Evidence of boosted 13CO/12CO ratio in early-type galaxies in dense
  environments
  We present observations of $^{13}$CO(1-0) in 17 Combined Array for Research
in Millimeter Astronomy (CARMA) Atlas3D early-type galaxies (ETGs), obtained
simultaneously with $^{12}$CO(1-0) observations. The $^{13}$CO in six ETGs is
sufficiently bright to create images. In these 6 sources, we do not detect any
significant radial gradient in the $^{13}$CO/$^{12}$CO ratio between the
nucleus and the outlying molecular gas. Using the $^{12}$CO channel maps as 3D
masks to stack the $^{13}$CO emission, we are able to detect 15/17 galaxies to
$>3\sigma$ (and 12/17 to at least 5$\sigma$) significance in a spatially
integrated manner. Overall, ETGs show a wide distribution of
$^{13}$CO/$^{12}$CO ratios, but Virgo cluster and group galaxies preferentially
show a $^{13}$CO/$^{12}$CO ratio about 2 times larger than field galaxies,
although this could also be due to a mass dependence, or the CO spatial extent
($R_{\rm CO}/R_{\rm e}$). ETGs whose gas has a morphologically-settled
appearance also show boosted $^{13}$CO/$^{12}$CO ratios. We hypothesize that
this variation could be caused by (i) the extra enrichment of gas from
molecular reprocessing occurring in low-mass stars (boosting the abundance of
$^{13}$C to $^{12}$C in the absence of external gas accretion), (ii) much
higher pressure being exerted on the midplane gas (by the intracluster medium)
in the cluster environment than in isolated galaxies, or (iii) all but the
densest molecular gas clumps being stripped as the galaxies fall into the
cluster. Further observations of $^{13}$CO in dense environments, particularly
of spirals, as well as studies of other isotopologues, should be able to
distinguish between these hypotheses.
