Deep Hubble Space Telescope WFPC2 photometry of M31's thick disk (?)

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 (similar toV) and F814W (similar toI) 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 Population II horizontal branch. There is also the hint of a similar to2 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 +/- 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) is not sufficient to account for this shape. Instead, an additional Gaussian component with [[Fe/H]] = -0.22 +/- 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 greater than or similar to1.5 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.