A new window of exploration in the mass spectrum: strong lensing by galaxy groups in the SL2S ^*,**

¹ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 57 avenue d'Azereix, 65000 Tarbes, France e-mail: marceau.limousin@ast.obs-mip.fr
² Laboratoire d'Astrophysique de Marseille, UMR 6110, CNRS-Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
³ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁴ CNRS, UMR 7095, Institut d'Astrophysique de Paris, 75014 Paris, France
⁵ UPMC Université Paris 06, UMR 7095, Institut d'Astrophysique de Paris, 75014 Paris, France
⁶ Universidad de Valparaíso, Departamento de Física y Astronomia, Avenida Gran Bretaña 1111, Valparaíso, Chile
⁷ Durham University, Physics and Astronomy Department, South Road, Durham DH3 1LE, UK
⁸ Department of Astronomy, California Institute of Technology, 105-24, Pasadena, CA91125, USA
⁹ Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8W 3P6, Canada
¹⁰ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
¹¹ Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹² Laboratoire d'Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹³ Department of Physics, University of California, Santa Barbara, CA 93106, USA
¹⁴ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁵ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹⁶ Physics Department & Shanghai Key Lab for Astrophysics, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China
¹⁷ Observatoire de Paris, GEPI, CNRS-UMR 8111, 5 place Jules Janssen, 92195 Meudon Cedex, France

Received: 4 December 2008
Accepted: 25 May 2009

Abstract

The existence of strong lensing systems with Einstein radii covering the full mass spectrum, from ∼ (produced by galaxy scale dark matter haloes) to > (produced by galaxy cluster scale haloes) have long been predicted. Many lenses with Einstein radii around and above  have been reported but very few in between. In this article, we present a sample of 13 strong lensing systems with Einstein radii in the range (or image separations in the range ), i.e. systems produced by galaxy group scale dark matter haloes. This group sample spans a redshift range from 0.3 to 0.8. This opens a new window of exploration in the mass spectrum, around 10¹³–10¹⁴ , a crucial range for understanding the transition between galaxies and galaxy clusters, and a range that have not been extensively probed with lensing techniques. These systems constitute a subsample of the Strong Lensing Legacy Survey (SL2S), which aims to discover strong lensing systems in the Canada France Hawaii Telescope Legacy Survey (CFHTLS). The sample is based on a search over 100 square degrees, implying a number density of ~0.13 groups per square degree. Our analysis is based on multi-colour CFHTLS images complemented with Hubble Space Telescope imaging and ground based spectroscopy. Large scale properties are derived from both the light distribution of elliptical galaxies group members and weak lensing of the faint background galaxy population. On small scales, the strong lensing analysis yields Einstein radii between 2.5″ and 8″. On larger scales, strong lens centres coincide with peaks of light distribution, suggesting that light traces mass. Most of the luminosity maps have complicated shapes, implying that these intermediate mass structures may be dynamically young. A weak lensing signal is detected for 6 groups and upper limits are provided for 6 others. Fitting the reduced shear with a Singular Isothermal Sphere, we find  km s^-1 with large error bars and an upper limit of ~900 km s^-1 for the whole sample (except for the highest redshift structure whose velocity dispersion is consistent with that of a galaxy cluster). The mass-to-light ratio for the sample is found to be  ~ 250 (solar units, corrected for evolution), with an upper limit of 500. This compares with mass-to-light ratios of small groups (with  km s^-1) and galaxy clusters (with  km s^-1), thus bridging the gap between these mass scales.
The group sample released in this paper will be complemented with other observations, providing a unique sample to study this important intermediate mass range in further detail.