Cold gas in the inner regions of intermediate redshift clusters

TL;DR

This study presents CO observations of LIRGs in intermediate redshift clusters, revealing environmental effects on cold gas content and its relation to star formation, with evidence of gas depletion in cluster environments.

Contribution

First CO measurements of LIRGs within intermediate redshift clusters, demonstrating environmental depletion of cold gas and extending local galaxy relations to higher IR luminosities.

Findings

Lower frequency of high L'CO galaxies in clusters compared to the field.

Star formation activity correlates with cold gas amount, not galaxy mass.

Evidence of decreased CO towards cluster centers indicating environmental impact.

Abstract

Determining gas content and star formation rate has known remarkable progress in field galaxies, but has been much less investigated in galaxies inside clusters. We present the first CO observations of luminous infrared galaxies (LIRGs) inside the virial radii of two intermediate redshift clusters, CL1416+4446 (z=0.397) and CL0926+1242 (z=0.489). We detect three galaxies at high significance (5 to 10 sigma), and provide robust estimates of their CO luminosities, L'CO. In order to put our results into a general context, we revisit the relation between cold and hot gas and stellar mass in nearby field and cluster galaxies. We find evidence that at fixed LIR (or fixed stellar mass), the frequency of high L'CO galaxies is lower in clusters than in the field, suggesting environmental depletion of the reservoir of cold gas. The level of star formation activity in a galaxy is primarily linked to the amount of cold gas, rather than to the galaxy mass or the lookback time. In clusters, just as in the field, the conversion between gas and stars seems universal. The relation between LIR and L'CO for distant cluster galaxies extends the relation of nearby galaxies to higher IR luminosities. Nevertheless, the intermediate redshift galaxies fall well within the dispersion of the trend defined by local systems. Considering that L'CO is generally derived from the CO(1-0) line and sensitive to the vast majority of the molecular gas in the cold interstellar medium of galaxies, but less to the part which will actually be used to form stars, we suggest that molecular gas can be stripped before the star formation rate is affected. Combining the sample of Geach et al. (2009, 2011) and ours, we find evidence for a decrease in CO towards the cluster centers. This is the first hint of an environmental impact on cold gas at intermediate redshift.