Properties of Lyαemitters 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 Lyαemitters with a rest-frame equivalent width of >15 Å were selected in a ∼7´$\,\times\,7$´ field around the radio galaxy. Spectroscopy of 40 candidate emitters resulted in the discovery of 33 emission line galaxies of which 31 are Lyαemitters with redshifts similar to that of the radio galaxy, while the remaining two galaxies turned out to be [$\ion{O}{ii}$] emitters. The Lyαprofiles 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 Lyαprofiles were found to be asymmetric, with apparent absorption troughs blueward of the profile peaks, indicative of absorption along the line of sight of an $\ion{H}{i}$mass of at least $2 \times 10^2 {-} 5 \times 10^4$M$_{\odot}$. 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 Lyαemission ($\beta \sim -1.09$). A large fraction of the confirmed emitters (∼2/3) have colors consistent with that of dust-free starburst galaxies. Observations with the Advanced Camera for Surveys on the Hubble Space Telescopeshow that the emitters that were detected in the ACS image have a range of different morphologies. Four Lyαemitters (∼25%) were unresolved with upper limits on their half light radii of $r_{\rm h} < 0.6-1.3$kpc, three objects (∼19%) show multiple clumps of emission, as does the radio galaxy, and the rest (∼56%) are single, resolved objects with $r_{\rm h} < 1.5$kpc. A comparison with the sizes of Lyman break galaxies at $z \sim 3$suggests that the Lyαemitters are on average smaller than LBGs. The average star formation rate of the Lyαemitters is 2.6 M$_{\odot}\;{\rm yr}^{-1}$as measured by the Lyαemission line or <3.9 M$_{\odot}\;{\rm yr}^{-1}$as measured by the UV continuum. The properties of the Lyαgalaxies (faint, blue and small) are consistent with young star forming galaxies which are still nearly dust free.
The volume density of Lyαemitting galaxies in the field around MRC 0316–257 is a factor of $3.3^{+0.5}_{-0.4}$larger compared with the density of field Lyαemitters at that redshift. The velocity distribution of the spectroscopically confirmed emitters has a dispersion of 640 km s-1, corresponding to a FWHMof 1510 km s-1, which is substantially smaller than the width of the narrow-band filter (${\it FWHM} \sim 3500$km s-1). The peak of the velocity distribution is located within 200 km s-1of the redshift of the radio galaxy. We conclude that the confirmed Lyαemitters are members of a protocluster of galaxies at $z \sim 3.13$. The size of the protocluster is unconstrained and is larger than $3.3 \times 3.3$Mpc2. The mass of this structure is estimated to be >$3{-}6 \times 10^{14}$M$_{\odot}$and could be the progenitor of a cluster of galaxies similar to e.g. the Virgo cluster.