High noon for cosmic giants: beacons for massive galaxy formation from South Pole Telescope protocluster cores

The most massive local galaxies sit in the centers of galaxy clusters, surrounded by red early types. While these dense environments are well studied out to redshifts 1-2, the formation process in the first two billion years remain enshrouded in cosmic history. Contrary to the successful hierarchical structure formation scenario, giant elliptical galaxy precursors are observed in compact groups of vigorously star-forming, dust obscured galaxies. Characterized by enormous molecular reservoirs, protocluster cores show correlated star-formation on the scales of the emerging cosmic web. Moreover, they represent the sites for energy injection and enrichment of the intracluster medium around the brightest cluster galaxies (BCGs) observable already by cosmic noon. Selected from the 25000 deg^2 large South Pole Telescope (SPT) survey, the sensitivity and angular resolution of ALMA is instrumental to characterize physical conditions of the cold, star-forming medium of protocluster cores. Guided by 870um APEX/LABOCA maps, the redshift determination to the eight brightest cores is now completed. Strikingly, SPT2349-56, at z=4.3, is the record holder for the highest star-formation surface density known, with 15 ULIRGs within only 50 kpc in projection. The majority of core galaxies sit on the same cluster caustic in phase space, hinting at a core-wide collapse process caught in action. A beacon of this 'mega merger' are bright, massive tidal gas streamers, serendipitously discovered with ALMA. Clumpy ionized carbon [CII]158 arcs extend over 50 kpc, connecting individual galaxies to the larger collapsing core. Numerical simulations suggest that this is a signpost event for the formation of a red proto-BCG with ten times the Milky Way's halo mass, completed within the next few 100 million years. In this talk, I will present the collapse of SPT2349-56's exceptional core, and explain why characterizing the full sample will allow to test the paradigm of massive galaxy formation at z>3.