Program: GN-2015B-Q-26

Our collaboration is conducting a large, 107 orbits, HST program to image 37 fields of strong-lensing clusters. The goal of the HST program is to answer two questions: What was the morphology of star formation in galaxies at the epoch when most stars formed? What were the fundamental size scales of that star formation? We will be able to answer these questions, for the first time, thanks to the unique combination of HST’s exquisite image quality and the magnification boost of strong lensing that will overcome the normal diffraction limit of HST. A critical step for the interpretation of the observations, is accurate lens models, all the intrinsic properties of the background galaxies depend on their magnification by lensing. The lens modeling effort will be led by PI Sharon, and conducted at the University of Michigan. Here we propose to support the HST program with ground-based spectroscopy of lensed galaxies. The robustness of lens models strongly relies on correct identification of multiply-lensed galaxies, and an accurate measurement of their redshift. The proposed observations will allow us to more than double the number of lensed galaxies with spectroscopic redshift in each cluster, thus enabling their use as constraints in the lens modeling process, which can improve the statistical lensing uncertainties by a factor of four, and reduce the systematic uncertainties. We will target new arc candidates that have been discovered in the HST data. We note that this proposal has been approved for execution in 2014A, but due to technical failures of the observatory the bulk of the program has not been executed, It was approved for two targets in 2015A, here we request to continute this program in 2015B.