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Issue A&A
Volume 431, Number 3, March I 2005
Page(s) 793 - 812
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20042038



A&A 431, 793-812 (2005)
DOI: 10.1051/0004-6361:20042038

Properties of Ly $\alpha$ emitters around the radio galaxy MRC 0316-257 ,

B. P. Venemans1, H. J. A. Röttgering1, G. K. Miley1, J. D. Kurk2, C. De Breuck3, R. A. Overzier1, W. J. M. van Breugel4, C. L. Carilli5, H. Ford6, T. Heckman6, L. Pentericci7 and P. McCarthy8

1  Sterrewacht Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
    e-mail: venemans@strw.leidenuniv.nl
2  INAF, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125, Firenze, Italy
3  European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany
4  Lawrence Livermore National Laboratory, PO Box 808, Livermore CA, 94550, USA
5  NRAO, PO Box 0, Socorro NM, 87801, USA
6  Dept. of Physics & Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore MD, 21218-2686, USA
7  Dipartimento di Fisica, Università degli studi Roma Tre, via della Vasca Navale 84, Roma, 00146, Italy
8  The Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena CA, 91101, USA

(Received 21 September 2004 / Accepted 21 October 2004 )

Abstract
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 Ly $\alpha$ emitters with a rest-frame equivalent width of >15 Å were selected in a $\sim $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 Ly $\alpha$ emitters with redshifts similar to that of the radio galaxy, while the remaining two galaxies turned out to be [ $\ion{O}{ii}$] emitters. The Ly $\alpha$ 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 Ly $\alpha$ 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 $\ion{H}{i}$ 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 Ly $\alpha$ emission ( $\beta \sim -1.09$). A large fraction of the confirmed emitters ( $\sim $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 ( $\sim $25%) were unresolved with upper limits on their half light radii of $r_{\rm h} < 0.6-1.3$  kpc, three objects ( $\sim $19%) show multiple clumps of emission, as does the radio galaxy, and the rest ( $\sim $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 Ly $\alpha$ emitters are on average smaller than LBGs. The average star formation rate of the Ly $\alpha$ emitters is 2.6 M $_{\odot}\;{\rm yr}^{-1}$ as measured by the Ly $\alpha$ emission line or <3.9 M $_{\odot}\;{\rm yr}^{-1}$ as measured by the UV continuum. The properties of the Ly $\alpha$ galaxies (faint, blue and small) are consistent with young star forming galaxies which are still nearly dust free.

The volume density of Ly $\alpha$ emitting 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 Ly $\alpha$ 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 ( ${\it FWHM} \sim 3500$ 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 Ly $\alpha$ emitters are members of a protocluster of galaxies at $z \sim 3.13$. The size of the protocluster is unconstrained and is larger than $3.3 \times 3.3$  Mpc 2. 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.


Key words: galaxies: active -- galaxies: high-redshift -- galaxies: evolution -- galaxies: clusters: general -- cosmology: observations -- cosmology: early Universe

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