The ground-breaking photometric and kinematic surveys carried out in the past two decades have significantly advanced our knowledge of haloes and their substructures within LG galaxies. Nonetheless, the MW and M31 may not be representative of generic MW-sized haloes, given the stochasticity of the hierarchical assembly process: several marked differences in the stellar populations of their haloes underline the need for observations of a statistically significant sample of galaxy haloes with different morphologies, with surveys targeting large portions of their haloes.
Cosmological simulations of MW-mass analogues show a wide variation in the properties of their haloes. As already mentioned, the relative contribution of in-situ star formation and disrupted satellites remains unclear: depending on the models (e.g., full hydrodynamical simulations, N-body models with particle tagging), they can vary from a negligible number of accretion events for a MW-sized halo, to making up for most of a stellar halo content (e.g., Lu et al 2014, Tissera and Scannapieco 2014). Even within the same set of simulations, the number, mass ratio and morphology of accretion and merger events span a wide range of possible values (Bullock and Johnston 2005, Johnston et al 2008, Garrison-Kimmel et al 2014). The chemical content of extended haloes can provide useful insights into their assembly history: mergers or accretion events of similar-mass satellites will generally tend to produce mild to flat gradients; in-situ populations will feature increasingly metal-poor populations as a function of increasing galactocentric radius, similarly to the accretion of one or two massive companions (e.g., Cooper et al 2010, Font et al 2011). More extended merger histories are also expected to return younger and relatively metal-rich populations with respect to those coming from a shorter assembly, and to produce more massive stellar haloes, with the final result that the mean halo metallicities of MW-mass spirals can range by up to 1 dex (e.g., Renda et al 2005).
Comprehensive observational constraints are key to guide future simulations of galaxy haloes: the past decade has seen a dramatic increase in the observational census of resolved galaxy haloes beyond the LG, thanks to deep imaging obtained with space facilities, as well as to the advent of wide-field imagers on large ground-based telescopes.
While the increasing target distance means that it is easier to survey larger portions of their haloes, the drawback is that the depth of the images decreases dramatically, and thus we are only able to detect the brightest surface brightness features in the haloes, i.e., the uppermost ∼ 2−3 mag below the TRGB in terms of resolved stars (see Fig. 6 in Radburn-Smith et al 2011 for a schematic visualization of the different stellar evolutionary phases recognizable in such shallow CMDs). A number of studies has surveyed relatively nearby and more distant galaxy haloes in integrated light despite the serious challenges posed by sky subtraction at such faint magnitudes, masking of bright stars, flat-fielding and scattered light effects, point spread function modelling, and/or spatially variable Galactic extinction.
A few early studies have been able to uncover a halo component and tidal debris in the target galaxies (e.g., Malin et al 1983, Morrison et al 1994, Sackett et al 1994), without, however, settling the questions about their existence, nature or ubiquity. Different approaches have been adopted to detect haloes and their substructures, i.e., targeting either individual galaxies (e.g., Zheng et al 1999, Pohlen et al 2004, Jablonka et al 2010, Janowiecki et al 2010, Martínez-Delgado et al 2010, Adams et al 2012, Atkinson et al 2013) or stacking the images of thousands of objects (e.g., Zibetti et al 2004, Tal et al 2009). A precise quantification of the occurrence of faint substructure in the outskirts of nearby galaxies seems as uncertain as it can be, ranging from a few percent to ∼ 70% (see, e.g., Atkinson et al 2013, and references therein). This is perhaps unsurprising given the heterogeneity of methods used, target galaxy samples, and surface brightness limits in such studies. Besides the identification of such features, the characterization of unresolved halo stellar populations constitutes an even harder challenge: integrated colours and spectra can at most reach a few effective radii, thus missing the outer haloes. Even for the available datasets, the degeneracies between age, metallicity and extinction are generally challenging to break (e.g., de Jong et al 2007); in addition, tidal features can rarely tell us about the mass ratio of a merger event or its orbit (with the exception of tails). Here, we do not intend to discuss the detection of haloes and the variety of fractions and morphologies for tidal features observed in integrated light studies; Knapen & Trujillo (this volume) treat this topic in detail, while this contribution focusses on resolved populations.
