Properties of Lyα emitters around the radio galaxy MRC 0316-257

Observations of the radio galaxy MRC 0316 - 257 at z = 3.13 and the surrounding field are presented. Using narrow- and broad-band imaging obtained with the VLT, 77 candidate Lyalpha emitters with a rest-frame equivalent width of > 15 Angstrom were selected in a similar to 7' x 7' field around the radio galaxy. Spectroscopy of 40 candidate emitters resulted in the discovery of 33 emission line galaxies of which 31 are Lyalpha emitters with redshifts similar to that of the radio galaxy, while the remaining two galaxies turned out to be [O II] emitters. The Lyalpha profiles have widths ( FWHM) in the range of 120 - 800 km s(-1), with a median of 260 km s(-1). Where the signal-to-noise was large enough, the Lyalpha profiles were found to be asymmetric, with apparent absorption troughs blueward of the profile peaks, indicative of absorption along the line of sight of an H. mass of at least 2 x 10(2)- 5 x 10(4) M-.. Besides that of the radio galaxy and one of the emitters that is a QSO, the continuum of the emitters is faint, with luminosities ranging from 1.3 L* to < 0.03 L*. The colors of the confirmed emitters are, on average, very blue. The median UV continuum slope is beta = - 1.76, bluer than the average slope of LBGs with Ly alpha emission (beta similar to - 1.09). A large fraction of the confirmed emitters (similar to 2/3) have colors consistent with that of dust-free starburst galaxies. Observations with the Advanced Camera for Surveys on the Hubble Space Telescope show that the emitters that were detected in the ACS image have a range of different morphologies. Four Ly alpha emitters (similar to 25%) were unresolved with upper limits on their half light radii of r(h) < 0.6 - 1.3 kpc, three objects (similar to 19%) show multiple clumps of emission, as does the radio galaxy, and the rest (similar to 56%) are single, resolved objects with r(h) < 1.5 kpc. A comparison with the sizes of Lyman break galaxies at z similar to 3 suggests that the Ly alpha emitters are on average smaller than LBGs. The average star formation rate of the Ly alpha emitters is 2.6 M-. yr(-1) as measured by the Ly alpha emission line or < 3.9 M-. yr(-1) as measured by the UV continuum. The properties of the Lyalpha galaxies ( faint, blue and small) are consistent with young star forming galaxies which are still nearly dust free. The volume density of Lyalpha emitting galaxies in the field around MRC 0316 - 257 is a factor of 3.3+(-0.4)+(0.5) larger compared with the density of field Lyalpha emitters at that redshift. The velocity distribution of the spectroscopically confirmed emitters has a dispersion of 640 km s(-1), corresponding to a FWHM of 1510 km s(-1), which is substantially smaller than the width of the narrow- band filter ( FWHM similar to 3500 km s(-1)). The peak of the velocity distribution is located within 200 km s(-1) of the redshift of the radio galaxy. We conclude that the confirmed Lyalpha emitters are members of a protocluster of galaxies at z similar to 3.13. The size of the protocluster is unconstrained and is larger than 3.3 x 3.3 Mpc(2). The mass of this structure is estimated to be > 3-6 x 10(14) M-. and could be the progenitor of a cluster of galaxies similar to e. g. the Virgo cluster.