Passive galaxies as tracers of cluster environments at z ~ 2

¹ Irfu/Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
² Department of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 München, Germany
e-mail: vstrazz@usm.lmu.de
³ INAF−IASF, via Bassini 15, 20133 Milano, Italy
⁴ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ Institute for Astronomy, ETH Zürich, Wolfgang-Pauli-strasse 27, 8093 Zürich, Switzerland
⁶ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Dipartimento di Fisica e Astronomia, Universitá di Bologna, Viale Berti Pichat 6/2, 30127 Bologna, Italy
⁸ Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2a, 0014 Helsinki, Finland
⁹ National Astronomical Observatory of Japan, Subaru Telescope, 650 North Aohoku Place, Hilo, HI 96720, USA
¹⁰ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
¹¹ IRAM−Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin dHères, France
¹² Institut dAstrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 75005 Paris, France

Received: 22 September 2014
Accepted: 17 January 2015

Abstract

Even 10 billion years ago, the cores of the first galaxy clusters are often found to host a characteristic population of massive galaxies with already suppressed star formation. Here we search for distant cluster candidates at z ~ 2 using massive passive galaxies as tracers. With a sample of ~40 spectroscopically confirmed passive galaxies at 1.3 < z < 2.1, we tuned photometric redshifts of several thousand passive sources in the 2 sq. deg COSMOS field. This allowed us to map their density in redshift slices, probing the large-scale structure in the COSMOS field as traced by passive sources. We report here on the three strongest passive galaxy overdensities that we identify in the range 1.5 < z < 2.5. While the actual nature of these concentrations still needs to be confirmed, we discuss their identification procedure and the arguments supporting them as candidate galaxy clusters (probably in the mid-10¹³ M_⊙ range). Although this search approach is probably biased toward more evolved structures, it has the potential of selecting still rare, cluster-like environments close to their epoch of first appearance, enabling new investigations of the evolution of galaxies in the context of structure growth.