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

Abstract

The existence of strong lensing systems with Einstein radii covering the full mass spectrum, from ∼ 1 − 2 (produced by galaxy scale dark matter haloes) to > 10 (produced by galaxy cluster scale haloes) have long been predicted. Many lenses with Einstein radii around 1 − 2 and above 10 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 − 8 (or image separations in the range 6 − 16 ), 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 , 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 σ SIS ∼ 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 σ SIS ∼ 300 km s −1 ) and galaxy clusters (with σ 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.

Planck’s dusty GEMS

“…We therefore tested 10 plausible associations of individual arclets into image families and adopted the single model that converges and places all images at positions matching those observed. This results in two image systems with two and four images, respectively, and is a configuration that is also commonly found in other strong lensing systems (e.g., Gavazzi et al 2008;Tu et al 2009;Limousin et al 2009;Bayliss et al 2011). We fitted the centroid position in right ascension, ∆RA, and declination, ∆Dec, the velocity dispersion of the PIEMD, σ PIEMD , as well as its ellipticity, , and position angle, PA (see also Limousin et al 2013;Bonamigo et al 2015).…”

Galaxy structure searches by photometric redshifts in the CFHTLS

“…This cluster is also detected with our method (D4-7, S /N = 2) at z = 0.90. A more complete list of group detections in the SL2S survey is given in Limousin et al (2009). Among the 13 group detections of this paper, a single one (SL2S J2140-0532 at z = 0.444) is included in our surveyed area (others are outside the area where photometric redshifts were computed), and we detect it at z = 0.45 with S /N = 6.…”

The dark matter distribution inz ~ 0.5 clusters of galaxies

“…To get a reliable faint-galaxy redshift distribution, we used the photometric redshifts of galaxies in reference fields, neglecting the cosmic variance between different fields (Cypriano et al 2004;Hoekstra 2007;Limousin et al 2009;Oguri et al 2009). Observations in the CFHTLS Deep fields obtained with the same instrument form the ideal data set for our study (Gavazzi & Soucail 2007).…”