The Lens Redshift and Group Environment for He 0435 - 1223

The redshift of the galaxy lensing HE 0435−1223 is 0.4546 ± 0.0002, based on observations obtained with the Low Dispersion Survey Spectrograph 2 (LDSS2) on the Magellan Consortium’s 6.5 m Clay telescope. Hubble Space Telescope/ACS observations of the system also reveal a spiral-rich group of 10 galaxies within 40 of the elliptical lensing galaxy. The redshifts for two of these galaxies were measured to be in the foreground (at z = 0.419) with respect to the lens, thus at least some of the nearby galaxies are not part of the same physical group as the lensing galaxy. Mass models of the system (assuming same-plane deflectors) that take the local group environment into account do better at explaining the observed emission-line flux ratios (which are presumably unaffected by microlensing) than single halo models, but the match is still not perfect. In particular, component A (a minimum of the light travel time) is observed to be 0.20 mag brighter than predicted and component C (also a minimum image) is observed to be 0.16 mag fainter than predicted. Mass models for the system predict an A–D time delay of either 15.8 or 17.6 days (Ho = 72 km s −1 Mpc) depending on the details of the local galaxy environment. Subject headings: gravitational lensing: individual (HE 0435−1223)