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

Abstract

We report the spectroscopic identification of galaxies in the neighborhood of the radio galaxy MRC 0316-257, at a redshift z ~ 3.14. Candidate cluster galaxies were selected from deep V and I images combined with narrowband imaging at the wavelength of redshifted Lyα. Follow-up multislit 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. The first is 0.3 h−150 Mpc from the radio galaxy, is resolved with an intrinsic size of 11.6 ± 1.1 h−150 kpc, and shows Lyα in emission with rest WLyα = 55 ± 14 A. In addition, its extremely blue V - I color might indicate a protogalaxy forming a first generation of stars in a low-dust medium. The second galaxy is 1.3 h−150 Mpc from the radio galaxy, is marginally resolved, and, in addition to Lyα in emission, shows C IV in emission with a broad component that indicates the contribution from an active galactic nucleus. The comoving density of galaxies with V < 23.8 and Lyα flux greater than 10-16 ergs cm-2 s-1 in the vicinity of MRC 0316-257 is ~2.5 × 10-3 h350 Mpc-3, significantly higher than the expected background density of field galaxies with similar properties, and may indicate a rich cluster or protocluster environment. These observations indicate that the environment of high-redshift radio galaxies may provide significant numbers of galaxies with which to study the early stages of cluster formation and galaxy evolution.