A galaxy group candidate at z~3.7 in the COSMOS field

TL;DR

This paper reports the discovery of a galaxy group candidate at redshift 3.7 in the COSMOS field, characterized by high galaxy density, significant star formation, and potential early stages of galaxy evolution in a dense environment.

Contribution

It presents the identification and detailed analysis of a high-redshift galaxy group candidate with multi-wavelength data, suggesting it is in a maturing phase of galaxy evolution, which is a novel observation at this epoch.

Findings

Contains ten candidate members with high sky density.

Total star formation rate approximately 900 solar masses per year.

Estimated halo mass around 10^13 solar masses.

Abstract

We report a galaxy group candidate HPC1001 at $z\approx3.7$ in the COSMOS field. This structure was selected as a high galaxy overdensity at $z>3$ in the COSMOS2020 catalog. It contains ten candidate members, of which eight are assembled in a $10''\times10''$ area with the highest sky density among known protoclusters and groups at $z>3$. Four out of ten sources were also detected at 1.2$~$mm with Atacama Large Millimeter Array continuum observations. Photometric redshifts, measured by four independent methods, fall within a narrow range of $3.5<z<3.9$ and with a weighted average of $z=3.65\pm0.07$. The integrated far-IR-to-radio spectral energy distribution yields a total UV and IR star formation rate ${\rm SFR}\approx 900~M_{\odot}~yr^{-1}$. We also estimated a halo mass of $\sim10^{13}~M_\odot$ for the structure, which at this redshift is consistent with potential cold gas inflow. Remarkably, the most massive member has a specific star formation rate and dust to stellar mass ratio of $M_{\rm dust}/M_{*}$ that are both significantly lower than that of star-forming galaxies at this redshift, suggesting that HPC1001 could be a $z\approx3.7$ galaxy group in maturing phase. If confirmed, this would be the earliest structure in maturing phase to date, and an ideal laboratory to study the formation of the earliest quiescent galaxies as well as cold gas accretion in dense environments.