Major Substructure in the M31 Outer Halo: the South-West Cloud

Abstract

We undertake the first detailed analysis of the stellar population and spatial properties of a diffuse substructure in the outer halo of M31. The South-West Cloud lies at a projected distance of ~100 kpc from the centre of M31, and extends for at least ~50 kpc in projection. We use Pan-Andromeda Archaeological Survey photometry of red giant branch stars to determine a distance to the South-West Cloud of 793 +/- 45 kpc. The metallicity of the cloud is found to be [Fe/H] = -1.3 +/- 0.1. This is consistent with the coincident globular clusters PAndAS-7 and PAndAS-8, which have metallicities determined using an independent technique of [Fe/H] = -1.35 +/- 0.15. We measure a brightness for the Cloud of M_V = -12.1 mag; this is ~75 per cent of the luminosity implied by the luminosity-metallicity relation. Under the assumption that the South-West Cloud is the visible remnant of an accreted dwarf satellite, this suggests that the progenitor object was amongst M31's brightest dwarf galaxies prior to disruption.

13 pages, 9 figures, accepted for publication in MNRAS

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• The outer halo globular cluster system of M31 - I. The final PAndAS catalogue
• The outer halo globular cluster system of M31 - II. Kinematics
• The Nature and Origin of Substructure in the Outskirts of M31 -- II. Detailed Star Formation Histories
• Kinematics and simulations of the stellar stream in the halo of the Umbrella Galaxy
• A detailed study of Andromeda XIX, an extreme local analogue of ultra diffuse galaxies
• The outer halo globular cluster system of M31 - III. Relationship to the stellar halo
• Fermi-LAT Observations of Gamma-Ray Emission Towards the Outer Halo of M31
• Major Substructure in the M31 Outer Halo: Distances and Metallicities along the Giant Stellar Stream
• Integrated Light Chemical Tagging Analyses of Seven M31 Outer Halo Globular Clusters from the Pan-Andromeda Archaeological Survey
• Substructure and Tidal Streams in the Andromeda Galaxy and its Satellites
• Accretion in action: phase space coherence of stellar debris and globular clusters in Andromeda's South-West Cloud
• Optical Spectroscopy of Supernova Remnants in M81 and M82
• Major Substructure in the M31 Outer Halo: the East Cloud
• A Search for Intergalactic Globular Clusters in the Local Group
• A Dwarf Disrupting -- Andromeda XXVII and the North West Stream
• Metallicities of Outer Halo M31 Globular Clusters from Integrated Light Calcium-II Triplet Spectroscopy
• COSTA: the COld STream finder Algorithm. Searching for kinematical substructures in the phase space of discrete tracers