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