We present deep color-magnitude diagrams ( CMDs ) for a field along the outer disk of M31 based on archival Hubble Space Telescope Wide Field Planetary Camera 2 observations in the F555W ( \sim V ) and F814W ( \sim I ) filters .
The CMDs , which contain a total of about 50,000 stars , feature a prominent red giant branch ( RGB ) along with a significant population of helium burning red clump stars .
In addition , they exhibit the rarely seen asymptotic giant branch clump as well as a weak ‘ Pop II ’ horizontal branch .
There is also the hint of a \sim 2 Gyr subgiant branch at the faintest levels of the CMDs .
After adopting an M31 distance of ( m - M ) _ { 0 } = 24.5 and a reddening of E ( B - V ) = 0.08 , we draw the following conclusions .
1 ) The I-band absolute magnitude of the helium burning red clump stars is M _ { I } ( RC ) = -0.29 \pm 0.05 , which is in accord with the value derived from Hipparcos parallaxes of solar neighborhood clump stars by Stanek & Garnavich .
2 ) The metallicity distribution function constructed from bright RGB stars shows a characteristic shape ; however , a pure halo population consisting of metal-poor and intermediate metallicity components ( as advocated in the literature ) are not sufficient to account for this shape .
Instead , an additional Gaussian component with \langle [ Fe / H ] \rangle = -0.22 \pm 0.26 , comprising 70 % of the total number of stars , is required .
3 ) A comparison of our CMD with theoretical isochrones indicates that the majority of stars in our M31 field have ages that are \mathrel { > \kern - 10.0 pt \lower 3.87 pt \hbox { $ \sim$ } } 1.5 Gyr .
4 ) These points , along with the physical location of our field in M31 , suggest that we are observing the thick disk population of this galaxy .