Program: GS-2021B-Q-206

We propose to obtain deep GMOS spectroscopy and Flamingos2 imaging towards the densest z>4 protocluster systems discovered so far, SPT2349, SPT0457 and SPT2052. These systems have been recently confirmed, using ALMA, having many ULIRGs/SMGs detected in dust-continuum and CO line emission within 20" (integrated SFR of ~10^4 Msun/yr). Since they were found in a blind 1.4-mm survey of 2500 deg^2, it is unlikely there are many other systems like these in the entire sky. The proposed observations are thus essential to obtain an unbiased, coherent census of galaxy population in these unique protocluster fields. Our main goals are to (i) characterize the environment, by identifying Lyman-break galaxies. Given the high density of ULIRGs, we expect to detect a plethora of LBGs at z=4-4.3. The rich complimentary data will enable a full characterization of their properties, and (ii) probe the stellar masses of both the ULIRGs and the LBGs in the protocluster through deep Ks-band imaging (rest G-band). In past Gemini GMOS programs, we have obtained the requisite g,r,i imaging for LBG selection. The focus of this program is spectroscopic followup. Studying these 3 clusters identified by SPT-PC through their central mega-mergers will provide an initial sample from which to assess the early growth of massive protoclusters.