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2. LOCAL GROUP

The galaxies closest to us give us the most detailed information because of the large number of stars that can be resolved. Here, I will summarize what we have learnt in the past two decades about our own Galaxy (even though an extensive picture of the MW outskirts goes beyond the scope of this contribution and can be found in Figueras, this volume), about its closest spiral neighbour M31 and about their lower-mass satellites.

2.1. Milky Way

The MW is traditionally divided into discrete components, i.e., the bulge, the disks (thin and thick) and the halo. The spheroidal portion of the MW is given by the central bulge, which consists mainly of metal-rich populations, and an extended diffuse component, which has a lower mean metallicity. Overall, stars and GCs in the halo have ages ∼ 11−13 Gyr (Carollo et al 2007). The halo can be further deconstructed into an inner halo and an outer halo, even though the distinction could partly arise from observational biases (Schönrich et al 2014). The inner and outer haloes also seem to have different chemical composition ([Fe/H] ∼ −1.6 and [Fe/H] ∼ −2.2, respectively; Ryan and Norris 1991, Carollo et al 2007). According to simulations, the two halo components should also have formed on different timescales: the inner halo (< 20 kpc) is partly constituted by early-formed in-situ stars, partly due to a violent relaxation process, and partly assembled from early, massive merging events that provide metal-rich populations (Abadi et al 2006, Font et al 2011, Tissera et al 2013, Pillepich et al 2014, Cooper et al 2015); the outer halo is assembled more recently, with its mass beyond ∼ 30 kpc being mainly accreted in the past ∼ 8 Gyr (Bullock and Johnston 2005, Cooper et al 2010). These predictions are, however, still not sufficient at a quantitative level, and unconstrained as to the exact ratio of accreted stars versus in-situ populations. At the same time, observations of the MW halo with better statistics and precision are needed to inform them.

Our position within the MW puts us at a clear disadvantage for global studies of its outskirts: the distant and sparse halo stars are observed from within the substantial disk component, which produces contamination both in terms of extinction and numerous disk stars along the line of sight, which completely “obscure” the sky at low Galactic latitudes. Nonetheless, thanks to the advent of wide-field imagers, the past two decades have revolutionized the large scale view of our Galaxy. Several stellar tracers can be used to dig into the MW halo at different distance ranges: old main sequence turnoff (MSTO) stars are identified mostly out to ∼ 20 kpc, brighter RGB stars out to ∼ 40−50 kpc, while RR Lyrae and blue horizontal branch (BHB) stars can be detected out to 100 kpc. Spatial clustering of these stellar components indicate non-mixed substructure, which is often confirmed to be kinematically coherent.

2.1.1. The Emergence of Streams

After the cornerstone discovery of the disrupting Sagittarius dwarf, it became clear that substructure is not only present in the MW halo, but it also might constitute a big portion of it. To put it in S. Majewski's words (Majewski 1999),

“There is good reason to believe that within a decade we will have a firm handle on the contribution of satellite mergers in the formation of the halo, as we move observationally from serendipitous discoveries of circumstantial evidence to more systematic surveys for the fossils left behind by the accretion process” .

In the following decade, several stream-like features have indeed emerged from a variety of multi-band photometric and spectroscopic surveys indeed, and the Sloan Digital Sky Survey (SDSS) proved to be an especially prolific mine for such discoveries around the northern Galactic cap. The Sagittarius stream has been traced further, including in the Galactic anti-centre direction (e.g., Mateo 1998, Majewski et al 2003), and independent substructures have been uncovered (Ivezić et al 2000, Yanny et al 2000, Newberg et al 2002, Grillmair 2006, Jurić et al 2008), most notably the Monoceros ring, the Virgo overdensity, the Orphan stream and the Hercules-Aquila cloud. Some of these have later been confirmed to be coherent with radial velocities (Duffau et al 2006). Note that most of these substructures are discovered at Galactocentric distances > 15 kpc, while the inner halo is smooth due to its shorter dynamical timescales.

