Lens galaxy properties of SBS 1520+530: Insights from Keck spectroscopy and AO imaging

We report on an investigation of the SBS 1520+530 gravitational lens system. We have used archival Hubble Space Telescope (HST) imaging, Keck spectroscopic data, and Keck adaptive optics (AO) imaging to study the lensing galaxy and its environment. The AO imaging has allowed us to fix the lens galaxy properties with a high degree of accuracy when performing the lens modeling, and the data indicate that the lens has an elliptical morphology and perhaps a disk. The new spectroscopic data suggest that previous determinations of the lens redshift may be incorrect, and we report an updated, although inconclusive, value z(lens) = 0.761. We have also spectroscopically confirmed the existence of several galaxy groups at approximately the redshift of the lens system. We create new models of the lens system that explicitly account for the environment of the lens, and we also include improved constraints on the lensing galaxy from our AO imaging. Lens models created with these new data can be well fit with a steeper than isothermal mass slope (alpha = 2.29, where rho alpha r(-alpha)) if H-0 is fixed at 72 km s(-1) Mpc(-1); isothermal models require H-0 similar to 50 km s(-1) Mpc(-1). The steepened profile may indicate that the lens is in a transient perturbed state caused by interactions with a nearby galaxy.