Gas, Dust, and Young Stars in the Outer Disk of M 31

Using the Canada-France-Hawaii Telescope we have obtained deep high-resolution CCD images in V and I of a 28'x28' field in the outer disk of M 31 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 HI features recorded near the HI 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 HI 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 HI distribution. Evidently, star formation is both ongoing and wide spread in the outer disk of M 31. According to the current view of the star formation process, molecular gas is therefore also expected to be present.