"Good things come in threes:" cosmic web-scale protocluster assembly behind SPT0303-59

Structures in the Universe evolve hierarchically, up to the largest scales of the cosmic web. However, contrary to the successful hierarchical scenario, giant elliptical galaxy precursors are observed already at z>4 in groups of vigorously star-forming, dust obscured galaxies. The most extreme examples are hyper-luminous IR galaxies (HyLIRGs): starbursts feeding on massive cold gas reservoirs, often found in protocluster (PC) cores. Thus far, it is not clear what role the emerging cosmic web plays in triggering HyLIRGs and correlated star-formation of up to 10'000 solar masses per year. Now, for the first time, we can report the distances to all eight PC candidates selected from the South Pole Telescope (SPT) survey. SPT2349-56 (z=4.3) is the poster child of this sample, with 15 ULIRGs within a 50 kpc radius, ejecting giant arcs of cold gas into the circumgalactic medium. The brightest object of the sample, however, is SPT0303-59, totalling S_870~180 mJy. The CO line-redshifts show a broad distribution with five dusty galaxies at z~2.8, possibly akin in structure to the famous "Hyperion" proto-supercluster, and pairs at z~3.24 and ~4.65. Apart from two CO-rich HyLIRGs (plus seven dusty galaxies) just in the central 1', we detect ten more bright S_870um>7 mJy satellites within 10'. Surprisingly, their photo-z's agree well with the three subgroups -- but spread out over tens of cMpc! In this talk, I will present the curious case of SPT0303, a line-of-sight projection of three PC cores, rich in HyLIRGs. SPT0303's structure also implies enormous discovery potential, considering that hundreds of high-z sources detected in the next generation SPT-3G survey could be composed of non-lensed HyLIRG groups.