Molecular gas properties of star-forming brightest group galaxies at $z \sim 0.3$

TL;DR

This study investigates molecular gas content and star formation in brightest group galaxies at z~0.3, revealing varied gas reservoirs and implications for galaxy evolution and environmental effects.

Contribution

First targeted CO observations of BGGs at intermediate redshift, linking molecular gas properties with star formation and environmental influences.

Findings

One BGG shows significant molecular gas mass (~3x10^10 M_sun)

Two BGGs have undetected CO emission, with upper limits on gas mass

Gas depletion timescales are estimated to be less than 0.5-1.5 Gyr

Abstract

Recent efforts to characterise the molecular gas content of brightest cluster galaxies (BCGs) at intermediate redshift have revealed a sub-population of gas-rich systems, whose star formation activity is likely influenced by environmental processing. In this study, we aim to investigate the molecular gas reservoirs and star formation fuelling of central galaxies in groups, also known as brightest group galaxies (BGGs), at intermediate redshifts. We present targeted carbon monoxide (CO) line observations of three BGGs in the COSMOS field at $z \sim 0.3$, obtained with the IRAM 30m telescope. The galaxies exhibit disturbed morphologies, extended blue substructures, and interaction signatures. Furthermore, they exhibit significant star formation rates derived from multiwavelength diagnostics. We detect CO(1$\rightarrow$0) emission in one system, revealing a substantial molecular gas mass of $M_{H_2} \sim 3 \times 10^{10}$ M$_\odot$, while for the other two BGGs, CO emission lines remain undetected, yielding stringent upper limits of $M_{H_2} \lesssim 10^{10}$ M$_\odot$. By combining molecular gas constraints with fiducial star formation rates derived from total infrared emission, we infer gas depletion timescales in the range of $\lesssim 0.5-1.5$ Gyr. These results may indicate that, despite their active star formation and interaction signatures, some BGGs could already experience efficient gas exhaustion or suppressed gas replenishment, suggesting that gas depletion precedes star formation quenching. Our findings hint that environmental processes in galaxy groups could strongly regulate the availability of cold gas and drive rapid evolutionary phases in central galaxies, possibly bridging the gap between gas-rich BCGs and passively evolving systems.