During the past decade, one of the most stunning vizualisations of the ongoing accretion events in the MW halo was provided by the Field of Streams (Belokurov et al 2006), reproduced in Fig. 1. The stunning stellar density map is derived from SDSS data of stars around the old MSTO at the distance of Sagittarius, with a range of magnitudes to account for a range in distances. This map not only shows the Sagittarius stream and its distance gradient, but also a plethora of less massive streams, as well as an abundance of previously unknown dwarf satellites (see Sect. 2.1.3). The Field of Streams has been now complemented with results from the latest state-of-the-art surveys, most notably the all-sky Panoramic Survey Telescope and Rapid Response System (PanSTARRS), which covers an area significantly larger than that of SDSS. In Fig. 2 the first stellar density maps from PanSTARRS are shown, obtained in a similar way as the Field of Streams (Bernard et al 2016). The map highlights the fact that the deeper and wider we look at the Galaxy halo, the more substructures can be uncovered and used to constrain its past accretion history and the underlying DM halo properties. From this kind of maps, for example, the halo stellar mass that lies in substructures can be estimated, amounting to ∼ 2−3 × 108 M (see Belokurov 2013). Using SDSS, Bell et al (2008) also highlight the predominant role of accretion in the formation of the MW's halo based on MSTO star counts, adding to up to ∼ 40% of the total halo stellar mass (note that, however, different tracers could indicate much smaller values; e.g., Deason et al 2011).

Figure 1

Figure 1. Spatial density of SDSS stars around the Galactic cap, binned in 0.5 × 0.5 deg2; the colour scale is such that blue indicates the nearest stars while red is for the furthest ones. Labelled are the main halo substructures, which are in some cases streams associated with a GC or a dwarf galaxy; the circles show some newly discovered dwarf satellites of the MW. Plot adapted from Belokurov et al (2006) (http://www.ast.cam.ac.uk/~vasily/sdss/field_of_streams/dr6/)

Figure 2

Figure 2. Stellar density maps of the whole PanSTARRS footprint, obtained by selecting MSTO stars at a range of heliocentric distances (as indicated in each panel). The map is on a logarithmic scale, with darker areas indicating higher stellar densities. The many substructures are highlighted in each panel. Reproduced from Bernard et al (2016), their Fig. 1, with permission of MNRAS

Many of the known halo streams arise from tidally disrupting GCs, of which Palomar 5 is one of the most obvious examples (Odenkirchen et al 2001). This demonstrates the possible role of GCs, besides dwarf satellites, in building up the halo stellar population, and additionally implies that some of the halo GCs may be stripped remnants of nucleated accreted satellites (see Freeman and Bland-Hawthorn 2002, and references therein). In order to discern between a dwarf or a cluster origin of halo stars, we need to perform chemical “tagging”, i.e., obtain spectroscopic abundances for tens of elements for these stars (e.g., Martell and Grebel 2010): stars born within the same molecular cloud will retain the same chemical composition and allow us to trace the properties of their birthplace. A number of ambitious ongoing and upcoming spectroscopic surveys (SEGUE, APOGEE, Gaia-ESO, GALAH, WEAVE, 4MOST) is paving the path for this promising research line, even though theoretical models still struggle to provide robust predictions for the fraction of GC stars lost to the MW halo (e.g., Schiavon et al 2016, and references therein).

2.1.2. The Smooth Halo Component

Once the substructure in the halo is detected, it is important that it is “cut out” in order to gain insights into the smooth, in-situ stellar component (note that, however, the latter will inevitably suffer from residual contamination from accreted material that is now fully dissolved). The stellar profile of the Galactic halo is, in fact, not smooth at all: several studies have found a break at a radius ∼ 25 kpc, with a marked steepening beyond this value (Watkins et al 2009, Sesar et al 2013), in qualitative agreement with halo formation models. Some of the explanations put forward suggest that a density break in the halo stellar profile is the likely consequence of a massive accretion event, corresponding to the apocentre of the involved stars (Deason et al 2013).

