The mass distribution of a moderate redshift galaxy group and brightest group galaxy from gravitational lensing and kinematics

TL;DR

This study combines gravitational lensing and kinematic data to analyze the mass distribution of a galaxy group and its brightest galaxy at moderate redshift, revealing a complex, non-virialized structure with an isothermal core and external convergence effects.

Contribution

It provides a detailed mass and composition analysis of a galaxy group using new optical and spectroscopic data, highlighting the importance of external convergence in lens modeling.

Findings

The main lensing galaxy has a redshift of 0.3648 and velocity dispersion of 325 km/s.

The galaxy group at z ~ 0.365 is large, non-virialized, and contains 51 member galaxies.

The mass density profile is approximately isothermal inside the Einstein radius but steepens at that radius.

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.