Clustering around the radio-galaxy MRC0316-257 at z=3.14

We report here the spectroscopic identification of galaxies in the neighborhood of the radio-galaxy MRC0316-257, at a redshift $z\sim 3.14$. Candidate cluster galaxies were selected from deep V and I images combined with narrow band imaging at the wavelength of redshifted Ly$\alpha$. Follow-up multi-slit spectroscopy has allowed confirmation of the redshift of the radio-galaxy, $z=3.1420\pm 0.0020$, and identification of two associated galaxies at redshifts $z=3.1378\pm 0.0028$ and $z=3.1351\pm 0.0028$ respectively. The first galaxy is 0.3 $h_{50}^{-1}$ Mpc from the radio-galaxy, is resolved with an intrinsic size $11.6\pm h_{50}^{-1}$ kpc, and shows $Ly\alpha$ in emission with rest $W_{Ly\alpha }=55\pm 14$\AA. 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 $h_{50}^{-1}$ Mpc away from the radio-galaxy, is marginally resolved and, in addition to Ly$\alpha$ 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 $Ly\alpha$ flux $>{10}^{-16}$ ergcm${}^{-2}$sec${}^{-1}$ in the vicinity of MRC0316-257 is $\sim 2.5\times {10}^{-3}{h}_{50}^{3}Mp{c}^{-3}$, significantly higher than the expected background density of field galaxies with similar properties, and might indicate a rich cluster or proto-cluster environment.