Star formation activity in a young galaxy cluster at z=0.866

TL;DR

This study investigates star formation activity in a high-redshift galaxy cluster at z=0.866, finding a weak SFR-density relation and a mild dependence of SFR on stellar mass, with implications for cluster assembly and galaxy infall.

Contribution

It provides the first detailed analysis of star formation rates and their relation to environment in a high-redshift galaxy cluster using multi-method approaches.

Findings

Weak SFR-density relation observed.

More massive galaxies have higher SFR but lower SSFR.

No clear correlation between SSFR and cluster-centric distance.

Abstract

The galaxy cluster RXJ1257$+$4738 at $z=0.866$ is one of the highest redshift clusters with a richness of multi-wavelength data, and thus a good target to study the star formation-density relation at early epochs. Using a sample of spectroscopically-confirmed cluster members, we derive the star formation rates of our galaxies using two methods, (I) the relation between SFR and total infrared luminosity extrapolated from the observed \textit{Spitzer} MIPS 24$\mu$m imaging data, and (II) spectral energy distribution (SED) fitting using the MAGPHYS code, including eight different bands. We show that, for this cluster, the SFR-density relation is very weak and seems to be dominated by the two central galaxies and the SFR presents a mild dependence on stellar mass, with more massive galaxies having higher SFR. However, the specific SFR (SSFR) decreases with stellar mass, meaning that more massive galaxies are forming less stars per unit of mass, and thus suggesting that the increase in star-forming members is driven by cluster assembly and infall. If the environment is somehow driving the SF, one would expect a relation between the SSFR and the cluster centric distance, but that is not the case. A possible scenario to explain this lack of correlation is the contamination by infalling galaxies in the inner part of the cluster which may be on their initial pass through the cluster center. As these galaxies have higher SFRs for their stellar mass, they enhance the mean SSFR in the center of the cluster.