High-redshift 3CR: witnessing the formation of the most massive galaxies, clusters and AGN in the Bright Ages

At redshifits between 1 and 2, massive elliptical galaxies are assembling and black holes (BHs) are in the final stages of merging: the AGN phenomenon is about 1000 times more common and star formation is vastly more vigorous than it is now. As the Universe evolves through this enormous transition, important questions arise concerning the causal relationships and interplay between these powerful physical phenomena. Does the AGN fundamentally influence its environment, triggering or quenching star formation on a galactic or galaxy-cluster scale? Do the physical processes of this transition result in the fundamental relationships seen in galaxies today? We propose a complete two-filter WFC3 survey of the 3CR radio source host galaxies at z>1. These are the most powerful radio-loud AGNs that exist at such redshifts, hosted by the progenitors of some of the most massive ellipticals. HST is the only great observatory that has not achieved complete coverage of this landmark sample of quasars and radio galaxies, yet statistical completeness is essential. We will reveal with unprecedented clarity the star formation at this critical moment in the evolution of galaxies, AGN and clusters. We will reveal the cluster environment of these systems in the final stages of hierarchical merging. We will probe the influence of the powerful AGN on young and evolved stellar populations. We will determine whether all radio-loud AGNs are triggered by major galaxy and black hole mergers, a fundamental piece of information for galaxy-BH co-evolution models, and strive to articulate the fundamental interrelationships that lead to the Universe of galaxies, black holes and clusters we see today.