A 100 Mpc² structure traced by hyperluminous galaxies around a massive z = 2.85 protocluster

We present wide-field mapping at 850 μm and 450 μm of the z = 2.85 protocluster in the HS1549$+$19 field using the Submillimetre Common User Bolometer Array 2 (SCUBA-2). Spectroscopic follow-up of 18 bright sources selected at 850 μm, using the Nothern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter Array (ALMA), confirms the majority lies near z ~ 2.85 and are likely members of the structure. Interpreting the spectroscopic redshifts as distance measurements, we find that the SMGs span 90 Mpc² in the plane of the sky and demarcate a 4100 Mpc³ "pancake"-shaped structure in three dimensions. We find that the high star-formation rates (SFRs) of these SMGs result in a total SFR of 20,000 M_ yr⁻¹ only from the brightest galaxies in the protocluster. These rapidly star-forming SMGs can be interpreted as massive galaxies growing rapidly at large cluster-centric distances before collapsing into a virialized structure. We find that the SMGs trace the Lyman-α surface density profile. Comparison with simulations suggests that HS1549$+$19 could be building a structure comparable to the most massive clusters in the present-day Universe.