Program: GN-2006B-Q-22

We propose to measure the composition of the ISM in the spiral galaxy NGC 628, via spectroscopy of H II regions ranging from 0.3 to 1.8 times the galaxy photometric radius from the center of the galaxy. These measurements will provide metallicity data to complement a {\it Spitzer Space Telescope} program we are undertaking to map the PAH emission across the disks of NGC 628, NGC 2403, and M101. This program is designed to understand what environmental factors, including metallicity, determine the strength and behavior of PAH emission. Gemini GMOS is the ideal instrument for this work as we need high quality spectra of a minimum of ten HII regions to clearly define the metallicity gradient in NGC 628. The sensitivity, relatively wide field of view, multi-object capability, and spectral stability of GMOS will enable us to conduct this program efficiently and reliably.