Program: GS-2017B-Q-7

We propose to obtain deep GMOS spectroscopy and imaging and Flamingos2 imaging towards the densest protocluster system discovered so far, SPT2349-56 at z=4.3. This system has been recently confirmed, using ALMA in cycle-4, having 13 ULIRGs/SMGs detected in dust-continuum and 10 detected in CO emission within 20" (integrated SFR of ~10^4 Msun/yr). Since it was 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 this truly unique protocluster field. 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=4.3. 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).