  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.
