We present a deep photometric survey of the Andromeda galaxy, conducted with the wide-field cameras of CFHT and INT, that covers the inner 50 kpc of the galaxy and the southern quadrant out to $150 kpc and includes an extension to M33 at >200 kpc. This is the first systematic panoramic study of this very outermost region of galaxies. We detect a multitude of large-scale structures of low surface brightness, including several streams, and two new relatively luminous (M V $ À9) dwarf galaxies: And XV and And XVI. Significant variations in stellar populations due to intervening streamYlike structures are detected in the inner halo, which is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Underlying the many substructures lies a faint, smooth, and extremely extended halo component, reaching out to 150 kpc, whose stellar populations are predominantly metal-poor. We find that the smooth halo component in M31 has a radially decreasing profile that can be fitted with a Hernquist model of immense scale radius $55 kpc, almost 4 times larger than theoretical predictions. Alternatively a power law with AE V / R À1:91AE0:11 can be fitted to the projected profile, similar to the density profile in the Milky Way. If it is symmetric, the total luminosity of this structure is $10 9 L , again similar to the stellar halo of the Milky Way. This vast, smooth, underlying halo is reminiscent of a classical ''monolithic'' model and completely unexpected from modern galaxy formation models. M33 is also found to have an extended metal-poor halo component, which can be fitted with a Hernquist model also of scale radius $55 kpc. These extended slowly decreasing halos will provide a challenge and strong constraints for further modeling.

The Milky Way is a cannibal. Recent observations have revealed the detritus of its digestion of the Sagittarius Dwarf and Magellanic Clouds, apparent as streams of gaseous and stellar debris littering the Galactic Halo ? , ? , ? . The analysis of this material has shown that the dark matter distribution within our own Galaxy is nearly spherical ? , and that much of the Galactic Halo is made up of cannibalized satellite galaxies ? , ? . Yet it remains unclear whether such halo substructures really are as ubiquitous and as numerous as predicted by galaxy formation theory ? . Here we report the discovery of a giant stream of metal-rich stars within the halo of the Andromeda galaxy (M31), the Milky Way's nearest large neighbour. The source of this stream could be the dwarf galaxies M32 and NGC 205, close companions of M31, which have possibly lost a substantial amount of stars due to their tidal interactions with their large neighbour. These observations clearly demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that substructure in the form of huge, recently-deposited tidal streams, may be a generic feature of galaxy halos.Within the framework of hierarchical structure formation, large spiral galaxies like the Milky Way or Andromeda arose from the merger of many small galaxies and proto-galaxies ? . Later in their evolution, spiral galaxies become the dominant component in such mergers, digesting smaller systems that fall within their sphere of influence. The complete destruction of the victim is usually progressive and may take several orbits. However, the stellar debris from the destroyed dwarf galaxy follows a similar orbital trajectory to the progenitor, which preferentially will have started life far away from the place of its final demise, and so the tidally disrupted matter tends to be deposited over a broad range in distance from the larger galaxy. Over time, with the accumulation of many such mergers, large galaxies develop an extensive stellar and dark matter "halo", the latter being by far the most massive component of the galaxy. Meanwhile, part of the (dissipative) gas component of the smaller galaxies feeds the growth of the disk of the larger galaxy. This is clearly seen in numerical simulations of galaxy formation, which result in galactic halos comprised of clumps of dark matter ? .Is this prediction of 'dotty cosmology' correct, with halos possessing significant substructure, or are the dark and luminous components of halos distributed smoothly, as had previously been suggested ? ?The majority of the Milky Way halo is metal-poor and, to first order, smoothly distributed, however, recent studies have demonstrated that the halo contains non-negligible stellar substructure. Evidence for the phase-space clumping of halo stars has even been found in the Solar Neighbourhood, where ∼ 10% of stars may be associated to a single ancient accretion event ? . The discovery that the Milky Way is surrounded by a giant rosette-like stream originating from the Sagittarius d...

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.Comment: 20 pages, 10 figures. Accepted for publication in AJ (Sep 2002). Version with high resolution embedded figures available at http://www.astro.rug.nl/~ferguson/m31.ps.g

Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way. It has previously been suspected that dwarf galaxies may not be isotropically distributed around our Galaxy, because several are correlated with streams of H I emission, and may form coplanar groups. These suspicions are supported by recent analyses. It has been claimed that the apparently planar distribution of satellites is not predicted within standard cosmology, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion. Here we report the existence of a planar subgroup of satellites in the Andromeda galaxy (M 31), comprising about half of the population. The structure is at least 400 kiloparsecs in diameter, but also extremely thin, with a perpendicular scatter of less than 14.1 kiloparsecs. Radial velocity measurements reveal that the satellites in this structure have the same sense of rotation about their host. This shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way's disk and with the vector between the Milky Way and Andromeda.