Program: GS-2020B-Q-226

Quadruply imaged quasars (hereafter quads) are a particularly powerful manifestation of strong lensing for the study of the dark universe. The time delays between multiple images provide a direct measurement of cosmic distances, and thus of the Hubble Constant and other cosmological parameters. Recent results from the H0LiCOW collaboration based on the blind analysis of six systems presented limits on the Hubble constant H_0, dark energy equation of state parameter w, and flatness Omega_k that are competitive and complementary with established methods such as Cepheids, Baryonic Acoustic Oscillations and Supernovae Ia. One of the key components is characterization of the mass distribution along the line of sight. Our goal will be to use GMOS-S to target about half the i < 23 galaxies within a 3-arcmin radius around two quad systems located within the Dark Energy Survey (DES). Such a redshift survey will let us identify galaxies or galaxy groups that need to be explicitly included in the lensing model, as well as help us determine the mass density along the line of sight to a few percent precision, ingredients needed to enable high-precision H_0 constraints for our target quad systems.