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 \sim 34 kpc from the nucleus near the SW major axis .
This field is near the globular cluster G1 , and near one of the candidate tidal plumes identified by Ferguson et al .
( 2002 ) .
The F606W ( V ) and F814W ( I ) images were obtained in parallel with Space Telescope Imaging Spectrograph spectroscopy of G1 ( GO-9099 ) and total 7.11 hours of integration time — the deepest HST field in the outer disk of M31 to date , reaching to V \sim 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 ] \approx - 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 finding that some giants in the field have radial velocities close to that of the neutral hydrogen gas ( Reitzel , Guhathakurta , & Rich 2003 ) 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 .