Clustering around the Radio Galaxy MRC 0316−257 at [ITAL]z[/ITAL] = 3.14

We report here the spectroscopic identification of galaxies in the neighborhood of the radio-galaxy MRC0316-257, at a redshift z~3.14. Candidate cluster galaxies were selected from deep V and I images combined with narrow band imaging at the wavelength of redshifted Lyα. Follow-up multi-slit spectroscopy has allowed confirmation of the redshift of the radio-galaxy, z=3.1420±0.0020, and identification of two associated galaxies at redshifts z=3.1378±0.0028 and z=3.1351±0.0028 respectively. The first galaxy is 0.3 h₅₀⁻¹ Mpc from the radio-galaxy, is resolved with an intrinsic size 11.6± h₅₀⁻¹ kpc, and shows Lyα in emission with rest WLyα=55±14. In addition, its extremely blue $V-I$ color might possibly indicate a proto-galaxy forming a first generation of stars in a low dust medium. The second galaxy is 1.3 h₅₀⁻¹ Mpc away from the radio-galaxy, is marginally resolved and, in addition to Lyα in emission, shows CIV in emission with a broad component indicating the contribution from an AGN. The comoving density of galaxies with $V<23.8$ and a Lyα flux >10⁻¹⁶ ergcm⁻²sec⁻¹ in the vicinity of MRC0316-257 is ~2.5×10⁻³h³₅₀Mpc⁻³, significantly higher than the expected background density of field galaxies with similar properties, and might indicate a rich cluster or proto-cluster environment.