Program: GN-2016A-Q-30

We propose multi-object spectroscopy with GEMINI-N/GMOS to characterize the lensing structures of 4 of the brightest, most strongly gravitationally lensed high-redshift galaxies on the sub-mm sky seen with the Planck all-sky survey. These galaxies provide a unique opportunity to study the processes that drive, regulate, and limit the intense star formation in the most rapidly growing galaxies on the scales where local energy injection from winds, turbulence, and perhaps cosmic rays prevails over large-scale galaxy-wide phenomena. Translating their observed into physical properties requires robust lens modelling, which GMOS will help us provide. We will measure accurate spectroscopic redhifts of 10-30 foreground galaxies -- all in extended structures like intermediate-redshift groups or low-mass clusters -- to confirm their cluster/group membership, measure their velocity dispersion (a proxy of mass), and unravel the astrophysical nature of the overall structure: Single cluster? Merging groups? Substructure within a dense filament? Together with already scheduled HST/WFC3 imaging, the proposed observations will boost the accuracy of our lens modelling by factors of a few and are a vital and missing piece of our overall analysis.