  We present the largest and most detailed survey to date of the stellar
populations in the outskirts of M31 based on the analysis of 14 deep HST/ACS
pointings spanning the range 11.5-45.0 kpc. We conduct a quantitative
comparison of the resolved stellar populations in these fields and identify
several striking trends. The color-magnitude diagrams (CMDs), which reach ~3
magnitudes below the red clump, can be classified into two main categories
based on their morphologies. `Stream-like' fields, so named for their
similarity to the CMD of the giant stellar stream, are characterized by a red
clump that slants bluewards at fainter magnitudes and an extended horizontal
branch. On the other hand, `disk-like' fields exhibit rounder red clumps with
significant luminosity width, lack an obvious horizontal branch and show
evidence for recent star formation (~0.25 - 2.0 Gyr ago). We compare the
spatial and line-of-sight distribution of stream-like fields with a recent
simulation of the giant stream progenitor orbit and find an excellent
agreement. These fields, found across the face of M31, attest to the high
degree of pollution caused by this event. Disk-like material resides in the
extended disk structure of M31 and is detected out to 44 kpc (projected); the
uniform populations in these fields, including the ubiquitous presence of young
populations, and the strong rotation reported elsewhere are most consistent
with a scenario in which this structure has formed through heating and
disruption of the existing thin disk, perhaps due to the impact of the giant
stream progenitor. Our comparative analysis sheds new light on the likely
composition of two of the ultra-deep pointings formerly presented as pure outer
disk and pure halo in the literature.

Astrophysics