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

The existence of strong lensing systems with Einstein radii covering the full mass spectrum, from ∼$1{-}2\arcsec$ (produced by galaxy scale dark matter haloes) to >$10\arcsec$ (produced by galaxy cluster scale haloes) have long been predicted. Many lenses with Einstein radii around $1{-}2\arcsec$ and above $10\arcsec$ 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 $3\arcsec{-}8\arcsec$ (or image separations in the range $6\arcsec{-}16\arcsec$), 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 $M_{\odot}$, 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 $\sigma_{\rm SIS}\,\sim 500$ 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 $M/L_i$ ~ 250 (solar units, corrected for evolution), with an upper limit of 500. This compares with mass-to-light ratios of small groups (with $\sigma_{\rm SIS} \sim 300$ km s-1) and galaxy clusters (with $\sigma_{\rm SIS} > 1000$ 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.