Program: GS-2012A-C-1

We propose to continue and complete our Gemini/GMOS program to obtain spectroscopic redshifts and dynamical mass estimates for a large redshift-independent sample of galaxy clusters detected by the Atacama Cosmology Telescope (ACT) via the Sunyaev-Zeldovich effect (SZE) for multiple high-impact science goals. The proposed GMOS observations of 10 additional newly-discovered SZE massive clusters combined with the scheduled observations (Oct 24-28) of 10 clusters will provide a 50% improvement on our current 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 an improved 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 a complete, SZE-selected sample of massive clusters over all redshifts within a cosmologically-significant survey volume. The final overall sample of ~35 massive clusters will be used to calibrate the Y-mass scaling relation, with a scatter of around 10% on the slope, which in turn allows us to constrain the dark energy via cluster mass function, in conjunction with a cluster correlation function analysis.