Obtaining resolved photometry beyond the LG is a daunting task as well, due to the very faint luminosities involved — the brightest RGB stars for galaxies at ∼ 4−10 Mpc have magnitudes of I ∼ 24−28.5, and thus this approach is so far really limited to the Local Volume. Early attempts to perform photometry of individual stars in the outskirts of nearby galaxies have been made using large photographic plates and the first CCDs (e.g., Humphreys et al 1986, Davidge and Jones 1989, Georgiev et al 1992). The brightest populations (i.e., the youngest) could often be reconciled with being members of the parent galaxy, but the critical information on the faint, old stars was still out of reach. With the advent of wide-format CCDs in the mid 90s, photometry finally became robust enough to open up new perspectives on the resolved stellar content of our closest neighbours.
The first studies of this kind date back to twenty years ago and mainly focus on the inner regions of the target galaxies, most commonly their disks or inner haloes, with the goal of studying their recent star formation and of deriving TRGB distances (see, e.g., Soria et al 1996 for CenA, Sakai and Madore 1999 for M81 and M82). Elson (1997), in particular, resolved individual stars in the halo of the S0 galaxy NGC 3115 with HST. By analysing the uppermost 1.5 mag of the RGB at a galactocentric distance of 30 kpc, they derived a distance of ∼ 11 Mpc, and additionally discovered for the first time a bimodality in the photometric metallicity distribution function of this early-type galaxy. Tikhonov et al (2003) studied for the first time the resolved content of the nearest (∼ 3.5 Mpc) S0 galaxy NGC 404 with combined ground-based and HST imaging. Their furthermost HST pointings (∼ 20 kpc in projection) contain RGB stars that are clearly older than the main disk population, with similar colour (metallicity). The authors conclude that the disk of NGC 404 extends out to this galactocentric distance, but they do not mention a halo component.
Beyond these early studies of individual galaxies, the need for systematic investigations of resolved stellar haloes was soon recognized. Next we describe the design and results of some systematic surveys targeting samples of galaxies in the Local Volume.
A decade ago, Mouhcine et al (2005a, b, c) started an effort to systematically observe the haloes of eight nearby (< 7 Mpc) spiral galaxies with the resolution of HST. In particular, they utilized WFPC2 to target fields off of the galaxies' disks (2 to 13 kpc in projection along the minor axis) with the goal of investigating their stellar populations, and obtaining accurate distance estimates as well as photometric metallicity distribution functions, to gain insights into the halo formation process. Mouhcine et al (2005c) find the haloes to predominantly contain old populations, with no younger components and little to no intermediate-age populations. Interestingly, Mouhcine et al (2005b) find a correlation between luminosity and metallicity for the target galaxies, where the metallicity is derived from the mean colour of the resolved RGB. Both the spiral galaxies from their sample (NGC 253, NGC 4244, NGC 4945, NGC 4258, NGC 55, NGC 247, NGC 300, and NGC 3031 or M81) and the two ellipticals (NGC 3115 and NGC 5128 or Centaurus A, included in their comparison from previous literature data) fall on the same relation, indicating that haloes might have a common origin regardless of the galaxy morphological type. Interestingly enough, the MW halo turns out to be substantially more metal-poor than those of the other galaxies of comparable luminosity, based on kinematically selected pressure-supported halo stars within ∼ 10 kpc above the disk (see also Sect. 2.2). This relation is consistent with a scenario where halo field stars form in the potential well of the parent galaxy in a gradual way from pre-enriched gas. Moreover, the relatively high metallicities of the target haloes seem to suggest that they likely originate from the disruption of intermediate-mass galaxies, rather than smaller metal-poor dwarf galaxies (Mouhcine et al 2005c).
Interestingly, the dataset presented and studied in Mouhcine et al (2005a, b, c) is further analyzed by Mouhcine (2006) to find that each spiral of the sample presents a bimodal metallicity distribution. In particular, both a metal-poor and a metal-rich component are present in the outskirts of the target galaxies, and both components correlate with the host's luminosity. This is taken as a hint that these populations are born in subgalactic fragments that were already embedded in the dark haloes of the host galaxy; the metal-poor component additionally has a broader dispersion than that of the metal-rich population. These properties show similarities with GC subpopulations in the haloes of early-type galaxies (e.g., Peng et al 2006). Mouhcine (2006) argues that the metal-poor component may arise from the accretion of low-mass satellites, while the metal-rich one could be linked to the formation of the bulge or the disk.
