Program: GS-2016B-Q-68

The South Pole Telescope has discovered a new population of extraordinarily millimetre-bright objects in a 2500 deg^2 survey. Our extensive follow-up campaign has demonstrated that they are high-redshift, strongly lensed, dusty, star-forming galaxies, lensed by factors of 5 to 30, providing a unique opportunity to study these objects in greater detail and at lower cost than otherwise possible. We propose to complete our crucial optical followup of a new sample of 16 ALMA Cy3-observed SPT-SMGs, extending to higher redshifts (including z=6.9!). We propose to use GMOS-S and FLAMINGOS2 to study optical/nearIR counterparts and obtain spectroscopic redshifts for the lenses of sub-mm bright sources, the sources themselves, and surrounding environments. Our past Gemini and VLT followup has made lens models possible, uncovering lens redshifts and masses for 35 of the SPT-SMGs, placing the SMGs in context with unlensed populations.