Searching for the sources responsible for cosmic reionization: Probing the redshift range 7 < z < 10 and beyond

We review recent observations that suggest that the global star formation rate density of UV-bright galaxies is declining monotonically with redshift over 3 < z < 7 and illustrate the challenges that a continuation of this decline poses in explaining the assembled stellar mass in several z ∼ 6 galaxies deduced from recent Spitzer data. A plausible conclusion is a vigorous period of yet earlier star formation. Prior to JWST and TMT, strong lensing offers a unique probe of the extent of this earlier activity. We discuss the first results of a blind spectroscopic survey of 10 lensing clusters for 8.5 < z < 10 Ly α emitters using NIRSPEC on Keck. We demonstrate this survey is achieving an (unlensed) sensitivity equivalent to a star formation rate of 0.1 M⊙ yr−1. A companion survey, now beginning with HST and Spitzer, will target lensed z and J-band drop-outs and probe a ≃1 arcmin2 region 1 magnitude deeper than the UDF/NICMOS observations. In combination, both surveys will provide the first constraints on the contribution of early, low luminosity, sources to cosmic reionization.