The kinematics of halo stars, of GCs and of satellite galaxies, as well as the spatial distribution of streams and tidal features in satellites, can be further used as mass tracers for the DM halo. The total MW mass is to date still poorly constrained, given the difficulty of evaluating it with a broad range of different tracers. The general consensus is for a virial mass value of ∼ 1.3 ± 0.3 × 1012 M, even though values discrepant up to a factor of two have recently been suggested (see Bland-Hawthorn and Gerhard 2016 for a compilation of estimates). Besides providing estimates for the total MW mass, studies of SDSS kinematical data, of the Sagittarius stream and of GCs tidal streams have provided discording conclusions on the shape of the MW DM halo: nearly spherical from the modelling of streams or strongly oblate from SDSS kinematics at Galactocentric distances < 20 kpc, while nearly spherical and oblate based on stream geometry or prolate from kinematical arguments for distances as large as ∼ 100 kpc (see Bland-Hawthorn and Gerhard 2016 for details). These constraints need a substantial improvement in the future to be able to inform cosmological models: the latter predict spherical/oblate shapes once baryons are included in DM-only flattened haloes (see Read 2014).

2.1.3. Dwarf Satellites

As mentioned above, the SDSS has revolutionized our notions of dwarf satellites of the MW. Bright enough to be easily recognized on photographic plates, a dozen “classical” MW dwarf satellites has been known for many decades before the advent of wide-field surveys (Mateo 1998, Grebel et al 2000). Starting with the SDSS, an entirely new class of objects has started to emerge with properties intermediate between the classical dwarfs and GCs (see Willman 2010, and references therein). The so-called ultra-faint satellites have magnitudes higher than MV ∼ −8 and surface brightness values so low that the only way to find them is to look for spatial overdensities of resolved main sequence/BHB stars. Their discovery ten years ago doubled the number of known MW satellites and revealed the most DM-dominated galaxies in the Universe, with mass-to-light ratios of up to several times 103 M / L (Simon and Geha 2007).

More recently, the interest in the low end of the galaxy LF has been revitalized once again with deep, wide-field surveys performed with CTIO/DECam, VST/Omegacam, and PanSTARRS: these have led to the discovery of more than 20 southern dwarfs in less than two years (Bechtol et al 2015, Koposov et al 2015, Kim et al 2015, Drlica-Wagner et al 2015, Torrealba et al 2016, and references therein). Some of these discoveries represent extremes in the properties of MW satellites, with surface brightness values as low as ∼ 30 mag arcsec−2, total luminosities of only a few hundred L and surprisingly low stellar density regimes. One of the perhaps most intriguing properties of the newly discovered dwarfs is that many of them appear to be clustered around the Large Magellanic Cloud (LMC): this might be the smoking gun for the possible infall of a group of dwarfs onto the MW, which is predicted by simulations (D'Onghia and Lake 2008, Sales et al 2016). Low-mass galaxies are expected to have satellites on their own and to provide a large fraction of a giant galaxy's dwarf companions (e.g., Wetzel et al 2015). The properties of the possible LMC satellites will give us a glimpse onto the conditions of galaxy formation and evolution in an environment much different from the LG as we know it today.

These faintest galaxies, or their accreted and fully dispersed counterparts, are also excellent testbeds to look for the very most metal-poor stars and to investigate the star formation process in the early stages of the Universe (e.g., Frebel and Norris 2015). The study of the lowest mass galaxies holds the promise to challenge our knowledge of galaxy physics even further and pushes us to explore unexpected and exciting new limits.

