Program: GN-2011B-Q-2

Radial gradients in metallicity are found in nearly every large galaxy. Accurate measurements of these gradients are of key importance for modelling chemical evolution, stellar populations, and star formation histories, as well as for calibration of metallicity-dependent distance indicators such as Cepheids. M31, the nearest large galaxy, is of major interest for the detailed modeling of galaxies, since so many observations of its stellar populations and interstellar medium are available. Surprisingly, the metallicity gradient of M31 is rather poorly known: the standard reference in the field is 29 years old and based on spectra of only 17 objects! We have recently completed a new catalog of HII regions in M31 and propose to use this as the basis for a much-improved measurement of the galaxy's metallicity gradient. GMOS-N spectra are sensitive enough to detect the auroral lines in extragalactic HII regions, allowing a direct estimate of the electron temperature. With spectra of about 50 HII regions in 3 fields, we will measure the abundance gradient measurement with an uncertainty of 0.001 dex. This precision is an improvement of a factor of 2-3 over current estimates and is sufficient to distinguish between different chemical evolution models.