Program: GS-2010B-C-2

We propose to use Gemini/GMOS to obtain spectroscopic redshifts and dynamical mass estimates for the first sample of high-redshift galaxy clusters detected by the Atacama Cosmology Telescope (ACT) via the Sunyaev-Zeldovich effect (SZE). The proposed GMOS observations of 14 massive newly-discovered SZE-clusters will provide a precision calibration of the SZE signal (Y) vs. mass correlation at redshifts out to 1. With this calibration, our existing SZE-selected cluster sample should enable a better measurement of $\sigma_8$ and $\Omega_M$. The observations will also allow us to study the evolution, star formation rates and stellar populations in the most massive clusters over $0.3 < z <1$. The proposed observations represents the first complete, SZE-selected sample of massive clusters over all redshifts within a cosmologically-significant survey volume. The overall sample (of $\sim 50$ clusters) will be used to calibrate the Y-mass scaling relation, which in turn allows us to constrain the dark energy via cluster mass function, in conjunction with a cluster correlation function analysis. This sample will also allows us to start studying the cluster gas physics responsible for the the SZE signature.