A study of the variation in the stellar population in the disc of M31

Abstract

CCD photometry has been performed on four fields in the outer disc on the north-eastern major axis and one in the halo of the Andromeda galaxy (M31). Colour-magnitude diagrams for each field are presented, which show the brightest 3 mag of the red giant branch. In all cases the giant branch is broad, suggesting a large metallicity dispersion or that mass funnelling is taking place. We use asymptotic giant branch (AGB) stars to investigate population differences between the fields, and find that all possess a strong intermediate-age component. No large differences are found between four fields in the outer disc and the field situated in the halo, which suggests that all fields have a similar star formation history. We have also resolved the low-surface-brightness spur in the north-eastern section of the disc, obtaining the first direct evidence that the stellar populations follow the warp in the gaseous (HI) disc. We derive a distance modulus of 24.3 ± 0.2.