Environmental effects at early cosmic times: galaxy evolution in the first massive clusters

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This project is based on recently obtained, deep, homogeneous imaging follow-up with HST and Spitzer of the complete sample of five high-significance (S/N>5) galaxy clusters at z>~1.4 detected through the Sunyaev-Zeldovich effect in the SPT-SZ survey over 2500 square degrees. The large survey area and depth of SPT-SZ allow the identification of this still unique sample of clusters among the rarest, most massive known at this cosmic epoch. The cluster selection, tightly related to cluster mass and totally independent of cluster galaxies (unlike most cluster surveys at these redshifts), makes this sample uniquely suited for unbiased galaxy population studies probing environmental signatures on galaxy evolution in early cluster environments. We designed the specific combination of HST and Spitzer observations to provide a first "essential data set" to start efficiently probing galaxy populations in terms of star formation and quenching, stellar mass assembly, passive fractions vs. stellar mass, formation of the red sequence, formation and evolution of Brightest Cluster Galaxies, morphologies vs. stellar population properties, structural evolution of bulge- and disk-dominated galaxies, and merger rates and properties in these extreme environments, as well as the galaxy stellar mass function, halo occupation distribution and stellar mass fraction in most massive halos at z~1.5. All HST and Spitzer observations are now completed and reduced. First analyses are ongoing with a DLR-supported program focused on the HST observations, confirming the high-redshift nature of these clusters, with the most distant system being consistently estimated at z~1.8 from both the (totally independent) red-sequence and X-ray modeling analyses. At this redshift, it is not only the most distant cluster discovered in the SPT-SZ survey, but the most distant massive (M500>10^14 Msun) ICM-selected cluster discovered thus far, and potentially the most distant cluster known in this extreme mass range irrespective of selection. We ask here for support for continuing this work, in particular fully exploiting the synergy between HST and Spitzer observations which is critical to the goals of this investigation.The proposed project will allow us to complete the first study of this kind of such a well defined, homogeneous massive distant cluster sample, and exploit its pilot-project nature for an efficient design of follow-up of distant clusters for galaxy evolution studies for new and coming surveys (SPTpol, SPT-3G, eROSITA), bridging today's state-of-the-art work and its powerful future extensions.