GMASS ultradeep spectroscopy of galaxies at z ~ 2 ^*,**

V. Witnessing the assembly at z = 1.6 of a galaxy cluster

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: kurk@mpia.de
² INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
⁴ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ CEA, Laboratoire AIM - CNRS - Université Paris Diderot, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁷ NOAO-Tucson, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁸ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA
⁹ INAF - Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
¹⁰ Università di Padova, Dipartimento di Astronomia, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
¹¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching bei München, Germany

Received: 14 April 2008
Accepted: 10 June 2009

Abstract

Context. Clusters of galaxies represent important laboratories for studying galaxy evolution and formation. Well established and relaxed clusters are known below , as well as, clusters in formation found close to radio galaxies at , but the in-between redshift range, during which clusters are expected to undergo significant changes, is almost unexplored.

Aims. By studying a galaxy overdensity in redshift and angular distribution at , uncovered in the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS), we provide insight into the evolution of cluster galaxies at high redshift.

Methods. We present a study of the significance of the galaxy overdensity at , Cl 0332-2742, its velocity dispersion, and X-ray emission. We identify the colour bimodality of the cluster members and compare the properties of members of Cl 0332-2742 with galaxies outside the overdensity.

Results. From the redshifts of the 42 overdensity members, we measure a velocity dispersion of 500 km s^-1. We conservatively estimate the overdensity in redshift space for the spike at in the GMASS field to be . A map of the surface density of galaxies at in the GMASS field shows that its structure is irregular with several filaments and local overdensities. The differences in the physical properties of Cl 0332-2742 member and field galaxies agree with the latest hierarchical galaxy formation models: for overdensity members, the star formation rate (), and specific , is approximately 50% lower than for the field galaxies; overdensity galaxies are twice the age, on average, of field galaxies; and there is a higher proportion of both massive (), and early-type galaxies, inside Cl 0332-2742 than in the field. Among the 42 members, seven have spectra consistent with being passively evolving, massive galaxies. These are all located within an area where the surface density of galaxies is highest. In a colour–magnitude diagram, the photometric data of these early-type galaxies are in close agreement with a theoretical red sequence of a galaxy cluster at redshift , which formed most of its stars in a short burst of star formation at .

Conclusions. We conclude that the redshift spike at in the GMASS field represents a sheet-like structure in the cosmic web, and that the area with the highest surface density within this structure, containing already seven passively evolving galaxies, will evolve into a cluster of galaxies at a later time.