A search for clusters at high redshift - III. Candidate H-alpha emitters and EROs in the PKS 1138-262 proto-cluster at z=2.16

TL;DR

This study uses deep multi-wavelength imaging to identify H-alpha emitters and EROs around the high-redshift radio galaxy PKS 1138-262, providing evidence of a proto-cluster environment at z=2.16.

Contribution

It reports the discovery of candidate H-alpha emitters and EROs in the proto-cluster, highlighting their increased density near the radio galaxy, and suggests this region will evolve into a galaxy cluster.

Findings

40 candidate H-alpha emitters within 1.8 Mpc of the radio galaxy.

44 extremely red objects (EROs) detected, twice the number in blank fields.

Density of emitters and EROs increases towards the radio galaxy.

Abstract

In this paper we present deep VLT multi wavelength imaging observations of the field around the radio galaxy PKS 1138-262 aimed at detecting and studying a (potential) proto-cluster centered at this radio source. PKS 1138-262 is a massive galaxy at z=2.16, located in a dense environment as indicated by optical, X-ray and radio observations. We had already found an over-density of Ly-alpha emitting galaxies in this field, consistent with a proto-cluster structure associated with the radio galaxy. In addition, we find 40 candidate H-alpha emitters that have nominal rest frame equivalent width > 25 Angstrom within 1.8 Mpc and 2000 km/s of the radio galaxy. Furthermore, we find 44 objects with I - K > 4.3. This number of extremely red objects (EROs) is about twice the number found in blank field ERO surveys, suggesting that some EROs in this field are part of the proto-cluster. The density of H-alpha emitters and extremely red objects increases towards the radio galaxy, indicating a physical association. From comparisons with other K band, ERO, H-alpha and Ly-alpha surveys, we conclude that PKS 1138-262 is located in a density peak which will evolve into a cluster of galaxies.