Characterizing the First Galaxy Clusters at the Epoch of their Formation with STAR-X

The next generation survey with the South Pole Telescope (SPT-3G) will discover all galaxy clusters with masses above 10^14 Msun.  Due to the comparable angular resolution of these SZ observations and the size of these high redshift clusters, we require higher resolution observations across the electromagnetic spectrum to reveal their morphology and other detailed properties.  At present, such information on the ICM is most accessible at X-ray energies.  We have proposed a new MidEx-class X-ray observatory, the Survey and Time-domain Astrophysical Research eXplorer (STAR-X), capable of surveying 300 z>1.5 clusters and a thousand more at lower redshifts in its prime 2-year mission.  With 5" angular resolution, over 2500 cm^2 of effective area, and soft X-ray sensitivity, STAR-X will be able to resolve the ~8" spatial structures that XMM-Newton cannot quite disentangle and gather photons >4x faster than Chandra.  Its large 1 deg^2 FOV and low background will also allow the outskirts of many nearby clusters to be mapped out to the virial radius.  I will discuss how this new mission will expand our current understanding of how galaxy clusters form and evolve over all of cosmic time.