We present the largest and most detailed survey to date of the stellar populations in the outskirts of M31 based on the homogeneous analysis of 14 deep HST/ACS pointings spanning the range 11.5 kpc \lesssim R _ { proj } \lesssim 45 kpc .
Many of these pointings sample coherent substructure discovered in the course of the INT/WFC imaging survey of M31 while others sample more diffuse structure in the extended disk .
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 \gtrsim 3 magnitudes below the red clump with high completeness , 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 .
They show no evidence for young populations .
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 ( \sim 0.25 - 2 ~ { } { 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 are found across much of the inner halo of M31 , and 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 here up to { R _ { proj } } \sim 44 ~ { } { kpc } ; 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 .