2.2. M31 (Andromeda)

Our nearest giant neighbour has received growing attention in the past decade. Having a remarkable resemblance with the MW and a comparable mass (e.g., Veljanoski et al 2014), it is a natural ground of comparison for the study of spiral haloes. In terms of a global perspective, the M31 halo is arguably known better than that of the MW: our external point of view allows us to have a panoramic picture of the galaxy and its surrounding regions. The other side of the medal is that, at a distance of ∼ 780 kpc, we can only resolve the brightest evolved stars in M31, and we are mostly limited to a two-dimensional view of its populations. Its proximity also implies a large angular size on the sky, underlining the need for wide field-of-view imagers to cover its entire area.

At the distance of M31, ground-based observations are able to resolve at best the uppermost ∼ 3−4 magnitudes below the TRGB, which is found at a magnitude i ∼ 21. The RGB is an excellent tracer for old (> 1 Gyr) populations, but suffers from a degeneracy in age and metallicity: younger, metal-rich stars overlap in magnitude and colour with older, metal-poor stars (Koch et al 2006). Despite this, the RGB colour is often used as a photometric indicator for metallicity, once a fixed old age is assumed (VandenBerg et al 2006, Crnojević et al 2010). This assumption is justified as long as a prominent young and intermediate-age population seems to be absent (i.e., as judged from the lack of luminous main sequence and asymptotic giant branch, AGB, stars), and it shows very good agreement with spectroscopic metallicity values where both methods have been applied.

The very first resolved studies of M31's halo introduced the puzzling evidence that the M31 halo stellar populations along the minor axis have a higher metallicity than that of the MW at similar galactocentric distances (e.g., Mould and Kristian 1986). This was further confirmed by several studies targeting projected distances from 5 to 30 kpc and returning an average value of [Fe/H] ∼ −0.8: in particular, Durrell et al (2001) study a halo region at a galactocentric distance of ∼ 20 kpc and underline the difference between the properties of M31 and of the MW, suggesting that our own Galaxy might not represent the prototype of a typical spiral. In fact, it has later been suggested that the MW is instead fairly atypical based on its luminosity, structural parameters and the metallicity of its halo stars when compared to spirals of similar mass (Hammer et al 2007). This result was interpreted as the consequence of an abnormally quiet accretion history for the MW, which apparently lacked a major merger in its recent past.

The wide-area studies of M31's outskirts were pioneered ∼ 15 years ago with an Isaac Newton Telescope survey mapping ∼ 40 deg2 around M31, reaching significantly beyond its disk out to galactocentric distances of ∼ 55 kpc (Ibata et al 2001, Ferguson et al 2002). As mentioned before, the southern Giant Stream was first uncovered with this survey, and the halo and its substructures could be studied with a dramatically increased detail. A metal-poor halo component ([Fe/H] ∼ −1.5) was finally uncovered for regions beyond 30 kpc and out to 160 kpc (Irwin et al 2005, Kalirai et al 2006, Chapman et al 2006), similar to what had been observed for the MW both in terms of metallicity and for its stellar density profile. These studies do not detect a significant gradient in metallicity across the covered radial range. Nonetheless, the properties of the inner halo remained a matter of debate: while Chapman et al (2006) found a metal-poor halo population within 30 kpc above the disc, Kalirai et al (2006) analysed a kinematically selected sample of stars within 20 kpc along the minor axis and derived a significantly higher value of [Fe/H] ∼ −0.5. At the same time, Brown et al (2006) used deep, pencil beam Hubble Space Telescope (HST) pointings in M31's inner halo to conclude that a significant fraction of its stellar populations have an intermediate age with an overall high metallicity. These results were later interpreted by Ibata et al (2007) in light of their wider-field dataset: the samples from Kalirai et al (2006) and Brown et al (2006) are simply part of regions dominated by an extended disc component and with a high contamination from various accretion events, respectively. This underlines, once again, the importance of wide-field observations to reach a global understanding of halo properties.

