| Abstract | The Canada-France-Hawaii 3.6 m telescope has been used to study the stellar population of a field in the halo of M31 down to B_lim_ = 26.4 and V_lim_ = 25.4. The area studied is situated on the SE minor axis of M31 at a distance of 40' (8.6 kpc) from the nucleus. From the location of the red giant branch the stellar population in this field is seen to be dominated by stars with metallicities intermediate between those of M5 and 47 Tuc; we assign a mean metallicity [Fe/H] ~ -1.0 to the bulk of the stellar population in our field. This metallicity is very similar to that derived for globular clusters surrounding M31 (van den Bergh), but significantly higher than the mean abundance of globular clusters and field stars in the Galactic halo. The blue horizontal branch in our field is weak, even though this field was previously found to be rich in RR Lyrae variables. In both of these respects the halo of M31 at 9 kpc from the nucleus appears to resemble the stellar population found in the Galactic globular cluster NGC 6171. The resemblance between the mean properties of the M31 inner halo and NGC 6171 is further strengthened by the fact that the metallicity of NGC 6171 is [Fe/H] ~ -1.0-identical to that derived for M31. In addition the magnitude of the tip of the giant branch is M_V_ = -1.8 for both NGC 6171 and also for the dominant stellar population in the halo of M31. We conclude that there is an excellent Galactic counterpart to the average stellar population found in our M31 halo field, so that it is no longer necessary to postulate the existence of an unusual horizontal branch morphology to explain the large numbers of RR Lyrae stars found in M31. If the RR Lyrae variables in M31 resemble those in NGC 6171, then their colors are redder than assumed in an earlier paper. The effect of this would be to lower the true distance modules of M31 slightly to 24.23 +/- 0.15. A significant spread in abundance is observed in our halo field; from the width of the giant branch we estimate that σ_[Fe/H]_~ 0.3. This metallicity dispersion is smaller than predicted by simple one-zone chemical evolution models, but may be explained by infall of metal-enriched gas into the inner halo during the formation of M31. |