The shortcoming of this ambitious study is, however, twofold: first, the limited field of view (FoV) of HST hampers global conclusions on the galaxies' haloes, and the stellar populations at even larger radii may have different properties than those in the observed fields; second, perhaps most importantly, it is not obvious what structure of the galaxy is really targeted, i.e., the halo, the outer bulge or disk, or a mixture of these.
Along the same lines of these studies, Radburn-Smith et al (2011) present an even more ambitious HST survey of 14 nearby disk galaxies within 17 Mpc, with a range of luminosities, inclinations and morphological types. The Galaxy Halos, Outer disks, Substructure, Thick disks, and Star clusters (GHOSTS) survey aims at investigating radial light profiles, axis ratios, metallicity distribution functions (MDFs), SFHs, possible tidal streams and GC populations, all to be considered as a function of galaxy type and position within the galaxies. The 76 ACS pointings of the survey are located along both major and minor axes for most of the targets, and reach ∼ 2−3 mag below the TRGB, down to surface brightness values of V ∼ 30 mag arcsec−2. This dataset thus represents a very valuable resource for testing hierarchical halo formation models. Monachesi et al (2016) investigate six of the galaxies in this sample (NGC 253, NGC 891, M81, NGC 4565, NGC 4945, and NGC 7814) and conclude that all of them contain a halo component out to 50 kpc, and two of them out to 70 kpc along their minor axis. The colour (i.e., photometric metallicity) distribution of RGB stars in the target haloes is analysed and reveals a non-homogeneity which likely indicates the presence of non-mixed populations from accreted objects. The average metallicity out to the largest radii probed remains relatively high when compared to the values of the MW halo; metallicity gradients are also detected in half of the considered galaxies. Surprisingly, and in contrast to the results presented by Mouhcine et al (2005b), the spiral galaxies in this sample do not show a strong correlation between the halo metallicity and the total mass of the galaxies, highlighting instead the stochasticity inherent to the halo formation process through accretion events (e.g., Cooper et al 2010). The advantage of the GHOSTS dataset over the one from Mouhcine et al (2005b) is that the GHOSTS fields are deeper, there are several pointings per galaxy and they reach significantly larger galactocentric distances, thus offering a more global view of the haloes of the targets.
In an effort to increase the sample of nearby galaxies for which stellar haloes are resolved and characterized, several groups have individually targeted Local Volume objects with either ground-based or space-borne facilities: the low-mass spirals NGC 2403 (Barker et al 2012, with Subaru/SuprimeCam), NGC 300 (Vlajić et al 2009, with Gemini/GMOS), and NGC 55 (Tanaka et al 2011, with Subaru/SuprimeCam), the ellipticals NGC 3379 (Harris et al 2007b, with HST) and NGC 3377 (Harris et al 2007a, with HST), and the lenticular NGC 3115 (Peacock et al 2015, with HST). In most of these galaxies, a resolved faint halo (or at least an extended, faint and diffuse component) has been detected and is characterized by populations more metal-poor than the central/disk regions. Most of these haloes also show signs of substructure, pointing at past accretion/merger events as predicted by a hierarchical galaxy formation model. Even galaxies as far as the central elliptical of the Virgo cluster, M87, (∼ 16 Mpc) are starting to be targeted with HST, although pushing its resolution capabilities to the technical limits (Bird et al 2010).
While spectroscopically targeting individual RGB stars to obtain radial velocity and metallicity information is still prohibitive beyond the LG (see Sect. 2.3), some cutting-edge studies have pushed the limits of spectroscopy for dwarf galaxies within ∼ 1.5 Mpc (e.g., Kirby et al 2012, and references therein). At the same time, novel spectroscopic techniques are being developed to take full advantage of the information locked into galaxy haloes. One example is the use of co-added spectra of individual stars, or stellar blends, to obtain radial velocities, metallicities and possibly gradients in galaxies within ∼ 4 Mpc, as robustly demonstrated by Toloba et al (2016a). The development of new analysis methods and the advent of high-resolution spectrographs will soon allow for systematic spectroscopic investigations of nearby galaxy haloes which will importantly complement the available photometric studies, similarly to the studies of LG galaxies.
