Gas, Dust, and Young Stars in the Outer Disk of M31

Using the Canada-France-Hawaii Telescope we have obtained deep high-resolution CCD images in V and I of a 28 ′ × 28 ′ field in the outer disk of M31 at ≈ 116 ′ from the center along the major axis to the south–west, and covering a range of projected galactocentric distance from about 23 to 33 kpc. The field was chosen to correspond with extended H I features recorded near the H I edge of the galaxy. The many tens of thousands of objects detected in this large field have been classified using an automatic algorithm which distinguishes unresolved from resolved structures and provides photometry on them. For the most part the unresolved objects are stars in M 31. The V–I colors of these stars are highly correlated with the column density of H I in the field. Assuming a Galactic extinction law, this yields a minimum extinction/atomic-gas ratio about 1/3 of that in the Solar neighbourhood. The ISM in this outer disk of M 31 therefore contains substantial amounts of dust. We have identified a population of B stars in the field whose distribution is also well correlated with the extended H I distribution. Evidently, star formation is both ongoing and wide spread in