4. MAJOR TIDAL FEATURES IN THE HALO OF M31

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. 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. 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:

• Giant Stellar Stream (GSS): Discovered in the first quadrant of M31 that was mapped by the INT/WFC survey, the GSS is the most prominent overdensity in M31's halo and covers a large fraction of its south-east quadrant (Ibata et al. 2001). It can be traced as a coherent structure to a projected galactocentric radius of ∼ 100 kpc, and spans a width of ∼ 25 kpc. The stream has a linear morphology with a sharp eastern edge and an estimated absolute V-band magnitude of M_V ≈ −14 (Ibata et al. 2001). However, this is a crude estimate based on only that part of the stream which is visible in the earliest INT maps. As the stream is now known to be more than twice as long as this, and other debris features have been identified as forward wraps of the structure, the total luminosity of the GSS could easily be 1–2 magnitudes higher. Ibata et al. (2007) show that there is a large-scale stellar population gradient present, with the high surface brightness core region of the stream having relatively more metal-rich stars than the peripheral regions. Both photometric and spectroscopic studies reveal the core stream to have a moderately high metallicity of [Fe/H] ≥ −0.5 to −0.7, with the envelope dropping to [Fe/H] ∼ −1.4 (Guhathakurta et al. 2006, Ibata et al. 2007, Gilbert et al. 2009.
• G1 Clump: This feature was first recognised in the INT map published by Ferguson et al. (2002) and appears as a rather round clump of stars located at a projected radius of ∼ 30 kpc along the south-western major axis of M31. It has dimensions of 0.5^∘ × 0.7^∘, or 7 × 10 kpc at the distance of M31. Ferguson et al. (2002) estimate an absolute magnitude of M_V ≈ −12.6 and a V-band surface brightness of ≈ 28.5 mag arcsec⁻². The feature was originally named because the luminous M31 globular cluster G1 lies nearby. This star cluster is notable because it has been argued to have both an internal metallicity spread as well as an intermediate-mass black hole, characteristics that suggest it could be the remnant core of a nucleated dwarf elliptical galaxy (e.g. Meylan et al. 2001, Gebhardt et al. 2005). While the detection of tidal debris in the vicinity of this enigmatic object was very exciting, subsequent observations of the properties of stars in the G1 Clump appear to rule out any association between the two (Rich et al. 2004, Reitzel et al. 2004, Ibata et al. 2005, Faria et al. 2007.
• North-East Clump (NE Clump): Located at a projected radius of ∼ 40 kpc and near the north-eastern major axis, this substructure is one of the most nebulous features in the inner halo of M31. It subtends a diameter of ∼ 1^∘ or ∼ 14 kpc at the distance of M31, and appears to connect to the main body of the galaxy by a faint filament. It is estimated to have an absolute g-band magnitude of M_g ≈ −11.6 and a g-band surface brightness of ≈ 29.0 mag arcsec⁻² (Zucker et al. 2004a). Although it was initially suggested that the NE Clump was a disrupting dwarf satellite, subsequent observations have disfavoured this interpretation (Ibata et al. 2005, Richardson et al. 2008, Bernard et al. 2015a), since the stellar populations are more representative of the disk.
• North-Eastern (NE) and Western (W) Shelves: The NE Shelf is a diffuse but fairly sharp-edged extension lying north-east of M31's center, while the Western Shelf is a fainter feature of similar morphology and size on the opposite side of the galaxy. On the basis of their comparably high metallicities, Ferguson et al. (2002) suggested the NE Shelf could be an extension of the GSS. Using inferences from N-body simulations, Fardal et al. (2007) argued that both the NE and W Shelves were forward continuations of the stream, representing material stripped off during successive pericentric passages.
• Streams B, C, D: Identified by Ibata et al. (2007), Streams B–D are a series of approximately parallel tangential streams which cross the southern minor axis of M31 inside a radius of 100 kpc. Their metallicities are in the range −1.5 ≤ [Fe/H] ≤ −0.5. The eastern portions of Streams C and D appear to overlap in projection and all of the streams seem to terminate, or at least dramatically fade, once they reach the GSS. Stream C has been shown to be particularly complex, consisting of two distinct (but overlapping) metallicity and kinematic components (Chapman et al. 2008, Gilbert et al. 2009). Along with the GSS, the metal-rich component of Stream C is the only other outer halo feature visible in the most metal-rich maps of Fig. 4.
• Far Outer Halo Streams (R ≳ 100 kpc): Figs. 3 and 4 show that the outer halo is littered with various faint streams and clumps, including Stream A, the North-West (NW) Stream, the South-West (SW) Cloud))) and the (((Eastern (E) Cloud (Ibata et al. 2007, McConnachie et al. 2009, Richardson et al. 2011, Carlberg et al. 2011, Bate et al. 2014). With µ_V ≥ 31.5 mag arcsec⁻², these streams represent the faintest spatially-coherent debris yet identified around M31. Stream A crosses the minor axis at a projected radius of ∼ 120 kpc and aligns with the inner network of tangential streams. The E and SW Clouds appear as stellar arcs at slightly smaller radii on either side of Stream A. The NW Stream is a long (∼100 kpc) and narrow (∼3 kpc) radial feature; although there is no visible connection between the upper and lower branches of the stream, the fact that both trace out segments of a single ellipse and have a similar metallicity has supported the notion they are related (Carlberg et al. 2011). Although part of the NW Stream was also seen in the deep pencil beam study of Tanaka et al. (2010) (their (Stream F), their Stream E was not recovered in the PAndAS survey.

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).