The M31 INT survey was further extended out to 150 kpc (200 kpc in the direction of the low-mass spiral M33) with the Canada-France-Hawaii Telescope/Megacam and dubbed Pan-Andromeda Archaeological Survey (PAndAS; Ibata et al 2007, McConnachie et al 2009). This survey contiguously covered an impressive 380 deg2 around M31, reaching 4 mag below the TRGB. The PAndAS RGB stellar density map (see Fig. 3) is a striking example of an active accretion history, with a copious amount of tidal substructure at both small and large galactocentric radii. PAndAS also constituted a mine for the discovery of a number of very faint satellites and GCs (see below; Richardson et al 2011, Huxor et al 2014, Martin et al 2016). Fig. 4 further shows the RGB stellar map broken into bins of photometric metallicity. The parallel Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey (Guhathakurta et al 2006, Kalirai et al 2006) provides a comparison dataset with both photometric and spectroscopic information, the latter obtained with Keck/DEIMOS. The SPLASH pointings are significantly smaller than the PAndAS ones but strategically cover M31 halo regions out to ∼ 225 kpc. Deeper, pencil-beam photometric follow-up studies have further made use of the HST to target some of the substructures uncovered in M31's outskirts, resolving stars down to the oldest MSTO (e.g., Brown et al 2006, Bernard et al 2015). These observations reveal a high complexity in the stellar populations in M31, hinting at a high degree of mixing in its outskirts. Overall, M31 has evidently had a much richer recent accretion history than the MW (see also Ferguson and Mackey 2016).

Figure 3

Figure 3. Stellar density maps of metal-poor RGB populations at the distance of M31, as derived from the PAndAS survey. The large circles lie at projected radii of 150 kpc and 50 kpc from M31 and M33, respectively. Upper panel: The Andromeda satellites are visible as clear overdensities and are marked with circles. The vast majority of them were uncovered by the PAndAS survey. Reproduced by permission of the AAS from Richardson et al (2011), their Fig. 1. Lower panel: The main substructures around M31 are highlighted, showcasing a broad range of morphologies and likely progenitor type. Tidal debris is also present in the vicinities of the low-mass satellites M33 and NGC 147, indicating an ongoing interaction with M31. Reproduced by permission of the AAS from Lewis et al (2013), their Fig. 1

Figure 4

Figure 4. Stellar density map of M31 (akin to Fig. 3), this time subdivided into photometric metallicity bins (as indicated in each subpanel). The upper panels show high metallicity cuts, where the Giant Stream and Stream C are the most prominent features; note that the shape of the Giant Stream changes as a function of metallicity. The lower panels show lower metallicity cuts: the lower left panel is dominated by substructure at large radii, while the most metal-poor panel (lower right) is smoother and believed to mostly contain in-situ populations. Reproduced by permission of the AAS from Ibata et al (2014), their Fig. 9.

2.2.1. Streams and Substructures

As seen from the maps in Figs. 3 and 4, while the inner halo has a flattened shape and contains prominent, relatively metal-rich substructures (e.g., the Giant Stream), the outer halo (> 50 kpc) hosts significantly less extended, narrow, metal-poor tidal debris.

The features in the innermost regions of M31 can be connected to its disk populations (e.g., the north-east structure or the G1 clump): kinematic studies show that a rotational component is present in fields as far out as 70 kpc, and they retain a fairly high metallicity (Dorman et al 2013). This reinforces the possible interpretation as a vast structure, which can be explained as disk stars torn off or dynamically heated due to satellite accretion events. Deep HST pointings of these features indeed reveal relatively young populations, likely produced from pre-enriched gas in a continuous fashion, comparable to the outer disk (Ferguson et al 2005, Brown et al 2006, Bernard et al 2015).

