Within the context of the cold dark matter paradigm, structure formation proceeds hierarchically and galaxies like the Milky Way and Andromeda (M31) are predicted to arise from the merger and accretion of many smaller sub-systems as well as from the smooth accretion of intergalactic gas (e.g White & Rees 1978, White & Frenk 1991). Galaxy outskirts are of particular interest since the long dynamical timescales in these regions mean that coherent debris from past accretion events has the greatest longevity. The discovery of the Sagittarius dwarf galaxy and associated tidal stream (see Law & Majewski, this volume) demonstrated beyond a doubt that satellite accretion played an important role in the growth of the Milky Way's halo, but the veracity of this aspect of the hierarchical model could not rest solely on a single event observed within a single galaxy. In this spirit, the late 90s saw the quest begin to identify other galactic systems which were visibly in the process of devouring smaller satellites.
Our nearest giant neighbour, M31, provides the most obvious target for such studies. Lying at ∼780 kpc, it is in many respects a sister galaxy to the Milky Way. It has a similar total mass (e.g. Diaz et al. 2014, Veljanoski et al. 2014), is of a similar morphological type, and it resides in the same low-density environment – one which is deemed typical for much of the present-day galaxy population. M31's proximity means that individual stars near the tip of the red giant branch (RGB) can be resolved from the ground; this offers a powerful method for probing very low effective surface brightnesses, such as those expected for tidal debris streams. Furthermore, its high inclination to the line-of-sight (i ∼ 77∘) means that it is ideally suited for studies of its halo regions.
There are some disadvantages to studying tidal streams in M31 as opposed to our own Milky Way. Even with the world's largest telescopes, ground-based studies of M31's halo are limited to using luminous giant stars whereas Milky Way studies can harness the power of the much more numerous main sequence turn-off population (e.g. Belokurov et al. 2006). Additionally, in M31 we are mainly confined to analysis of projected positions on the sky (although occasionally some line-of-sight distance information is available), and we can only measure radial velocities. This can be contrasted with the situation in the Milky Way where it is possible to additionally determine line-of-sight distances 1 and proper motions, and thus probe the full six dimensional phase space. In effect, these differences imply that in M31 we are sensitive to tidal streams that are of higher surface brightness than those we can uncover in the Milky Way, and moreover typically only the subset retaining a high degree of spatial and/or kinematical coherence.
On the other hand, there are some clear advantages to studying an external system such as M31. Our vantage point largely alleviates complicated line-of-sight projection and extinction effects, such as must be endured in studies of the Milky Way. This means that we have a better understanding of the morphology of tidal features and where stellar substructures lie (at least in projection) with respect to each other, and with respect to additional halo tracers such as globular clusters (GCs) and dwarf satellites. Our bird's-eye view also makes it fairly straightforward to construct in situ samples of halo stars at various radii. Remarkably, we currently know more about the outer halo (R ≳ 50 kpc) of M31 than we do of the Milky Way. In the Milky Way, the outer halo is obscured by a dense veil of foreground stars making the robust identification of the low density population of outer halo stars difficult. In addition, while the main sequence turn-off method so extensively used by the Sloan Digital Sky Survey (SDSS) teams has probed out to distances of ∼ 40 kpc, such stars are not detected in SDSS imaging at distances beyond this. While some outer halo substructures have been uncovered using other tracers (e.g. Newberg et al. 2003, Watkins et al. 2009), the inhomogeneous and sparse nature of these studies precludes any meaningful conclusions about the global properties of the Milky Way's outer halo. In M31, we also have a far clearer view of the low latitude regions of the galaxy, enabling discrimination between perturbed disk features and accreted substructures.
Over the last 15 years, a multitude of studies have targeted the outskirts of M31; this chapter reviews the tremendous progress and exciting results that have emerged from this work.
1 In practice, individual distances to large samples of stars in the Milky Way are crude at the moment. ESA's Gaia mission will change this when it starts to deliver data in 2016 but the most accurate distances will be limited to stars within roughly 10 kpc of the Sun. Back.