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Figs. 2 and 3 show maps from the PAndAS survey of RGB stars in the inner and outer halo regions of M31, displayed to highlight prominent substructures. Additionally, Fig. 4 shows the distribution of RGB stars in four different metallicity bins, revealing how the morphology of the tidal debris changes as a function of metallicity. A striking feature of all of these visualisations is the non-uniformity of the stellar distribution in the outer regions. The most metal-poor map presented in Fig. 4 has a smoother appearance than the others, but it still exhibits a substantial degree of substructure.

Figure 2

Figure 2. The PAndAS map of metal-rich RGB stars in the inner halo of M31, upon which a typical textbook image of M31 is superposed. The map is constructed from stars with i0 ≤ 23.5, having −1 ≤ [Fe/H] ≤ 0. The large white ellipse has a semi-major axis of 27 kpc and delineates the full extent of the bright disk; the dashed blue circle has a radius of 50 kpc. Prominent inner halo substructure is outlined and labelled, as are the dwarf satellites M32 and NGC 205.

The inner halo (R < 50 kpc) of M31 appears as a flattened structure (axis ratio ∼ 0.5) in Fig. 2, around the edge of which bright tidal features (e.g. streams, clumps, spurs, shelves) can be seen. In the outer halo (R ≥ 50 kpc), the most prominent features are a multitude of faint narrow streams and arcs. Based on their appearance in Fig. 4, these outer streams are also considerably more metal-poor than the substructures which dominate the inner halo. Lewis et al. (2013) searched for a correlation between the tidal structures seen in stars, and features in the H i gas around M31. Interestingly, they found a general lack of spatial correlation between these two components on all scales, with very few potential overlaps.

Figure 3

Figure 3. A map of metal-poor RGB stars across the full extent of the PAndAS survey (i0 ≤ 23.5 and −2.5 ≤ [Fe/H] ≤ −1.1). The white ellipse is the same as in Fig. 2 while the dashed circles surrounding M31 have radii of 50 kpc (yellow) and 150 kpc (blue) respectively and that surrounding M33 has a radius of 50 kpc. Prominent outer halo substructure is outlined and labelled, as are tidal streams associated with the satellite galaxies M33 and NGC 147.

A brief description of some of the most prominent tidal features seen around M31 is given below:

Figure 4

Figure 4. The PAndAS map of RGB stars, as in Fig. 3, but presented this time as a function of metallicity. The two high metallicity bins (top panels) are dominated by the Giant Stellar Stream, although note that this structure changes morphology slightly between the two panels. While the low metallicity bottom left panel is dominated by numerous streams, the more metal-poor right-hand panel appears much smoother. Reproduced from Ibata et al. (2014).

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