Program: GS-2020B-Q-125

(Covid-19 shutdown, resubmission of last semester's B-rated program). SPT-selected protoclusters represent unique and massive examples of the progenitors of clusters of galaxies at z=4-6. The 9 brightest cluster members exhibit individual SFRs >1000 Msun/yr (measured from their far-IR emision), show signs of being major mergers with tidal disruption in the [CII] line maps, and have the weakest [CII]/far- IR ratios in the protoclusters, suggesting AGN dominated emission. We propose F2 observations to constrain the strength of MgII_2800 as an AGN indicator, in these most massive (>10^11 Msun) galaxies identified by ALMA, HST and Gemini. Bright AGN and starburst are expected during major mergers, but characterizing and separating the AGN/SF components is a difficult process. Robust indicators of AGN are abundant in the mid-IR, but are not accessible from the ground. F2 observations offer a unique route to characterizing the AGN strength in these galaxies.