Deep [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] 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 (∼ V ) and F814W (∼ 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 Population II horizontal branch. There is also the hint of a ∼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 ± 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 ≳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.