Besides the systematic studies presented here, which mostly involve deep space observations, an increasing effort is being invested in producing spatial density maps of outer haloes in some of the closest galaxies with ground-based observations, akin to the panoramic view of M31 offered by PAndAS. In the following Section we describe some of these efforts.
3.2. Panoramic Views of Individual Galaxies
Panoramic views of nearby galaxies can be obtained with the use of remarkable ground-based wide-field imagers such as Subaru/SuprimeCam and HyperSuprimeCam and CFTH/MegaCam in the northern hemisphere, and Magellan/Megacam, CTIO/DECam and VISTA/VIRCAM in the southern hemisphere. Clearly, such CMDs cannot reach the depth of those obtained for M31; these studies nevertheless represent cornerstones for our investigation of global halo properties, and serve as precursor science cases for the next generation of telescopes that will open new perspectives for this kind of studies to be performed on a significantly larger sample of galaxies. As mentioned in Sect. 2.3, the haloes of low-mass galaxies are also starting to be systematically investigated, to gain a more complete picture of galaxy formation at all mass scales. Here we further describe the few examples of spatially extended imaging obtained to date for some of the closest spiral and elliptical galaxies.
Despite its relatively large distance (∼ 9 Mpc, Radburn-Smith et al 2011), the “MW-twin” NCG 891 (van der Kruit 1984) is one of the first spirals to be individually investigated in resolved light. Its high inclination and absence of a prominent bulge make it an appealing target for halo studies.
Mouhcine et al (2007) exploit three HST pointings located approximately 10 kpc above the disk of NGC 891 to investigate the properties of this galaxy's halo. The broad observed RGB indicates a wide range of metallicities in this population, with metal-rich peaks and extended metal-poor tails. The three fields also show a decreasing mean metallicity trend as a function of increasing distance along the major axis. The mean metallicity of this sample of RGB stars ([Fe/H] ∼ −1) falls on the halo metallicity-galaxy luminosity relation pointed out by Mouhcine et al (2005b): this, together with the gradient mentioned before, is in contrast with the lower metallicities and absence of a gradient for non-rotating stars in the inner haloes of the MW and M31 (Chapman et al 2006, Kalirai et al 2006). Mouhcine et al (2007) thus suggest that not all massive galaxies' outskirts are dominated by metal-poor, pressure-supported stellar populations (because of the inclination and absence of a bulge, the studied RGB sample is thought to be representative of the true halo population). A possible explanation is suggested with the presence of two separate populations: a metal-rich one that is present in the most massive galaxies' outskirts, and one constituting the metal-poor, pressure-supported halo, coming from the accretion of moderate-mass satellites. For smaller-mass galaxies, the halo would instead be dominated by debris of small satellites with lower metallicities.
Follow-up analysis on the same HST dataset has been carried out by Ibata et al (2009) and Rejkuba et al (2009). After careful accounting for the internal reddening of the galaxy, a mild metallicity gradient is confirmed in NGC 891's spheroidal component, which is surveyed out to ∼ 20 kpc (assuming elliptical radii), and suggested to arise from the presence of a distinct outer halo, similarly to the MW (Ibata et al 2009). Most importantly, and for the first time, this refined analysis reveals a substantial amount of substructure not only in the RGB spatial distribution but also as metallicity fluctuations in the halo of NGC 891. This evidence points at multiple small accretion events that have not fully blended into the smooth halo.
