Optical dropout galaxies lensed by the cluster A2667^⋆

¹ Université de Toulouse, UPS-OMP, IRAP, 31400 Toulouse, France
e-mail: nicolas.laporte@ast.obs-mip.fr; roser@ast.obs-mip.fr
² CNRS, IRAP, 14 Avenue Edouard Belin, 31400 Toulouse, France
e-mail: Daniel.Schaerer@unige.ch
³ Geneva Observatory, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁴ Institute for Computational Cosmology, Department of Physics, University of Durham, DH1 3LE, UK
e-mail: johan.richard@durham.ac.uk
⁵ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
e-mail: eegami@as.arizona.edu
⁶ Laboratoire d’Astrophysique de Marseille, CNRS – Université Aix-Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
e-mail: jean-paul.kneib@oamp.fr
⁷ Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98 bis boulevard Arago, 75014 Paris, France
e-mail: hudelot@iap.fr; mellier@iap.fr

Received: 30 September 2010
Accepted: 13 April 2011

Abstract

Context. We investigate the nature and the physical properties of ten z, Y, and J-dropout galaxies selected in the field of the lensing cluster A2667.

Aims. This cluster is part of our project aimed at obtaining deep photometry at  ~0.8–2.5 microns with ESO/VLT HAWK-I and FORS2 on a representative sample of lensing clusters extracted from our multi-wavelength combined surveys with Spitzer, HST, and Herschel. The goal is to identify a sample of redshift z ~ 7–10 candidates accessible to detailed spectroscopic studies.

Methods. Our selection is performed using the usual dropout technique based on deep I, z, Y, J, H, and Ks-band images (AB  ~  26–27, 3σ), targeting z ≳ 7.5 galaxy candidates. We also include IRAC data between 3.6 and 8 μm, and MIPS 24 μm when available. In this paper, we concentrate on the complete Y and J-dropout sample among the sources detected with a high signal-to-noise ratio in both H and Ks bands, as well as the bright z-dropout sources fulfilling the color and magnitude selection criteria adopted by Capak and collaborators. SED-fitting and photometric redshifts were used to constrain the nature and the properties of these candidates.

Results. Ten photometric candidates are selected within the  ~7′ × 7′ HAWK-I field of view (~33 arcmin² of effective area once corrected for contamination and lensing dilution at z ~ 7–10). All of these are detected in H and Ks bands in addition to J and/or IRAC 3.6 μm/4.5 μm images, with H_AB ranging from 23.4 to 25.2, and have modest magnification factors between 1.1 and 1.4. Although best-fit photometric redshifts are obtained at high-z for all these candidates, the contamination by low-z interlopers is expected to be in the range  ~50–75% based on previous studies, and on comparison with the blank-field WIRCAM Ultra-Deep Survey (WUDS). The same result is obtained when photometric redshifts are computed using a luminosity prior, allowing us to remove half of the original sample. Among the remaining galaxies, two additional sources could be identified as low-z interlopers based on a detection at 24 μm and the HST z₈₅₀ band. These low-z interlopers are not accurately described by current spectral templates given the large break, and cannot be easily identified based on broad-band photometry in the optical and near-IR domains alone. A good fit at z ~ 1.7–3 is obtained when assuming a young stellar population together with a strong extinction. Given the estimated dust extinction and high SFRs, some of them could also be detected in the IR or sub-mm bands.

Conclusions. After correction for contaminants, the observed number counts at z ≳ 7.5 seem to agree with expectations for an evolving LF, and be inconsistent with a constant LF since z ~ 4. At least one and up to three candidates in this sample are expected to be genuine high-z sources, although spectroscopy is still needed to confirm this.