Optical and near-IR spectroscopy of candidate red galaxies in two z~2.5 proto-clusters

TL;DR

This study uses optical and near-IR spectroscopy to confirm the presence of massive, red galaxies in high-redshift proto-clusters, revealing their properties and formation history, and demonstrating the effectiveness of specific galaxy selection techniques.

Contribution

First spectroscopic confirmation of red, passively evolving galaxies in a z>2 proto-cluster, advancing understanding of galaxy evolution in dense environments.

Findings

Confirmed two massive red galaxies in PKS1138-262 at the proto-cluster redshift.

Identified a foreground structure at z=2.6 in MRC0943-242 field.

Validated JHK_s selection effectiveness for galaxies at 2.3<z<3.1.

Abstract

We present a spectroscopic campaign to follow-up red colour-selected candidate massive galaxies in two high redshift proto-clusters surrounding radio galaxies. We observed a total of 57 galaxies in the field of MRC0943-242 (z=2.93) and 33 in the field of PKS1138-262 (z=2.16) with a mix of optical and near-infrared multi-object spectroscopy. We confirm two red galaxies in the field of PKS1138-262 at the redshift of the radio galaxy. Based on an analysis of their spectral energy distributions, and their derived star formation rates from the H-alpha and 24um flux, one object belongs to the class of dust-obscured star-forming red galaxies, while the other is evolved with little ongoing star formation. This result represents the first red and mainly passively evolving galaxy to be confirmed as companion galaxies in a z>2 proto-cluster. Both red galaxies in PKS1138-262 are massive, of the order of 4-6x10^11 M_Sol. They lie along a Colour-Magnitude relation which implies that they formed the bulk of their stellar population around z=4. In the MRC0943-242 field we find no red galaxies at the redshift of the radio galaxy but we do confirm the effectiveness of our JHK_s selection of galaxies at 2.3<z<3.1, finding that 10 out of 18 (56%) of JHK_s-selected galaxies whose redshifts could be measured fall within this redshift range. We also serendipitously identify an interesting foreground structure of 6 galaxies at z=2.6 in the field of MRC0943-242. This may be a proto-cluster itself, but complicates any interpretation of the red sequence build-up in MRC0943-242 until more redshifts can be measured.