Program: GS-2010B-Q-28

We propose to obtain GMOS-S spectroscopy of 20 quasars as part of a multi-semester endeavour to study the z>4.4 IGM with Gemini (a related 2010B GMOS-N proposal is submitted). Ultimately, our 3-semester program will provide ~200 R~850 spectra of z>4.4 quasars, the largest homogeneous sample of science-grade high-redshift quasar spectra in existence. This sample will yield the first statistically meaningful measurements of the neutral gas content, the number density of optically thick absorbers, and the mean hydrogen absorption at z>4.5. In particular, spectral stacks will enable the first direct measurement of the mean free path to ionizing radiation at these redshifts, and probe its expected strong evolution near the epoch of reionization. This novel route to constrain reionization with the Lyman forest requires a large, homogeneous sample of higher-quality spectra than provided by SDSS. Gemini/GMOS can efficiently supply these data, which we will publicly release in reduced form. They will also permit (i) an unparalleled survey for damped Lya and Lyman limit systems at z>4.5, (ii) constraints on the mean opacity of Lyman series transitions, and (iii) a survey for variable emission/absorption in quasar spectra. With this request, we will complete the Fall semester of targets.