The most prominent feature in M31's outer halo, the Giant Stream, was initially thought to originate from the disruption of either M32 or NGC 205, the two dwarf ellipticals located at only ∼ 25−40 kpc from M31's centre (Ibata et al 2001, Ferguson et al 2002). While both these dwarfs shows signs of tidal distortion, it was soon clear that none of them could produce the vast structure extending ∼ 100 kpc into M31's halo. Great effort has been spent into mapping this substructure both photometrically and spectroscopically, in order to trace its orbit and define its nature: a gradient in its line-of-sight distance was first highlighted by McConnachie et al 2003, who found the outer stream regions to be located behind M31, the innermost regions at about the distance of M31, and an additional stream component on the opposite (northern) side of M31 to be actually in front of M31. The stream presents a metallicity gradient, with the core regions being more metal-rich and the envelope more metal-poor (see also Fig. 4), as well as a very narrow velocity dispersion, with the addition of a puzzling second kinematic component (Gilbert et al 2009); possible interpretations for the latter may be a wrap or bifurcation in the stream, as well as a component from M31's populations.

A number of increasingly sophisticated theoretical studies have tried to reproduce the appearance of the Giant Stream and picture its progenitor, which is undetected to date. The general consensus seems to be that a relatively massive (∼ 109 M) satellite, possibly with a rotating disk, impacted M31 from behind with a pericentric passage around 1−2 Gyr ago (most recently, Fardal et al 2013, Sadoun et alSadoun, Mohayaee, and Colin 2014). In particular, simulations can reproduce the current extension and shape of the stream and predict the progenitor to be located to the north-east of M31, just beyond its disk (Fardal et al 2013). This study also concludes that some of the substructures linked to M31's inner regions are likely to have arisen from the same accretion event, i.e., the north-east structure and the G1 clump (Fig. 3): these shelf features would trace the second and third passage around M31, which is also supported by their radial velocities. CMDs of the Giant Stream populations are in agreement with these predictions: its stellar populations have mixed properties, consistent with both disk and stream-like halo features (Ferguson et al 2005, Richardson et al 2008). Detailed reconstruction of its SFH indicate that most star formation occurred at early ages, and was possibly quenched at the time of infall in M31's potential (around 6 Gyr ago) (Bernard et al 2015). Again, these studies deduce a likely origin of these populations as a dwarf elliptical or a spiral bulge.

Besides the Giant Stream, the only other tidal feature with a relatively high metallicity is Stream C (see Fig. 3 and 4), which appears in the metal-poor RGB maps as well. The origin of this feature is obscure, even though it is tempting to speculate that it could be part of the Giant Stream event. The lower left panel of Fig. 4, showing metal-poor populations, encompasses all of the narrow streams and arcs beyond 100 kpc, which extend for up to several tens of kpc in length. All these substructures are extremely faint (µV ∼ 31.5 mag arcsec−2), and their origin is mostly unknown because of the difficulty in following up such faint and sparse populations. As part of the HST imaging of these features, Bernard et al 2015 find that their populations are mainly formed at early ages and undergo a more rapid chemical evolution with respect to the disk populations. Despite the metal-poor nature of these features, the hypothesis of a single accretion event producing most of the tidal features observed in the outer halo is not that unlikely, given the metallicity gradient present in the Giant Stream itself.

An efficient alternative to investigate the nature of these streams is to study the halo GC population: the wide-field surveys of M31 have allowed to uncover a rich population of GCs beyond a radius of ∼ 25 kpc (e.g., Huxor et al 2014, and references therein), significantly more numerous than that of the MW halo. Mackey et al (2010) first highlighted a high spatial correlation between the streams in M31's halo and the GC population, which would be extremely unlikely in a uniform distribution. Following the hypothesis that the disrupting satellites might be providing a high fraction of M31's halo GCs, Veljanoski et al (2014) obtained spectroscopic follow-up: they were able to confirm that streams and GCs often have correlated velocities and remarkably cold kinematics. This exciting result gives hope for studies of more distant galaxies, where halo populations cannot be resolved and GCs could be readily used to trace possible substructure.

2.2.2. Smooth Halo

One of the first spatially extended datasets to investigate the halo of M31 in detail is described in Tanaka et al (2010): their Subaru/SuprimeCam photometry along the minor axis in both directions are deeper, even though less extended, than PAndAS. The stellar density profile derived in this study extends out to 100 kpc and shows a consistent power law for both directions. The authors also suggest that, given the inhomogeneities in the stellar populations, the M31 halo is likely not fully mixed.

