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

TL;DR

This study provides a comprehensive analysis of the stellar populations in M31's outskirts, revealing distinct stream-like and disk-like features and their implications for galaxy formation and interaction history.

Contribution

It offers the largest detailed survey of M31's outskirts, classifies stellar populations based on CMD morphology, and links these features to galaxy interaction models.

Findings

Identification of two main stellar population types: stream-like and disk-like.

Good agreement between observed stream-like fields and simulation of the giant stream progenitor.

Detection of young populations and rotation in the extended disk out to 44 kpc.

Abstract

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.