2005AJ....129.2531M

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 on the Magellan Consortium's 6.5 m Clay Telescope. Hubble Space Telescope Advanced Camera for Surveys 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 lens. Mass models of the system (assuming same-plane deflectors) that take the local galaxy 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. The A-D time delay is predicted to be either 15.8 or 17.6 days (H₀=72 km/s/Mpc), depending on the details of the local galaxy environment.