Star formation activities of galaxies in the large-scale structures at z = 1.2^*

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: mtanaka@eso.org
² European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
³ Oskar Klein Center, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
⁴ Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
⁵ Department of Astronomy, Universidad de Concepción. Casilla 160-C, Concepción, Chile
⁶ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching bei München, Germany
⁷ University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁸ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
⁹ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA

Received: 10 June 2009
Accepted: 7 September 2009

Abstract

Recent wide-field imaging observations of the X-ray luminous cluster RDCS J1252.9-2927 at uncovered several galaxy groups that appear to be embedded in filamentary structure extending from the cluster core. We make a spectroscopic study of the galaxies in these groups using GMOS on Gemini-South and FORS2 on VLT with the aim of determining if these galaxies are physically associated to the cluster. We find that three groups contain galaxies at the cluster redshift and that they are probably bound to the cluster. This is the first confirmation of filamentary structure as traced by galaxy groups at . We then use several spectral features in the FORS2 spectra to determine the star formation histories of group galaxies. We find a population of relatively red star-forming galaxies in the groups that are absent from the cluster core. While similarly red star forming galaxies can also be found in the field, the average strength of the Hδ line is systematically weaker in group galaxies. Interestingly, these groups at are in an environment in which the on-going build-up of red sequence is happening. The unusual line strengths can be explained by star formation that is heavily obscured by dust. We hypothesize that galaxy-galaxy interactions, which is more efficient in the group environment, is the mechanism that drives these dust obscured star formation. The hypothesis can be tested by obtaining spectral observations in the near-IR, high resolution imaging observations and observations in the mid-IR.