Evidence of boosted 13CO/12CO ratio in early-type galaxies in dense environments

Abstract

We present observations of ¹³CO(1–0) in 17 Combined Array for Research in Millimeter Astronomy ATLAS^3D early-type galaxies (ETGs), obtained simultaneously with ¹²CO(1–0) observations. The ¹³CO in six ETGs is sufficiently bright to create images. In these six sources, we do not detect any significant radial gradient in the ¹³CO/¹²CO ratio between the nucleus and the outlying molecular gas. Using the ¹²CO channel maps as 3D masks to stack the ¹³CO 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 ¹³CO/¹²CO ratios, but Virgo cluster and group galaxies preferentially show a ¹³CO/¹²CO ratio about two times larger than field galaxies, although this could also be due to a mass dependence, or the CO spatial extent (R_CO/R_e). ETGs whose gas has a morphologically settled appearance also show boosted ¹³CO/¹²CO 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 ¹³C to ¹²C in the absence of external gas accretion), (ii) much higher pressure being exerted on the mid-plane 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 ¹³CO in dense environments, particularly of spirals, as well as studies of other isotopologues, should be able to distinguish between these hypotheses.