In the most metal-poor (lower right) panel of Fig. 4, the substructures in the outer halo fade away, displaying a smoother component that can be identified with the in-situ M31 halo. Once the substructures are decoupled based on the lack of obvious spatial correlation and with an additional photometric metallicity cut, Ibata et al (2014) derive a stellar density profile out to 150 kpc. Again, the profile follows a power-law, which turns out to be steeper when increasingly more metal-rich populations are considered. Ibata et al (2014) also conclude that only 5% of M31's total halo luminosity lies in its smooth halo, and the halo mass is as high as ∼ 1010 M, significantly larger than what estimated for the MW.

The SPLASH survey extends further out than PAndAS, and benefits from kinematical information that is crucial to decontaminate the studied stellar samples from foreground stars and decreases the scatter in the radial profiles. Based on this dataset, Gilbert et al (2012) find that the halo profile does not reveal any break out to 175 kpc. This is somewhat surprising given the prediction from simulations that accreted M31-sized stellar haloes should exhibit a break beyond a radius of ∼ 100 kpc (Bullock and Johnston 2005, Cooper et al 2010). Beyond a radius of 90 kpc, significant field-to-field variations are identified in their data, which suggests that the outer halo regions are mainly comprised of stars from accreted satellites, in agreement with previous studies. At the outermost radii probed by SPLASH (∼ 230 kpc), there is a tentative detection of M31 stars, but this is hard to confirm given the high contamination fraction. Finally, the Gilbert et al (2012) stellar halo profile suggests a prolate DM distribution, which is also consistent with being spherical, in agreement with Ibata et al (2014).

Both Ibata et al (2014) and Gilbert et al (2014) investigate the existence of a metallicity gradient in the smooth halo of M31: they found a steady decrease in metallicity of about 1 dex from the very inner regions out to 100 kpc. This might indicate the past accretion of (at least) one relatively massive satellite. At the same time, a large field-to-field metallicity variation could mean that the outer halo has been mainly built up by the accretion of several smaller progenitors.

2.2.3. Andromeda Satellites

Similarly to the boom of satellite discoveries around the MW, the vast majority of dwarfs in M31's extended halo has been uncovered by the SDSS, PAndAS, and PanSTARRS surveys in the past decade (see Martin et al 2016, and references therein). The M31 satellites follow the same relations between luminosity, radius and metallicity defined by MW satellites, with the exception of systems that are likely undergoing tidal disruption (Collins et al 2014). Once more, the characterization of the lowest-mass galaxies raises new, unexpected questions: from the analysis of accurate distances and kinematics, Ibata et al (2013) conclude that half of the M31 satellites lie in a vast (∼ 200 kpc) and thin (∼ 12 kpc) corotating plane, and share the same dynamical orbital properties. The extreme thinness of the plane is very hard to reconcile with ΛCDM predictions, where such structures should not survive for a Hubble time. While several theoretical interpretations have been offered (e.g., Fernando et al 2016), none is conclusive, and this reinforces the allure of mystery surrounding low-mass satellites.

2.3. Low-mass Galaxies In and Around the Local Group

Besides the detailed studies of the two LG spirals, increasing attention is being paid to lower-mass galaxies and their outskirts. Given the self-similar nature of DM, low-mass galaxies should naively be expected to possess haloes and satellites of their own; however, our difficulty in constraining star formation efficiency and physical processes affecting galaxy evolution at these scales blurs these expectations. In the last couple of years, the increasing resolution of cosmological simulations has allowed to make quantitative predictions about the halo and substructures in sub-MW-mass galaxies, and about the number of satellites around them (Wheeler et al 2015, Dooley et al 2016). Observations are thus much needed to test these predictions.

