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Issue A&A
Volume 428, Number 3, December IV 2004
Page(s) 817 - 821
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20041819



A&A 428, 817-821 (2004)
DOI: 10.1051/0004-6361:20041819

A search for clusters at high redshift

IV. Spectroscopy of H $\alpha$ emitters in a proto-cluster at z = 2.16
J. D. Kurk1, L. Pentericci2, R. A. Overzier1, H. J. A. Röttgering1 and G. K. Miley1

1  Sterrewacht Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
    e-mail: kurk@arcetri.astro.it
2  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117, Heidelberg, Germany

(Received 10 August 2004 / Accepted 7 October 2004 )

Abstract
Radio galaxy PKS 1138-262 is a massive galaxy at z = 2.16 , located in a dense environment. We have found an overdensity of Ly $\alpha$ emitting galaxies in this field, consistent with a proto-cluster structure associated with the radio galaxy. Recently, we have discovered forty candidate H $\alpha$ emitters by their excess near infrared narrow band flux. Here, we present infrared spectroscopy of nine of the brightest candidate H $\alpha$ emitters. All these candidates show an emission line at the expected wavelength. The identification of three of these lines with H $\alpha$ is confirmed by accompanying [ $\ion{N}{ii}$] emission. The spectra of the other candidates are consistent with H $\alpha$ emission at  $z \sim 2.15$, one being a QSO as indicated by the broadness of its emission line. The velocity dispersion of the emitters (360 km s -1) is significantly smaller than that of the narrow band filter used for their selection (1600 km s -1). We therefore conclude that the emitters are associated with the radio galaxy. The star formation rates (SFRs) deduced from the H $\alpha$ flux are in the range 6-44  $M_\odot$ yr -1 and the SFR density observed is 5-10 times higher than in the HDF-N at z = 2.23 . The properties of the narrow emission lines indicate that the emitters are powered by star formation and contain very young ( <100 Myr) stellar populations with moderately high metallicities.


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

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© ESO 2004


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