We have begun an exciting survey of the outskirts of z=0.8 clusters to find future cluster members. This will involve HST data, Spitzer data and thousands of spectra using a prism on the Magellan IMACS spectrograph.

For one of our target clusters, we have finished the prism spectroscopy. Below I show mass-selected sample (stellar masses greater than 1e10) of current and future cluster members. All of the objects are at the redshift of the cluster, and most will become a member by today. We are watching the galaxies fall in, and so are watching the buildup and formation of the cluster.

Blue dots are the 600 members from prism spectroscopy, while the open blue circles are members from traditional high resolution spectra. The red contours show galaxy overdensities, and the green circles are 1, 2 4 and 8 physical Mpc at the redshift of the cluster.

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