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 \pm 0.0002 and \sigma _ { v } = 325 \pm 25 km s ^ { -1 } , 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 \sim 0.365 associated with this system .
The width of the group velocity distribution is 694 \pm 93 km s ^ { -1 } , 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 .