The Gravitational Lens-Galaxy Group Connection. I. Discovery of a Group Coincident with CLASS B0712+472

Previous observations of the environments of the lensing galaxies in gravitational lens systems suggest that many of the lensing galaxies are associated with small groups of galaxies. As a result, we have begun a coordinated program to study the local environments of all known gravitational lens systems. In this paper, we present results on the gravitational lens system CLASS B0712+472, which has previously measured source and lens redshifts of (z_l,z_s)=(0.4060,1.339). Although we have not found a galaxy group associated with the primary lensing galaxy, we have found a foreground group that is spatially coincident with the lens system. Based on multiobject spectroscopy taken at the Keck 10 m telescope, we have confirmed 10 group members with a mean redshift of z=0.2909. The resulting velocity dispersion and estimated virial mass are 306⁺¹¹⁰_-58 km s^-1 and 3.0^+2.2_-1.2×10¹³ h^-1 M_solar, respectively, for (Ω_m,Ω_Λ)=(0.2,0.0). The dynamical properties of this moderate-redshift group are completely consistent with the range of values found in nearby groups of galaxies. Five of the group members are red elliptical-like galaxies, while the remaining five are active star-forming galaxies. Based on the spectroscopic results and the publicly available Hubble Space Telescope imaging of nine group members, we find that the early-type fraction is 40%. We estimate that the effect of this foreground group on the gravitational lensing potential of B0712+472 is small, producing an external shear that is only a few percent, although the shear could be larger if the group centroid is significantly closer to the lens system than it appears to be.