IMAGING OF FIELDS IN THE INNER HALO AND OUTER DISK OF M31

Moderately deep CCD images are used to investigate the stellar content of two fields which sample the outer disk and inner halo of M31. One field intersects the plane of the disk 20 kpc from the galaxy center. A bright main-sequence component is detected, and the brightness of the main-sequence tip indicates that star formation occurred up to 50 Myr in the past. Luminous (i.e., M(bol) less-than-or-equal-to - 5) asymptotic giant branch stars are also seen, indicating that star formation also occurred during intermediate epochs. The width of the red giant branch is greater than expected from observational uncertainties, and comparison with models of simple stellar systems suggests that this area contains a dispersion in both age and metallicity. The mean metallicity in the outer disk field does not appear to be different from that in Baade's Field IV, which is located at a similar distance from the center of M31, and the metallicity dispersion must be at least 0.3 dex. These results further support the suggestion made in 1990 by Richer, Crabtree, & Pritchet that the metallicity gradient in the outer disk of M31 may be relatively flat. The second field is located near the minor axis of M31 6.7 kpc from the galaxy center and samples the inner halo. The red giant branch is wider than expected from observational uncertainties, and it is concluded that the majority of stars have [Fe/H] between - 0.7 and - 1.3. The mean metallicity and metallicity dispersion in this field are in excellent agreement with what is seen in halo fields at larger radii, indicating that the metallicity of the M31 halo is spatially homogeneous, even in regions close to the disk and bulge.