Star Formation Activities of Galaxies in the Large-Scale Structures at z=1.2

TL;DR

This study confirms the presence of filamentary galaxy groups at z=1.2 associated with a cluster, revealing dust-obscured star formation and environmental effects on galaxy evolution in large-scale structures.

Contribution

First spectroscopic confirmation of filamentary structures at z>1 and analysis of star formation histories in group galaxies within these structures.

Findings

Galaxy groups at z=1.2 are physically associated with the cluster.

Group galaxies exhibit dust-obscured star formation with weaker spectral line strengths.

Environment influences star formation and galaxy evolution at high redshift.

Abstract

Recent wide-field imaging observations of the X-ray luminous cluster RDCSJ1252.9-2927 at z=1.24 uncovered several galaxy groups that appear to be embedded in filamentary structure extending from the cluster core. We make a spectroscopic study of the galaxies in these groups using GMOS on Gemini-South and FORS2 on VLT with the aim of determining if these galaxies are physically associated to the cluster. We find that three groups contain galaxies at the cluster redshift and that they are probably bound to the cluster. This is the first confirmation of filamentary structure as traced by galaxy groups at z>1. We then use several spectral features in the FORS2 spectra to determine the star formation histories of group galaxies. We find a population of relatively red star-forming galaxies in the groups that are absent from the cluster core. While similarly red star forming galaxies can also be found in the field, the average strength of the hd line is systematically weaker in group galaxies. Interestingly, these groups at z=1.2 are in an environment in which the on-going build-up of red sequence is happening. The unusual line strengths can be explained by star formation that is heavily obscured by dust. We hypothesize that galaxy-galaxy interactions, which is more efficient in the group environment, is the mechanism that drives these dust obscured star formation. The hypothesis can be tested by obtaining spectral observations in the near-IR, high resolution imaging observations and observations in the mid-IR.