Motivated by these studies, Mouhcine et al (2010) provide the first attempt to derive a PAndAS-like map of a MW-analogue beyond the LG: their wide-field map of NGC 891's halo is shown in Fig. 5. The panoramic survey, performed contiguously with Subaru/SuprimeCam, covers an impressive ∼ 90 × 90 kpc2 in the halo of NGC 891 with the V and i filters, reaching ∼ 2 mag below the TRGB. Among the abundant substructures uncovered by the RGB map around NGC 891, a system of arcs/streams reaches out some ∼ 50 kpc into the halo, including the first giant stream detected beyond the LG with ground-based imaging. The latter's shape does not rule out a single accretion event origin, but a possible progenitor cannot be identified as a surviving stellar overdensity. These structures appear to be old, given the absence of corresponding overdensities in the luminous AGB (i.e., intermediate-age populations) maps. Another surprising feature highlighted by the RGB map is a flattened, super-thick envelope surrounding the disk and bulge of NGC 891, which does not seem to constitute a simple extension of its thick disk but is instead believed to generate from the tidal disruption of satellites given its non-smooth nature (Ibata et al 2009).
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Figure 5. Surface density map of RGB stars in the halo of NGC 891, obtained with Subaru/SuprimeCam. The overdensities of old RGB stars reveal a large complex of arcing streams that loops around the galaxy, tracing the remnants of an ancient accretion. The second spectacular morphological feature is the dark cocoon-like structure enveloping the high surface brightness disk and bulge. Fig. 1 from Mouhcine et al (2010), reproduced by permission of the AAS |
Located at a distance of 3.6 Mpc (Radburn-Smith et al 2011) and with a dynamical mass inside 20 kpc of ∼ 1011 M⊙, M81 is one of the closest MW-analogues, and has thus been among the first targets for extended halo studies beyond the LG. The earliest Hi imaging of the galaxy group dominated by this spiral unambiguously shows a spectacular amount of substructure, most prominently a bridge of gas between M81 and its brightest companions NGC 3077 and M82, located at a projected distance of ∼ 60 kpc (van der Hulst 1979, Yun et al 1994).
Given the high level of interaction and Hi substructure in a group that can be considered as a LG-analogue, it is natural to pursue the investigation of this complex environment even further. The intergalactic gas clouds embedding this environment are traced by young stellar systems identified in resolved stellar studies (Durrell et al 2004, Davidge 2008, de Mello et al 2008). Some of them are classified as tidal dwarf galaxies, such as Holmberg IX and the Garland (Makarova et al 2002, Karachentsev et al 2004, Sabbi et al 2008, Weisz et al 2008), characterized by a predominance of young stellar populations. This type of galaxy has no counterpart in our own LG, and it is believed to be DM-free (see, e.g., Duc et al 2000).
The first detailed look into the resolved populations in the outskirts of M81 is through the eye of HST: the predominantly old halo RGB stars show a broad range of metallicities and a radial gradient (Tikhonov et al 2005, Mouhcine et al 2005c). The radial stellar counts (along several different directions) also reveal a break at a radius of ∼ 25 kpc, which is interpreted as the transition point between thick disk and halo (Tikhonov et al 2005). In a similar fashion, the ground-based wide-field imager Subaru/SuprimeCam has been used to uncover a faint and extended component beyond M81's disk with a flat surface brightness profile extending out to ∼ 0.5 deg (or ∼ 30 kpc) to the north of M81 (Barker et al 2009). This low surface brightness feature (∼ 28 mag arcsec−2) traced by the brightest RGB star counts appears bluer than the disk, suggesting a metallicity lower than that of M81's main body, but its true nature remains unclear. The authors suggest this component to have intermediate properties between the MW's halo and its thick disk, but the limited surveyed area (0.3 deg2) precludes any robust conclusions.
As part of a campaign to obtain panoramic views of nearby galaxy haloes, Mouhcine and Ibata (2009) present a 0.9 × 0.9 deg2 view of M81's surroundings obtained with the CFHT/MegaCam imager. The images resolve individual RGB stars down to ∼ 2 mag below the TRGB, but this study focusses on the younger, bright populations such as massive main sequence stars and red supergiants, which reveal further young systems tracing the Hi tidal distribution between M81 and its companions. These systems are younger than the estimated dynamical age of the large-scale interaction and do not have an old population counterpart, suggesting that they are not simply being detached from the main body of the primary galaxies but are instead formed within the Hi clouds.
