Major substructure in the M31 outer halo: the South-West Cloud

• Published in: Monthly notices of the Royal Astronomical Society 2014-02, Vol.437 (4), p.3362-3372
• Main Authors: Bate, N. F., Conn, A. R., McMonigal, B., Lewis, G. F., Martin, N. F., McConnachie, A. W., Veljanoski, J., Mackey, A. D., Ferguson, A. M. N., Ibata, R. A., Irwin, M. J., Fardal, M., Huxor, A. P., Babul, A.
• Format: Article
• Language: English
• Subjects: Astronomy; Dwarf stars; Luminosity; Star & galaxy formation
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stt2139

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  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.