A 100 Mpc² structure traced by hyperluminous galaxies around a massive z = 2. 85 protocluster

We present wide-field mapping at 850 m and 450 m of the z = 2. 85 protocluster in the HS1549+19 field using the Submillimetre Common User Bolometer Array 2 (SCUBA-2). Spectroscopic follow-up of 18 bright sources selected at 850 m, using the Nothern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter Array (ALMA), confirms the majority lies near z 2. 85 and are likely members of the structure. Interpreting the spectroscopic redshifts as distance measurements, we find that the SMGs span 90 Mpc² in the plane of the sky and demarcate a 4100 Mpc³ "pancake"-shaped structure in three dimensions. We find that the high star-formation rates (SFRs) of these SMGs result in a total SFR of 20, 000 M_ yr^-1 only from the brightest galaxies in the protocluster. These rapidly star-forming SMGs can be interpreted as massive galaxies growing rapidly at large cluster-centric distances before collapsing into a virialized structure. We find that the SMGs trace the Lyman- surface density profile. Comparison with simulations suggests that HS1549+19 could be building a structure comparable to the most massive clusters in the present-day Universe.