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Evidence for Stellar Substructure in the Halo and Outer Disk of M31

We report the discovery of significant stellar substructure in the halo and outer disk of our nearest large galactic neighbour, M31. Our deep panoramic survey with the Isaac Newton Telescope Wide Field Camera currently maps out an area of ~25 square degrees around M31, extending along the semi-major axis to 55 kpc, and is the first to allow an uninterrupted study of the density and color distribution of individual red giant branch stars across a large fraction of the halo of an external spiral galaxy. We find evidence for both spatial density and metallicity (as inferred from colour information) variations, which are often, but not always, correlated. In addition to the previously reported giant stellar stream (Ibata 2001b), the data reveal the presence of significant stellar overdensities at large radii close to the south-western major axis, in the proximity of the very luminous globular cluster G1, and near the north-eastern major axis, coinciding with and extending beyond the previously-known `northern spur'. The most prominent metallicity variations are found in the southern half of the halo, where two large structures with above average metallicites are apparent; one of these coincides with the giant stellar stream while the other corresponds to a much lower-level stellar enhancement. Our findings contrast with, but do not conflict with, past studies of the M31 halo and outer disk which have suggested a rather homogeneous stellar population at large radius: the bulk of our newly-detected substructure lies in the previously-uncharted far outer regions of the galaxy. We discuss the possible origin of the substructure observed and the implications it has for constraining the galaxy assembly process.