The CO(3-2)/CO(1-0) luminosity line ratio in nearby star-forming galaxies and AGN 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 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.