Durrell et al (2010) recently conducted a deeper, albeit spatially limited, HST study of a field at a galactocentric distance of ∼ 20 kpc. This field reveals an [M/H] ∼ −1.15 population with an approximate old age of ∼ 9 Gyr. This field thus contains the most metal-poor stars found in M81's halo to that date, which led the authors to the conclusion that they were dealing with an authentic halo component. This study is extended by Monachesi et al (2013) with the HST GHOSTS dataset (see Sect. 3.1): they construct a colour profile out to a radius of ∼ 50 kpc, and this dataset does not show a significant gradient. The mean photometric metallicity derived is [Fe/H]∼ −1.2, similarly to Durrell et al (2010). This result is found to be in good agreement with simulations and the authors suggest that the halo of M81 could have been assembled through an early accretion of satellites with comparable mass (e.g., Cooper et al 2010, Font et al 2006).
As a further step in the investigation of M81's halo, the Barker et al (2009) and Mouhcine and Ibata (2009) ground-based imaging of M81 is being improved by means of the Subaru/HyperSuprimeCam. The first ∼ 2 × 2 deg2 (∼ 100 × 115 kpc2) resolved stellar maps from different subpopulations (upper main sequence, red supergiants, RGB and AGB stars) are presented in Okamoto et al (2015) and constitute a preview of an even wider-field effort to map the extended halo of this group. These first maps (see Fig. 6) confirm a high degree of substructure, most interestingly: the youngest populations nicely trace the Hi gas content, confirming previous small FoV studies; the RGB distributions are smoother and significantly more extended than the young component, and show stream-like overlaps between the dominant group galaxies, e.g., M82's stars clearly being stripped by M81; a redder RGB distribution is detected for M81 and NGC 3077 compared to M82, indicating a lower metallicity in the latter; in addition, M82 and NGC 3077's outer regions present S-shaped morphologies, a smoking gun of the tidal interaction with M81 and typical of interacting dwarf galaxies with larger companions (e.g., Peñarrubia et al 2009).
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Figure 6. Isodensity contour map of red RGB stars in the M81 group, as observed by Subaru/HyperSuprimeCam. Structures up to 20σ above the background level are visible; the cross marks represent the centres of known M81 group members, while solid lines are R25 of galaxies. The high degree of substructure underlines the ongoing tidal interactions in this group; note in particular the S-shape of the outer regions in NGC 3077 and M82. Fig. 4 from Okamoto et al (2015), reproduced by permission of the AAS |
Not less importantly, the widest-field survey to date (∼ 65 deg2) of the M81 group has been performed by Chiboucas et al (2009) with CFHT/MegaCam, although with only one filter. The main goal of this survey was to identify new, faint dwarf galaxies and investigate the satellite LF in a highly interacting group environment as compared to the LG. This is the first survey to systematically search for faint dwarfs beyond the LG. Resolved spatial overdensities consistent with candidate dwarfs have been followed up with two-band HST/ACS and HST/WFPC2 observations. Fourteen of the 22 candidates turned out to be real satellites of M81 based on their CMDs and TRGB distances, extending the previously known galaxy LF in this group by three orders of magnitude down to Mr ∼ −9.0 (Chiboucas et al 2013), with an additional possibly ultra-faint member at Mr ∼ −7.0. The measured slope of the LF in the M81 group appears to be flatter than cosmological predictions (α ∼ −1.27, in contrast to the theoretical value of α ∼ −1.8), similar to what has been found for the MW and M31 satellites.
Another obvious MW-mass spiral target for halo studies is NCG 253 (∼ 3.5 Mpc, Radburn-Smith et al 2011). Its role of brightest object within the loose Sculptor filament of galaxies makes it ideally suited to investigate the effects of external environment on the assembly of haloes. As already apparent from old photographic plates, NGC 253's outskirts show faint perturbation signs, such as an extended shelf to the south of its disk (Malin and Hadley 1997), pointing at a possible accretion event. This spiral galaxy, despite its relative isolation, is experiencing a recent starburst and a pronounced nuclear outflow: the latter is believed to host local star formation extending as high as ∼ 15 kpc above the disk in the minor axis direction (see Comerón et al 2001, and references therein).
