Abstract
The gravitational lens system CLASS B2108+213 has two radio-loud lensed images separated by 4.56 arcsec. The relatively large image separation implies that the lensing is caused by a group of galaxies. In this paper, new optical imaging and spectroscopic data for the lensing galaxies of B2108+213 and the surrounding field galaxies are presented. These data are used to investigate the mass and composition of the lensing structure. The redshift and stellar velocity dispersion of the main lensing galaxy (G1) are found to be z = 0.3648 +/- 0.0002 and sigma_v = 325 +/- 25 km/s, respectively. The optical spectrum of the lensed quasar shows no obvious emission or absorption features and is consistent with a BL Lac type radio source. However, the tentative detection of the G-band and Mg-b absorption lines, and a break in the spectrum of the host galaxy of the lensed quasar gives a likely source redshift of z = 0.67. Spectroscopy of the field around B2108+213 finds 51 galaxies at a similar redshift to G1, thus confirming that there is a much larger structure at z ~ 0.365 associated with this system. The width of the group velocity distribution is 694 +/- 93 km/s, but is non-Gaussian, implying that the structure is not yet viralized. The main lensing galaxy is also the brightest group member and has a surface brightness profile consistent with a typical cD galaxy. A lensing and dynamics analysis of the mass distribution, which also includes the newly found group members, finds that the logarithmic slope of the mass density profile is on average isothermal inside the Einstein radius, but steeper at the location of the Einstein radius. This apparent change in slope can be accounted for if an external convergence gradient, representing the underlying parent halo of the galaxy group, is included in the mass model.
Highlights
In the standard model of galaxy formation, structure arises hierarchically through the mergers and interactions of smaller mass systems, leading to increasingly more massive structures
We show that the gravitational lens system B2108+213 is a part of a massive group of galaxies and that the multiple imaging is found around the large central galaxy, as opposed to the satellite members of the group
The flux-ratio of the two lensed images A and B could only be well-fitted if the density profile of G1 was steepened to a logarithmic density slope of γ = 2.45+−00..1198 (68 per cent confidence level). This is relatively steep, but not inconsistent with some of the systems found in the Sloan Lens Advanced Camera for Surveys (ACS) Survey (SLACS) (Koopmans et al 2006, 2009) or, for example, with PG 1115+080, another lens system closely associated with a compact group (e.g. Treu & Koopmans 2002; Momcheva et al 2006)
Summary
In the standard model of galaxy formation, structure arises hierarchically through the mergers and interactions of smaller mass systems, leading to increasingly more massive structures. We present for the first time an analysis of the total (baryonic and dark) inner density profile of a moderate redshift galaxy group using strong gravitational lensing and stellar kinematics. A simple lens model, which includes both G1 and G2 with isothermal density profiles and an external shear, can explain the positions of the lensed images A and B, but fails to reproduce the observed flux-ratio (McKean et al 2005; More et al 2008) This could be explained if there is a perturbation in the lensing mass model, for example, due to mass substructure in the parent dark matter halo The photometric catalogue was generated by SEXTRACTOR (Bertin & Arnouts 1996)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
