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 r31=L'CO(3-2)/L'CO(1-0) 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 AGN host galaxies from BASS and 37 infrared
luminous galaxies from SLUGS. 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 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 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 (nH > 10$^4$ 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.
