Program: GS-2018B-Q-123

We have new single-band HST images confirming strong gravitational lensing of the brightest sub-millimeter galaxies (SMGs) in the sky. Here we propose imaging in r' and z' in 2 fields and MOS spectroscopy of the foreground lenses towards 7 targets in 6 fields with Gemini. We know the redshifts of the background SMGs (z_med=2.263), but not of the foreground lenses. About half of all star formation at high redshift apparently takes place in dust-shrouded galaxies invisible to UV/optical surveys. We have identified some of the most FIR-luminous galaxies in the Universe and have imaged them at 1.1 mm and measured their redshifts $1<z<4$ via CO emission lines using the Large Millimeter Telescope (LMT). Our full sample of 31 SMGs in 29 fields, culled from the Planck all-sky survey, includes 14 of the 21 most luminous galaxies known, with L_FIR>10e14 Lsun and SFR>10e4 Msun/yr. Our long-term goal is to determine the nature and powering mechanism of these hyperluminous SMGs. We predicted that most or all are strongly gravitationally lensed, with typical magnification ~ 10x. Our Cycle 24 HST/WFC3 imaging program of 22 targets confirms this: almost every source shows a large (up to 10" diameter) Einstein ring caused by a massive intervening lensing galaxy, with most of the rest showing large gravitational arcs from massive galaxy clusters at z<1. The HST images in F160W provide spectacular spatial detail and confirmation of the lensing hypothesis, but no color information, preventing robust lensing models, and the field of view is smaller than a typical cluster diameter at z~1. Here we seek deep r' and z' images with GMOS of 2 sources available from GEM-S this semester to supplement 6 fields to be imaged in 16A, and GMOS spectroscopy of all 6 available fields. The new images will immediately allow us to discriminate between foreground and background sources, to discriminate between lensing clusters and isolated lensing galaxies, and to detect lensed images of the SMGs and other background galaxies. Redshifts of the lensing galaxies, groups, and clusters from GMOS spectroscopy then let us construct robust lensing models. With lensing models in hand, we can reconstruct the source SMGs to derive structural details of gas and star formation down to scales of 10 to 100 pc, a unique opportunity to understand the fueling and triggering mechanisms of these extreme starbursts.