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

2013; Oxford University Press; Volume: 437; Issue: 4 Linguagem: Inglês

10.1093/mnras/stt2139

1365-2966

N. F. Bate, A. R. Conn, B. McMonigal, Geraint F. Lewis, Nicolas F. Martin, Alan W. McConnachie, J. Veljanoski, Dougal Mackey, A. M. N. Ferguson, Rodrigo Ibata, M. J. Irwin, Mark A. Fardal, Avon Huxor, Arif Babul,

Astronomy and Astrophysical Research

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 |$793^{+45}_{-45}$| 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 MV = −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.