First Results from a Photometric Survey of Strong Gravitational Lens Environments
Abstract
Evidence is mounting that many strong gravitational lenses lie in complex environments, such as poor groups of galaxies, that bias conclusions from lens analyses in significant ways. We are undertaking a photometric survey of all known galaxy-mass strong lenses in order to characterize their environments and include them in careful lens modeling, and to build a large, uniform sample of galaxy groups at intermediate redshifts (out to z~1) for evolutionary studies. In this paper we present wide-field photometry of the environments of twelve lens systems with redshifts 0.24< z_lens<0.5. Using a red-sequence finding technique, we find that eight of the twelve lenses lie in group environments, and that seven of the twelve lenses have a total of ten line-of-sight structures. Follow-up spectroscopy of a subset of these fields confirms these results. The median lens galaxy luminosity is ~0.5 L*, contradicting conventional wisdom that lens galaxies are all super-L* ellipticals, but in agreement with a simple model of a Schechter luminosity function convolved with the lensing probability. Only two of the eight lenses found in groups are the brightest group galaxy, a number in qualitative agreement with theoretical predictions. As in the local Universe, the highest velocity-dispersion galaxy groups at intermediate redshifts contain a brightest member that is spatially coincident with the group centroid, whereas the lower-$\sigma$ groups tend to have an offset brightest group galaxy. This result suggests that higher velocity dispersion groups are more dynamically relaxed than lower velocity dispersion groups and that at least some evolved groups exist by z~0.5.Keywords
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