A Search for Young Stars in the S0 Galaxies of a Super-Group at z=0.37

TL;DR

This study uses UV data to investigate star formation in S0 galaxies within a super-group at z=0.37, finding most have ceased star formation long before cluster assembly, suggesting S0 formation occurs outside dense environments.

Contribution

First UV-based analysis of S0 galaxies in a super-group at z=0.37, providing constraints on the timing of star formation cessation and S0 formation outside cluster environments.

Findings

Most S0 galaxies show no recent star formation.

Star formation in S0s ceased at least 0.1 to 0.7 Gyr ago.

S0 formation likely occurs outside dense cluster environments.

Abstract

We analyze GALEX UV data for a system of four gravitationally-bound groups at z=0.37, SG1120, which is destined to merge into a Coma-mass cluster by z=0, to study how galaxy properties may change during cluster assembly. Of the 38 visually-classified S0 galaxies, with masses ranging from log(M_*)~10-11, we detect only one in the NUV channel, a strongly star-forming S0 that is the brightest UV source with a measured redshift placing it in SG1120. Stacking the undetected S0 galaxies (which generally lie on or near the optical red-sequence of SG1120) still results in no NUV/FUV detection (<2 sigma). Using our limit in the NUV band, we conclude that for a rapidly truncating star formation rate, star formation ceased *at least* ~0.1 to 0.7 Gyr ago, depending on the strength of the starburst prior to truncation. With an exponentially declining star-formation history over a range of time-scales, we rule out recent star-formation over a wide range of ages. We conclude that if S0 formation involves significant star formation, it occurred well before the groups were in this current pre-assembly phase. As such, it seems that S0 formation is even more likely to be predominantly occurring outside of the cluster environment.