, which are in many cases at least a\nfactor of two larger than derived dust temperatures, point to a problem with\nthe common assumption that dust and gas kinetic temperatures are equivalent. As\npreviously suggested, the use of dust emission at wavelengths greater than 160\n$\\mu$m to derive dust temperatures, or dust heating from older stellar\npopulations, may be skewing derived dust temperatures in these galaxies to\nlower values. We confirm the detection of high-excitation OH $^2\\Pi_{3/2}$\nJ=9/2 absorption toward Arp220 (Ott et. al. 2011). We also report the first\ndetections of non-metastable NH$_3$ inversion transitions toward external\ngalaxies in the (2,1) (NGC253, NGC660, IC342, and IC860), (3,1), (3,2), (4,3),\n(5,4) (all in NGC660) and (10,9) (Arp220) transitions.\n