SPT2349-56: The Most Massive and Active Proto-Cluster Known at z=4.3

As part of the follow-up of bright 1.4mm sources from the South Pole Telescope 2500 square degree survey, we have identified a unique proto-cluster at z=4.3. Using ALMA CO(4-3) and [CII] spectroscopy, we have spectroscopically confirmed at least 12 members within a radius of 13 arcseconds. These 12 members have a combined star formation rate of 6800 M_sol/yr, which may increase up to 22000 M_sol/yr if the entire LABOCA 870µm flux of 100 mJy is accounted for. The 12 members have a velocity dispersion of 500 km/s, implying a virial mass of 2x10<SUP>13</SUP> M_sol. This makes SPT2349-56 a truly unique proto-cluster both in terms of mass and star-formation rate.