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

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.