Title: A Large Population of High Redshift Galaxy Clusters


Using a probabilistic photometric redshift-based approach combining Spitzer mid-IR and NOAO optical data, we have discovered nearly 300 new galaxy clusters and groups in the NDWFS Bootes field, nearly 100 of which are at 13) formation redshift. Deep follow-up IRAC and ACS data, currently being obtained for 18 of the z>1 clusters, allows detailed studies of the mass assembly history in massive galaxies at this key epoch.