Program: GS-2015B-LP-5

In the Dark Energy Survey (DES) we expect to discover many new strongly lensed galaxies and quasars. One of the main objectives of the strong lensing science program in DES is to derive constraints on dark energy. The two key components of this part of the program will be exploiting (1) lenses with background sources at multiple redshifts, and (2) lensed quasars. In order to realize this program, we will need spectroscopic redshifts of the lens and source(s), as well as redshifts for members of the environment. We are therefore proposing a program of follow-up observations using GMOS-S to obtain redshifts for our most promising candidates. Another vital element to extracting cosmology from the lensing systems is the availability of high-resolution imaging of the systems. This is required to obtain reliable models of the gravitational potential of the lens. We therefore also propose to obtain AO imaging in K-band using GSAOI under good imaging conditions for a subset of the confirmed systems that are suitable. Also, we will simultaneously observe about 50 additional galaxy targets in the mask of each of the strong lensing targets. The resulting sample of galaxy redshifts will substantially reduce the biases caused by photometric redshifts and sample variance on DES measurements of the dark energy parameter w.