Program: GN-2009A-Q-46

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 approved GO--4 and GO--5 Spitzer Space Telescope programs. The lensing magnification of our galaxies ranges from factors of 2 to 30, and span the complete range of star-forming galaxy types, making it possible to study ``typical'' galaxies at these redshifts directly with Spitzer. With our cycle 5 Spitzer proposal, we were awarded 30.9 hours of Gemini near-IR spectroscopy to measure \ha\ or \hb\ line luminosities for five 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 measure the \ha\ (or \hb)--\paa\ decrement, measuring the nebular extinction. We will then compare the dust--corrected \paa\ line luminosity against other SFR tracers commonly used at high redshifts: e.g., the UV luminosity, the rest-frame mid-IR and far--IR emission, and \ha\ emission. These measurements will provide our best SFR comparison yet obtained at high redshift.