A search for clusters at high redshift

III. Candidate Hα emitters and EROs in the PKS 1138–262 proto-cluster at z = 2.16

¹ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: kurk@arcetri.astro.it
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 21 January 2003
Accepted: 19 February 2004

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 , located in a dense environment as indicated by optical, X-ray and radio observations. We had already found an over-density of Lyα emitting galaxies in this field, consistent with a proto-cluster structure associated with the radio galaxy. In addition, we find 40 candidate Hα emitters that have nominal rest frame equivalent width >25 Å within 1.8 Mpc and 2000 km s^-1 of the radio galaxy. Furthermore, we find 44 objects with . 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 Hα emitters and extremely red objects increases towards the radio galaxy, indicating a physical association. From comparisons with other K band, ERO, Hα and Lyα surveys, we conclude that PKS 1138-262 is located in a density peak which will evolve into a cluster of galaxies.