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We present the largest and most detailed survey to date of the stellarpopulations in the outskirts of M31 based on the analysis of 14 deep HST/ACSpointings spanning the range 11.5-45.0 kpc. We conduct a quantitativecomparison of the resolved stellar populations in these fields and identifyseveral striking trends. The color-magnitude diagrams (CMDs), which reach ~3magnitudes below the red clump, can be classified into two main categoriesbased on their morphologies. `Stream-like' fields, so named for theirsimilarity to the CMD of the giant stellar stream, are characterized by a redclump that slants bluewards at fainter magnitudes and an extended horizontalbranch. On the other hand, `disk-like' fields exhibit rounder red clumps withsignificant luminosity width, lack an obvious horizontal branch and showevidence for recent star formation (~0.25 - 2.0 Gyr ago). We compare thespatial and line-of-sight distribution of stream-like fields with a recentsimulation of the giant stream progenitor orbit and find an excellentagreement. These fields, found across the face of M31, attest to the highdegree of pollution caused by this event. Disk-like material resides in theextended disk structure of M31 and is detected out to 44 kpc (projected); theuniform populations in these fields, including the ubiquitous presence of youngpopulations, and the strong rotation reported elsewhere are most consistentwith a scenario in which this structure has formed through heating anddisruption of the existing thin disk, perhaps due to the impact of the giantstream progenitor. Our comparative analysis sheds new light on the likelycomposition of two of the ultra-deep pointings formerly presented as pure outerdisk and pure halo in the literature.

The Nature and Origin of Substructure in the Outskirts of M31. I. Surveying the Stellar Content with HST/ACS