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

We report the discovery of significant stellar substructure in the halo andouter disk of our nearest large galactic neighbour, M31. Our deep panoramicsurvey with the Isaac Newton Telescope Wide Field Camera currently maps out anarea of ~25 square degrees around M31, extending along the semi-major axis to55 kpc, and is the first to allow an uninterrupted study of the density andcolor distribution of individual red giant branch stars across a large fractionof the halo of an external spiral galaxy. We find evidence for both spatialdensity and metallicity (as inferred from colour information) variations, whichare often, but not always, correlated. In addition to the previously reportedgiant stellar stream (Ibata 2001b), the data reveal the presence of significantstellar overdensities at large radii close to the south-western major axis, inthe proximity of the very luminous globular cluster G1, and near thenorth-eastern major axis, coinciding with and extending beyond thepreviously-known `northern spur'. The most prominent metallicity variations arefound in the southern half of the halo, where two large structures with aboveaverage metallicites are apparent; one of these coincides with the giantstellar stream while the other corresponds to a much lower-level stellarenhancement. Our findings contrast with, but do not conflict with, past studiesof the M31 halo and outer disk which have suggested a rather homogeneousstellar population at large radius: the bulk of our newly-detected substructurelies in the previously-uncharted far outer regions of the galaxy. We discussthe possible origin of the substructure observed and the implications it hasfor constraining the galaxy assembly process.