A comparison of tracers of cool gas in galaxies and the (CO)-C-12/(CO)-C-13 luminosity ratio in luminous infrared galaxies

We present new observations of (CO)-C-12 (J=2-1), (CO)-C-13 (J=2-1), and 1. 3 mm dust continuum emission toward seven diverse galaxies (IC 10, IRAS 105 65+2448, the Antennae-NGC 4038 and 4039, NGC 4736, Mrk 231, and NGC 6090). Compared with the other galaxies in the sample, the two ultraluminous infra red galaxies (ULIGs) exhibit reduced (CO)-C-13 line luminosities with respe ct to the (CO)-C-12 line luminosities and 1.3 mm dust luminosities. These o bservations parallel the established trend of large R-values in ULIGs compa red with more quiescent spiral galaxies. The two luminous merging pairs in our sample with moderate starburst activity, the Antennae and NGC 6090, do not appear to show the large R-values. The results are consistent with mode ls in which (CO)-C-12 emission is dominated in the most luminous mergers by warm regions of ISM with moderate optical depths For the other galaxies, ( tau (12CO)similar to1). R is more typical of the optically thick case 10<R< 20, and most of the emission probably arises from within cooler molecular c louds. We compare these data with previous (CO)-C-12 (J=3-2) and [C I] (P-3 (1)-P-3(0)) observations from the literature.