The existence of strong lensing systems with Einstein radii covering the full mass spectrum , from \sim 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 10 ^ { 13 } - 10 ^ { 14 } M _ { \sun } , 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 \sim 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 \arcsec and 8 \arcsec .
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 _ { SIS } \sim 500 km s ^ { -1 } with large error bars and an upper limit of \sim 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 } \sim 250 ( solar units , corrected for evolution ) , with an upper limit of 500 .
This compares with mass-to-light ratios of small groups ( with \sigma _ { SIS } \sim 300 km s ^ { -1 } ) and galaxy clusters ( with \sigma _ { SIS } > 1 000 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 .