Molecular gas in two companion cluster galaxies at z=1.2

TL;DR

This study reports the first CO detection of two star-forming galaxies in a z=1.2 galaxy cluster, revealing their molecular gas properties and suggesting limited gas refueling compared to field galaxies.

Contribution

First CO(2-1) detection of two cluster galaxies at z=1.2 with no AGN, providing insights into their molecular gas and star formation in a cluster environment.

Findings

Molecular gas mass ~2.2×10^{10} M_sun

Depletion time ~390 million years

Gas-to-stellar mass ratio ~0.17

Abstract

We study the molecular gas properties of two star-forming galaxies separated by 6 kpc in the projected space and belonging to a galaxy cluster selected from the Irac Shallow Cluster Survey, at a redshift $z=1.2$, i.e., $\sim2$ Gyr after the cosmic star formation density peak. This work describes the first CO detection from $1<z<1.4$ star forming cluster galaxies with no reported clear evidence of AGN. We exploit observations taken with the NOEMA interferometer at $\sim3$ mm to detect CO(2-1) line emission from the two selected galaxies, unresolved by our observations. Based on the CO(2-1) spectrum we estimate a total molecular gas mass $M({\rm H_2})=(2.2^{+0.5}_{-0.4})\times10^{10}$ $M_\odot$ and dust mass $M_{\rm dust}<4.2\times10^8~M_\odot$ for the two blended sources. The two galaxies have similar stellar masses and a large relative velocity of $\sim$400 km/s estimated from the CO(2-1) line width. These findings tend to privilege a scenario where both sources contribute to the observed CO(2-1). By using the archival Spitzer MIPS flux at 24$\mu$m we estimate an ${\rm SFR(24\mu m)}=(28^{+12}_{-8})~M_\odot$/yr for each of the two galaxies. Assuming that the two sources equally contribute to the observe CO(2-1) our analysis yields a depletion time scale $\tau_{\rm dep}=(3.9^{+1.4}_{-1.8})\times10^8$ yr, and a molecular gas to stellar mass ratio $0.17\pm0.13$ for each of two sources, separately. Our results are in overall agreement with those of other distant cluster galaxies. The two target galaxies have molecular gas mass and depletion time that are marginally compatible with, but smaller than those of main sequence field galaxies, suggesting that the molecular gas has not been refueled enough. Higher resolution and higher frequency observations will enable us to spatially resolve the two sources and possibly distinguish between different gas processing mechanisms.