A search for clusters at high redshift

In this paper we present deep VLT multi wavelength imaging observations of the field around the radio galaxy PKS 1138-262aimed at detecting and studying a (potential) proto-cluster centered at this radio source. PKS 1138-262is 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 Lyαemitting galaxies in this field, consistent with a proto-cluster structure associated with the radio galaxy. In addition, we find 40 candidate Hαemitters that have nominal rest frame equivalent width >25 Å within 1.8 Mpc and 2000 km s-1of 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 Hαemitters and extremely red objects increases towards the radio galaxy, indicating a physical association. From comparisons with other Kband, ERO, Hαand Lyαsurveys, we conclude that PKS 1138-262is located in a density peak which will evolve into a cluster of galaxies.