We have conducted a wide-field CCD-mosaic study of the resolved red-giant branch stars of M31 , in a field located 20 kpc from the nucleus along the SE minor axis .
In our ( I,V - I ) color-magnitude diagram , red-giant branch ( RGB ) stars in the top three magnitudes of the M31 halo are strongly present .
We use photometry of a more distant control field to subtract field contamination and then to derive the “ cleaned ” luminosity function and metallicity distribution for this outer-halo region of M31 .
From the color distribution of the foreground Milky Way halo stars , we find a reddening E ( V - I ) = 0.10 \pm 0.02 for this field , and from the luminosity of the RGB tip , we determine a distance modulus ( m - M ) _ { o } = 24.47 \pm 0.12 ( = 783 \pm 43 kpc ) .
The metallicity distribution function ( MDF ) is derived from interpolation within an extensive new grid of RGB models ( Vandenberg et al .
2000 ) .
We find that the MDF is dominated by a moderately high-metallicity population ( [ m/H ] \sim - 0.5 ) that has previously been found in more interior M31 halo/bulge fields , and is very much more metal-rich than the [ m/H ] \sim - 1.5 level which characterizes the Milky Way halo .
In addition , a significant ( \sim 30 % - 40 % , depending on AGB star contribution ) metal-poor population is also present .
To first order , the total shape of the MDF resembles that predicted by a simple , single-component model of chemical evolution starting from primordial gas with an effective yield y = 0.0055 .
It strongly resembles the MDF recently found for the outer halo of the giant elliptical NGC 5128 ( Harris et al .
2000 ) , though NGC 5128 has an even lower fraction of low-metallicity stars .
Intriguingly , in both NGC 5128 and M31 , the metallicity distribution of the globular clusters in M31 does not match the halo stars , in the sense that the clusters are far more heavily weighted to metal-poor objects .
We suggest similarities in the formation and early evolution of massive , spheroidal stellar systems .