paper

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

arXiv:astro-ph/0111205 · doi:10.1086/338648

Abstract

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 which is spatially coincident with the lens system. Based on multi-object spectroscopy taken at the Keck 10-m telescope, we have confirmed ten group members with a mean redshift of <z> = 0.2909. The resulting velocity dispersion and estimated virial mass are 306(+110/-58) km/s and 3.0(+2.2/-1.2) x 10^13 h^-1 M_sun, respectively, for (Omega_m,Omega_Lambda) = (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 publically-available HST 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 which 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.

Accepted for publication in the Astronomical Journal. 16 pages, 7 figures

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