A Search for clusters at high redshift. I. Candidate Lyalpha emitters near 1138-262 at z=2.2

Radio, optical and X-ray observations of the powerful radio galaxy PKS 1138-262 at z=2.156 have suggested that this galaxy is a massive galaxy in the center of a forming cluster. We have imaged 1138-262 and the surrounding 38 square arcminute field with the Very Large Telescope in a broad band and a narrow band encompassing the redshifted Lyalpha emission. We detect 50 objects with rest equivalent width larger than 20 Ä and a luminous, highly extended Lyalpha halo around 1138-262. If the radio galaxy is at the center of a forming cluster, as observations at other wavelengths suggest, these objects are candidate Lyalpha emitting cluster galaxies. Based on observations carried out at the European Southern Observatory, Paranal, Chile, programme P63.O-0477(A).