Gravitational lens environments: Improving lensing constraints on cosmology and galaxy evolution

Strong gravitational lenses are becoming increasingly more useful probes of cosmology and galaxy structure as observations and models of lenses improve. However, ignoring the environments of these lenses can lead to substantial biases in obtaining and interpreting science results from strong lenses. I have taken a census of lens environments to statistically quantify this bias; this is a critical step toward analyzing future datasets of hundreds of strong lenses i.e. from SNAP and LSST). I have found that, while nearly all lenses have at least one group along the line of sight to the lens, only 50% of lenses lie in group environments. The contributions of these groups to the lensing, as quantified by the external convergence (k) at the location of the lens due to the group and its members, vary from negligible to k ~ 0.15. The observed distribution of external convergences from my survey of the environments of the CLASS lenses approximately follows the distributions predicted by simple models of groups in the local universe. This indicates that these models can be used as prior distributions for ensemble models of the thousands of lenses that will be found during the next decade.

Additionally, I have found that knowledge of the lens environments can help in the interpretation of structural parameters of the lenses. In particular, I have found that lenses that are best modeled with steep mass slopes are likely to be associated with a perturbing companion, as is suggested by N-body simulations. These lenses may serve as a unique probe of galaxy-galaxy interactions. In addition to studying the lens galaxy, discovering groups associated with gravitational lenses enables the study of group environments beyond the local universe. My survey of the CLASS lenses has found at least19 moderate redshift groups, a number that is comparable to the number found in the CNOC2 blind redshift survey. These groups range in redshift from z = 0.26 to z = 0.81 and represent an excellent dataset for studying galaxy evolution in group environments at moderate redshifts.