A large-scale galaxy structure at z = 2.02 associated with the radio galaxy MRC 0156-252^⋆,⋆⋆

¹ INAF – Osservatorio di Roma, via Frascati 33, 00040 Monteporzio, Italy
e-mail: audrey.galametz@oa-roma.inaf.it
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁴ University of Nottingham, School of Physics and Astronomy, Nottingham NG7 2RD, UK
⁵ Max-Planck-Institut für extraterrestrische Physik (MPE), Postfach 1312, Giessenbachstr., 85741 Garching, Germany
⁶ Observatório Nacional, Rua José Cristino 77, CEP 20921-400 São Cristóvão, Rio de Janeiro, RJ, Brazil
⁷ Department of Astronomy, The University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
⁸ Department of Astrophysics, California Institute of Technology, MS 249-17, Pasadena, CA 91125, USA
⁹ CSIRO Astronomy & Space Science, PO Box 76, NSW 1710 Epping, Australia

Received: 23 July 2013
Accepted: 25 September 2013

Abstract

We present the spectroscopic confirmation of a structure of galaxies surrounding the radio galaxy MRC 0156-252 at z = 2.02. The structure was initially discovered as an overdensity of both near-infrared selected z > 1.6 and mid-infrared selected z > 1.2 galaxy candidates. We used the VLT/FORS2 multi-object spectrograph to target ~80 high-redshift galaxy candidates, and obtain robust spectroscopic redshifts for more than half the targets. The majority of the confirmed sources are star-forming galaxies at z > 1.5. In addition to the radio galaxy, two of its close-by companions (<6″) also show AGN signatures. Ten sources, including the radio galaxy, lie within | z − 2.020 | <0.015 (i.e., velocity offsets <1500 km s^-1) and within projected 2 Mpc comoving of the radio galaxy. Additional evidence suggests not only that the galaxy structure associated with MRC 0156-252 is a forming galaxy cluster but also that this structure is most probably embedded in a larger-scale structure.