Strong lensing as a probe of the mass distribution beyond the Einstein radius

Mass and light in SL2S J08544-0121, a galaxy group at z = 0.35

¹ Laboratoire d’Astrophysique de Marseille, UMR 6610, CNRS-Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
e-mail: marceau.limousin@oamp.fr
² Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 57 avenue d’Azereix, 65000 Tarbes, France
³ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁵ Durham University, Physics and Astronomy Department, South Road, Durham DH3 1LE, UK
⁶ Department of Astronomy, California Institute of Technology, 105-24 Pasadena, CA 91125, USA
⁷ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching, Germany
⁹ CNRS, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
¹⁰ UPMC Université Paris 06, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France
¹¹ Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France

Received: 22 June 2010
Accepted: 9 September 2010

Abstract

Strong lensing has been employed extensively to obtain accurate mass measurements within the Einstein radius. We here use strong lensing to probe mass distributions beyond the Einstein radius. We consider SL2S J08544-0121, a galaxy group at redshift z = 0.35 with a bimodal light distribution and with a strong lensing system located at one of the two luminosity peaks separated by  ~54″. The main arc and the counter-image of the strong lensing system are located at  ~5″ and  ~8″ from the lens galaxy centre. We find that a simple elliptical isothermal potential cannot satisfactorily reproduce the strong lensing observations. However, with a mass model for the group built from its light-distribution with a smoothing factor s and a mass-to-light ratio M/L, we obtain an accurate reproduction of the observations. We find M/L = 98 ± 27 (i band, solar units, not corrected for evolution) and s = 20″ ±  9 (2σ confidence level). Moreover, we use weak lensing to independently estimate the mass of the group, and find a consistent M/L in the range 66-146 (1-σ confidence level). This suggests that light is a good tracer of mass. Interestingly, this also shows that a strong lensing-only analysis (on scales of  ~10″) can constrain the properties of nearby objects (on scales of  ~100″). We characterise the type of perturbed strong lensing system that allows such an analysis: a non dominant strong lensing system used as a test particle to probe the main potential. This kind of analysis needs to be validated with other systems because it could provide a quick way of probing the mass distribution of clusters and groups. This is particularly relevant in the context of forthcoming wide-field surveys, which will yield thousands of strong lenses, some of which perturbed enough to pursue the analysis proposed in this paper.