The CO(3-2)/CO(1-0) Luminosity Line Ratio in Nearby Star-forming Galaxies and Active Galactic Nuclei from xCOLD GASS, BASS, and SLUGS

We study the ${r}_{31}={L}_{\mathrm{CO}(3\mbox{--}2)}^{{\prime} }/{L}_{\mathrm{CO}(1\mbox{--}0)}^{{\prime} }$ luminosity line ratio in a sample of nearby (z < 0.05) galaxies: 25 star-forming galaxies (SFGs) from the xCOLD GASS survey, 36 hard X-ray-selected active galactic nucleus (AGN) host galaxies from the BAT AGN Spectroscopic Survey, and 37 infrared-luminous galaxies from the SCUBA Local Universe Galaxy Survey. We find a trend for r₃₁ to increase with star formation efficiency (SFE). We model r₃₁ using the UCL-PDR code and find that the gas density is the main parameter responsible for the variation of r₃₁, while the interstellar radiation field and cosmic-ray ionization rate play only a minor role. We interpret these results to indicate a relation between SFE and gas density. We do not find a difference in the r₃₁ value of SFGs and AGN host galaxies, when the galaxies are matched in SSFR (<r₃₁> = 0.52 ± 0.04 for SFGs and <r₃₁> = 0.53 ± 0.06 for AGN hosts). According to the results of the UCL-PDR models, the X-rays can contribute to the enhancement of the CO line ratio, but only for strong X-ray fluxes and for high gas density (n_H > 10⁴ cm^-3). We find a mild tightening of the Kennicutt-Schmidt relation when we use the molecular gas mass surface density traced by CO(3-2) (Pearson correlation coefficient R = 0.83), instead of the molecular gas mass surface density traced by CO(1-0) (R = 0.78), but the increase in correlation is not statistically significant (p-value = 0.06). This suggests that the CO(3-2) line can be reliably used to study the relation between SFR and molecular gas for normal SFGs at high redshift and to compare it with studies of low-redshift galaxies, as is common practice.