Clues about the star formation history of the M31 disc from WFPC2 photometry

Abstract

Over the past several years, the Hubble Space Telescope (HST) has acquired many broad-band images of various regions in the M31 disc. I have obtained 27 such fields from the HST data archive in order to produce colour-magnitude diagrams (CMDs) of the stellar populations contained within these areas of the disc. I have attempted to reproduce these CMDs using theoretical stellar evolution models in conjunction with statistical tools for determining the star formation history that best fits the observations. The wide range of extinction values within any given field makes the data difficult to reproduce accurately; nevertheless, I have managed to find star formation histories that roughly reproduce the data. These statistically determined star formation histories reveal that, like the disc of the Galaxy, the disc of M31 contains very few old metal-poor stars. The histories also suggest that the star formation rate of the disc as a whole has been low over the past ~1Gyr.