A Comparison of Tracers of Cool Gas in Galaxies and the 12 CO/ 13 CO Luminosity Ratio in Luminous Infrared Galaxies

Abstract

We present new observations of 12CO (J=2-1), 13CO(J=2-1), and 1.3 mm dust continuum emission toward seven diversegalaxies (IC 10, IRAS 10565+2448, the Antennae-NGC 4038 and 4039, NGC4736, Mrk 231, and NGC 6090). Compared with the other galaxies in thesample, the two ultraluminous infrared galaxies (ULIGs) exhibit reduced13CO line luminosities with respect to the 12COline luminosities and 1.3 mm dust luminosities. These observationsparallel the established trend of large R-values in ULIGs compared withmore quiescent spiral galaxies. The two luminous merging pairs in oursample with moderate starburst activity, the Antennae and NGC 6090, donot appear to show the large R-values. The results are consistent withmodels in which 12CO emission is dominated in the mostluminous mergers by warm regions of ISM with moderate optical depths(τ12CO~1). For the other galaxies, R is moretypical of the optically thick case 10<20, and most of theemission probably arises from within cooler molecular clouds. We comparethese data with previous 12CO (J=3-2) and [C I](3P1-3P0) observations fromthe literature.