Environments of Strong Gravitational Lenses

This dissertation focuses on determining the properties of the environments and the line-of-sight mass distributions for a sample of strong gravitational lenses as well as establishing their effects on the observed lens properties and the Hubble constant.Strong gravitational lenses ought to be able to provide important constraints for cosmology, however the lack of understanding of their large--scale environments has hindered their use. Here, we present a spectroscopic survey of the environments and lines of sight of 28 strong galaxy--mass lenses. We determine redshifts for 9698 galaxies in the fields of these lenses and identify 163 structures with at least five members in 26 fields. We find that 12 of 26 lenses are group members. Six of these groups are newly discovered. Overall, between 38--67% of lenses are in groups, and in 8--31% of the lenses the group makes a significant contribution to the lens potential. Line-of-sight structures are present in virtually every lens field, and in 19% of lenses the structures appear to be a significant perturbation to the lens potential. We consider the effect of the environment on H0 derived from gravitational lenses. We find that, when the environment is ignored, lenses in groups predict a systematically higher value of H0, inconsistent at the 1 sigma level with H0 derived from isolated lenses. Correcting for the environment and line of sight structures brings the two values into agreement and lowers the combined value (H=66-3.2+3.4). Without correction for the environment, the H0 values from strong lenses should be considered a strict upper limit. We explore the correlation between the observed lens properties and external perturbations from the observational perspective. We find that four-image lenses are statistically more likely to be in groups than two-image lenses. Furthermore, the distributions of convergences are statistically different for quads and doubles. This finding strongly supports the idea that the high quad-to-double ratios are at least partially due to the effect of the environment. We also examine the connections between flux anomalies and environment and the correlations between image separations and convergence. We find no significant links due to the limitations of our sample