Deep Photometry in a Remote M31 Major-Axis Field Near G1

We present photometry from Hubble Space Telescope (HST) Wide Field Planetary Camera 2 parallel imagery of a remote M31 field at a projected distance of ∼34 kpc from the nucleus near the southwest major axis. This field is near the globular cluster G1, and near one of the candidate tidal plumes identified in the survey of Ferguson et al. (2002). The F606W (V) and F814W (I) images were obtained in parallel with STIS spectroscopy of G1 (GO-9099) and total 7.11 hr of integration time—the deepest HST field in the outer disk of M31 to date, reaching to V ∼ 28. The color-magnitude diagram of the field shows a clearly defined red clump at V = 25.25 and a red giant branch consistent with [Fe/H] ≈ -0.7. The lack of a blue horizontal branch contrasts with other M31 halo fields, the Andromeda dwarf spheroidals, and with the nearby globular cluster G1. Comparing the observed luminosity function to the Padova models, we find that at least some of the stellar population must be younger than 6–8 Gyr. The outermost detected neutral hydrogen disk of M31 lies only 2 kpc in projection from our field. The recent finding by Reitzel, Guhathakurta, & Rich that some giants in the field have radial velocities close to that of the neutral hydrogen gas leads us to conclude that our field samples the old, low surface brightness disk rather than the true Population II spheroid or the remnants of a disrupted M31 satellite.