Program: GN-2008A-Q-40

All studies of high redshift galaxies rely on the star formation rates (SFRs) based on local calibrations, usually hydrogen recombination lines. Of these, Paschen-$\alpha$ ($\lambda$1.8571~\micron) suffers the least from dust extinction, and is a direct tracer of the ionizing radiation from young stars. We are conducting a study of \paa\ in gravitationally lensed galaxies at $2 < z < 3$ using spectroscopy from an approved GO--4 Spitzer Space Telescope program. The lensing magnification of our galaxies ranges from factors of 4 to 30, making it possible to study ``typical'' star-forming galaxies at these redshifts directly with Spitzer. As part of our Spitzer proposal, we were awarded 26.3 hours of Gemini near-IR spectroscopy to measure \ha\ and (where possible) \hb\ line luminosities for four galaxies in our sample. \textit{The purpose of the current proposal is to rank our awarded Gemini allocation either as band 1 or band 2.} The near--IR spectra are crucial to correct the observed luminosities for dust extinction. We will then compare the \paa\ line luminosity against other SFR tracers commonly used at high redshifts: e.g., the UV luminosity, the rest-frame mid-IR emission features and luminosity, the far-IR luminosity, and the luminosity from the \ha\ emission line. Our galaxy sample spans the complete range of star-forming galaxy types at $2 < z < 3$. Therefore these measurements will provide our best SFR comparison yet obtained at high redshift.