On the evolution of environmental and mass properties of strong lens galaxies in COSMOS^⋆

¹ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
² Centro de Astro-Ingeniería, Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago, Chile
³ Max Planck Institut für Astronomia, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Laboratoire AIM, CEA/DSM-CNRS-Universite Paris Diderot, IRFU/SEDI-SAP, Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁵ Hubble Fellow, Department of Astronomy, UC Berkeley, 601 Campbell Hall, Berkeley, CA 94720-3411, USA
⁶ Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁷ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley CA 94720, USA
⁸ Berkeley Center for Cosmological Physics, University of California, Berkeley, CA 94720, USA
⁹ Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
¹⁰ Laboratoire d’Astrophysique de Marseille, CNRS Université de Provence, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
¹¹ Jet Propulsion Laboratory, MS 169-327,4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA

Received: 19 October 2009
Accepted: 12 February 2011

Abstract

Context. Nearly 100 new strong-lens candidates have been discovered in the COSMOS field. Among these, 20 lens candidates with 0.34 ≲ z_lens ≲ 1.13 feature multiple images of background sources.

Aims. Using the multi-wavelength coverage of the field and its spectroscopic follow-up, we characterize the evolution with redshift of the environment and of the dark-matter (DM) fraction of the lens galaxies.

Methods. We present spectroscopic and new photometric redshifts of the strong-lens candidates. The lens environment is characterized in the following way: we account for the projected 10 closest galaxies around each lens and for galaxies with a projected distance less than 1 Mpc at the lens galaxy redshift. In both cases, we perform similar measurements on a control sample of “twin” non-lens early-type galaxies (ETGs). In addition, we identify group members and field galaxies in the X-ray and optical catalogs of galaxy groups and clusters. From those catalogs, we measure the external shear contribution of the groups/clusters surrounding the lens galaxies. The systems are then modeled using a singular isothermal ellipsoid for the lens galaxies plus the external shear produced by the groups/clusters.

Results. We observe that the average stellar mass of lens galaxies increases with redshift. In addition, we measure that the environment of lens galaxies is compatible with that of the twins over the whole redshift range tested here. During the lens modeling, we notice that when let free, the external shear points in a direction which is the mean direction of the external shear produced by the groups/clusters and of the closest galaxy to the lens. We also notice that the DM fraction of the lens galaxies measured within the Einstein radius significantly decreases as the redshift increases.

Conclusions. Given these observations, we conclude that while the environment of lens galaxies is compatible with that of non-lens ETGS over a wide range of redshifts, their mass properties evolves significantly with redshift: it is still not clear whether this advocates in favor of a stronger lensing bias toward massive objects at high redshift or if it is simply representative of the high proportion of massive and high stellar density galaxies at high redshift.