Peak Efficiency: Mass Assembly in a Forming Supercluster at the Peak of Cosmic Star Formation Activity

The Hyperion proto-supercluster at z~2.5 is the largest and best-studied large-scale structure observed during the peak of the cosmic star formation activity. Hyperion is the most massive overdensity yet discovered in the high-redshift universe, comparable in size and mass to present-day superclusters, and is comprised of seven distinct matter peaks, each corresponding to a proto-group or -cluster exhibiting its own unique activity. These peaks span over an order of magnitude in total mass and show a variety of levels of dynamical maturity. A small portion of Hyperion was serendipitously observed by the 3D-HST survey, revealing tantalizing indications that stellar mass assembly was proceeding rapidly in member galaxies both through star formation and merging processes.

We propose to use HST's unique capabilities, combined with existing data, to perform unbiased studies of environmentally-driven evolution in the early universe. Our program will produce a near complete map of Hyperion with WFC3 spectroscopy and imaging, confirming ~150 members (~50 in the densest peaks) - doubling the member sample and uniquely sampling the dusty star-forming and quiescent members. With this incredible sample, we will constrain the relationships between average galaxy star formation rates, stellar mass functions, and merging activity and environment. In conjunction with simulations and semi-empirical models, we will place strong constrains on different avenues of stellar mass assembly at a time when environmental quenching should be imposing itself in earnest and the vast majority of the stellar content of galaxies should be forming.