Properties of CO Molecular Gas in IR Luminous Galaxies

We present the properties of the 12CO(1-0) and (3-2) line emission from the nuclei of 60 IR luminous SLUGS galaxies. This subsample is flux limited at S60 > 5.24 Jy with FIR luminosities mostly at LFIR > 10^10 Lo. The emission line strengths of 12CO(1-0) and (3-2) transitions were compared at a common resolution of ~ 15", and the line ratios r31 vary from 0.22 to 1.72 with a mean value of 0.66 for the sources observed, indicating a large spread of the degree of excitation of CO in the sample. Our analysis shows that (1) there is a non-linear relation between CO and FIR luminosities, such that their ratio LCO/LFIR decreases linearly with increasing LFIR, (2) we find a possible dependence of the degree of CO gas excitation on the efficiency of star forming activity, (3) using the large velocity gradient (LVG) approximation to model the observed data, the results show that the mean value of the CO-to-H2 conversion factor $X$ for the SLUGS sample is lower by a factor of 10 compared to the conventional value derived for the Galaxy, assuming that the abundance of CO relative to H2 is 10^-4, (4) due to a contribution to the SCUBA brightness measurement by 12CO(3-2) emission, the average dust mass is reduced by 25-38%, and the mean global gas-to-dust mass ratio is reduced from 430 to 360, but is further reduced to 100 when applied to the nuclear regions of the SLUGS galaxies, (5) for a subset of 12 galaxies with H I maps, we derive a mean total face-on surface density of H2+H I of about 42 Mo pc^-2 within ~ 2 kpc of the nucleus, which is intermediate between that in galaxies like our own and those with strong star formation