Program: GN-2012B-C-4

The basic statistical properties of galaxies at 4<z<7 have now been measured accurately using large samples of photometrically selected samples (e.g., Lyman-break galaxies). Making substantive progress now requires deep spectroscopy of these galaxies to deliver redshifts, stellar masses and star-formation rates (SFRs) accurate enough to test theoretical evolutionary paths of individual galaxies. We propose to obtain GMOS spectra of a magnitude limited, H(AB)<26.5, sample of 4.0<z<6.5 galaxies in the UDS and COSMOS CANDELS fields: these are two fields with very deep HST and Spitzer data over the largest areas. Our principal science goal is to measure redshifts of >~200 galaxies at 4<z<6.5, which allows us to measure with high accuracy their SFR/stellar-mass relation. Current observations show the SFR/stellar mass relation is constant at z>2, while theory predicts it should decline with decreasing redshift. We will refute (or confirm) this emerging contradiction. Our second main science goal is to measure accurately the evolving frequency of Ly-alpha emission in these galaxies as a measure of the rising cosmic hydrogen neutral fraction at increasing redshift. This is a resubmission of a 2011B proposal, which was highly ranked but not schedulable in the queue. It was recommended we reapply, requesting classical time, which we do here.