Hubble, Chandra and Keck Constraints on Massive Galaxy Clusters at z=0.2 and z=0.5

I present recent observations from two Hubble Space Telescope(HST)/ACS programs that target the most X-ray luminous and thus (presumably) most massive galaxy clusters at z{=}0.5 - the highest redshift at which complete, well-defined samples of such rare systems are available. The first program (GO:9836, PI: R.S. Ellis) exploits a huge mosaic of 41 ACS pointings spanning a 10 Mpc region centered on MS0451-03. This is the largest contiguous space-based image of a cluster to date. I describe a preliminary weak-lensing analysis and a new Keck/DEIMOS redshift catalog of 1000 galaxies in this field. The second program (GO:9722, PI: H. Ebeling) studies the core regions of the twelve most luminous clusters at z≥0.5 from the MAssive Cluster Survey (MACS; Ebeling et al. 2001). Multi-color ACS observations in combination with recent Keck/LRIS spectroscopy of gravitational arcs constrain the distribution of mass in the cluster cores, thus laying the foundation for detailed multi-diagnostic (lensing, X-ray, near-infrared, SZE) investigation of this sample. For example, it is of particular interest to explore how the structure and state of relaxation of massive clusters evolved between this sample at z≥0.5 that measured by Smith et al. (2004, astro-ph/0403588) at z{=}0.2.