Program: GS-2019A-FT-205

The Massive and Distant Clusters of WISE Survey (MaDCoWS) is an infrared all-sky survey to detect and classify the most massive galaxy clusters at z~1 and above. The resulting cluster sample covers a large range of mass at high redshift, making it ideal for studies of galaxy formation and evolution in the most extreme environments. We have recently measured the stellar mass fractions in these IR-selected clusters and compared them to identical measurements in clusters selected via their Sunyaev Zel'dovich decrements. While we see a tentative difference in stellar mass fractions based on selection---a potentially important bias to be identified and accounted for in cluster evolution and cosmology studies---this difference is not statistically significant due to the large statistical and systematic uncertainties in our initial measurement. We propose GMOS r- and z-band observations of a sample of MaDCoWS clusters with SZ detections. These observations will double our sample size, reducing our statistical errors by 40%, and enable photometric redshift-based cluster membership determinations that will dramatically lower our systematic errors by a factor of three. The proposed data will allow us to (i) establish whether or not selection method induces a bias in the cluster stellar mass fraction, and (ii) measure the slope of the relation between stellar mass fraction and cluster halo mass in MaDCoWS clusters at z~1.