Evidence of boosted CO/CO ratio in early-type galaxies in dense environments

We present observations of CO(1–0) in 17 Combined Array for Research in Millimeter Astronomy (CARMA) ATLAS early-type galaxies (ETGs), obtained simultaneously with CO(1–0) observations. The CO in six ETGs is sufficiently bright to create images. In these 6 sources, we do not detect any significant radial gradient in the CO/CO ratio between the nucleus and the outlying molecular gas. Using the CO channel maps as 3D masks to stack the CO emission, we are able to detect 15/17 galaxies to > 3σ (and 12/17 to at least 5σ) significance in a spatially integrated manner. Overall, ETGs show a wide distribution of CO/CO ratios, but Virgo cluster and group galaxies preferentially show a CO/CO ratio about 2 times larger than field galaxies, although this could also be due to a mass dependence, or the CO spatial extent (RCO/Re). ETGs whose gas has a morphologically-settled appearance also show boosted CO/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 C to 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 CO in dense environments, particularly of spirals, as well as studies of other isotopologues, should be able to distinguish between these hypotheses.