Since the late 90s, numerous studies of star-forming dwarfs within or just beyond the LG have claimed the detection of an RGB component extending beyond the blue, young stars (see Stinson et al 2009, and references therein), hinting at a generic mode of galaxy formation independent on galaxy size. Such envelopes, however, were not characterized in detail, and in fact could not be identified uniquely as the product of hierarchical merging without, e.g., accurate age and metallicity estimates.

The presence of extended haloes in the most luminous satellites of the MW and M31, i.e., the irregular LMC and the low-mass spiral M33, respectively, has not been confirmed to date despite the availability of exquisite datasets. Gallart et al (2004) demonstrate how, out to a galactocentric distance of 7 kpc, the stellar density profile of the LMC disk does not show a clear break, in contrast to previous tentative claims. Clearly, the question is complicated by the fact that the LMC is undergoing tidal disruption, and stripped stellar material could easily be misinterpreted as a halo component. Nonetheless, McMonigal et al (2014) suggest to have found a sparse LMC halo population from a wide-field dataset around the nearby dwarf galaxy Carina, at galactocentric distances as large as 20 deg. The question might be settled in the near future with the help of wide-field surveys such as the Survey of MAgellanic Stellar History (Martin et al 2015). With regard to possible low-mass satellites, there is now tantalizing indication that the LMC might have fallen onto the MW with its own satellite system, as mentioned in Sect. 2.1.3. As part of the PAndAS survey, deep imaging of M33 has revealed prominent substructure in its outer disk reminiscent of a tidal disturbance, and a faint, diffuse substructure possibly identified as a halo component (Cockcroft et al 2013). This result was, however, carefully reconsidered by McMonigal et al (2016), who claim that a definitive sign of a halo structure cannot be confirmed, and if present it must have a surface brightness below µV ∼ 35 mag arcsec−2.

Besides the investigation of haloes and satellites, deep and wide-field views of low-mass galaxies are crucial to, e.g., assess the presence of tidal disturbances, which in turn are key to estimate mass values and constrain DM profiles (e.g., Sand et al 2012). As demonstrated by Crnojević et al (2014), a striking similarity in the global properties (luminosity, average metallicity, size) of two low-mass galaxies, such as the M31 satellites NCG 185 and NGC 147, can be quite misleading: once deep imaging was obtained around these galaxies (within PAndAS), NCG 147 revealed extended, symmetric tidal tails, returning a much larger extent and luminosity for this dwarf than what was previously thought. This dataset further showed a flat metallicity gradient for NGC 147, in contrast with the marked gradient found in NGC 185. All these pieces of evidence point at an ongoing interaction of NGC 147 with M31. Large-scale studies of LG dwarfs also provide useful insights into their evolutionary history: by studying CMDs reaching below the MSTO, Hidalgo et al (2013) trace significant age gradients that advocate an outside-in mode of star formation for dwarf galaxies.

Clearly, systematic deep searches are needed to detect and characterize the outskirts of low-mass satellites. With this goal in mind, wide-field surveys of nearby (< 3 Mpc) dwarfs have started to be pursued. The first of these efforts targets NGC 3109, a sub-LMC-mass dwarf located just beyond the boundaries of the LG: several candidate satellites of NGC 3109 are identified from a CTIO/DECam survey targeting regions out to its virial radius (Sand et al 2015). One of them, confirmed to be at the distance of NGC 3109, is relatively bright (MV ∼ −10), and is already in excess of the predicted number by Dooley et al (2016) for this system. Other ongoing surveys are similarly looking for halo substructures and satellites in several relatively isolated dwarfs, e.g., the SOlitary LOcal dwarfs survey (Higgs et al 2016) and the Magellanic Analog Dwarf Companions And Stellar Halos survey (Carlin et al 2016), by using wide-field imagers on large telescopes such as CFHT/MegaCam, Magellan/Megacam, CTIO/DECam and Subaru/HyperSuprimeCam. These datasets will constitute a mine of information to constrain the role of baryonic processes at the smallest galactic scales.

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