The resolved near-infrared study of Davidge (2010) allowed them to detect bright AGB stars, but not RGB stars, extending out to ∼ 13 kpc from the disk plane in the south direction: these are interpreted as being expelled from the disk into the halo as consequence of a recent interaction. Subsequently, Bailin et al (2011) exploited a combination of HST data from the GHOSTS survey and ground-based Magellan/IMACS imaging, the former being deeper while the latter have a more extended FoV (out to ∼ 30 kpc in the halo NGC 253 in the south direction). The authors are able to estimate NGC 253's halo mass as ∼ 2 × 109 M⊙, or 6% of the galaxy's total stellar mass: this value is broadly consistent with those derived from the MW and M31 but higher, reminiscent of the halo-to-halo scatter seen in simulations. A power law is fit to the RGB radial profile which is found to be slightly steeper than that of the two LG spirals, and appears to be flattened in the same direction as the disk component. This is the one of the few studies to date to quantitatively measure such parameters for a halo beyond the LG, and it sets the stage for the possibilities opened by similar studies of other nearby galaxies. The RGB density maps derived in Bailin et al (2011) from IMACS imaging confirm the early detection of a shelf structure, and uncover several additional kpc-scale substructures in the halo of this spiral.
A more recent wide-field study of NGC 253 is presented by Greggio et al (2014), who exploit the near-infrared VISTA/VIRCAM imager to study the RGB and AGB stellar content of this galaxy out to ∼ 40−50 kpc, covering also the northern portion which was not included in previous studies. This portion, in particular, reveals an RGB substructure symmetric (and likely connected) to the one in the south. A prominent arc (∼ 20 kpc in length) to the north-west of the disk is detected and estimated to arise from a progenitor with a stellar mass of roughly ∼ 7 × 106 M⊙. The RGB radial density profile shows a break at a radius of ∼ 25 kpc, indicative of the transition from disk to halo. The elongated halo component already discussed in Bailin et al (2011) is confirmed here, but is considered to be an inner halo: an outer, more spherical and homogeneous component extends at least out to the galactocentric distances covered by this survey. Intriguingly, the AGB density map reveals that 25% of this intermediate-age (i.e., up to a few Gyr old) population is spread out to ∼ 30 kpc above the disk: this component cannot easily be explained with either an in-situ or an accreted origin.
NGC 253 is also one of the two targets of the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), recently initiated with the wide-field imager Magellan/Megacam. This ambitious survey aims at obtaining RGB stellar maps of this galaxy and of the elliptical Centaurus A (Cen A; see next Section) out to galactocentric radii of ∼ 150 kpc, similarly to the PAndAS survey of M31. Early results from this survey include the discovery of two new faint satellites of NGC 253, one of which is clearly elongated and in the process of being disrupted by its host (Sand et al 2014, Toloba et al 2016b).
It is important to target galaxies of different morphologies and environments to thoroughly investigate the assembly of haloes. The closest (∼ 3.8 Mpc; Harris et al 2010) elliptical galaxy is Centaurus A (Cen A; technically speaking, Maffei 1 is slightly closer but it lies behind the Galactic disk and is thus heavily reddened, see Wu et al 2014). Cen A is the dominant galaxy of a rich and dense group, which also has a second subgroup component centred on the spiral M83 (e.g., Karachentsev et al 2007).
Despite having often been referred to as a peculiar galaxy, due to its pronounced radio activity, its central dust lanes, and a perturbed morphology, the luminosity of Cen A is quite typical of field elliptical galaxies: a recent (< 1 Gyr) merger event is believed to be the culprit for its peculiar features (see Israel 1998, and references therein). Besides this main merger event, Peng et al (2002) uncover a system of faint shells and an arc within ∼ 25 kpc of Cen A's centre from integrated light observations; the arc is believed to have been produced by the infall of a low-mass, star forming galaxy around ∼ 300 Myr ago.
This elliptical galaxy has been the subject of a systematic study conducted with HST/ACS and HST/WFPC2 throughout the past couple of decades: a number of pointings at increasingly large galactocentric radii (from a few out to ∼ 150 kpc) have been used to investigate the properties and gradients of Cen A's halo populations (Rejkuba et al 2014, and references therein). The considered pointings out to 40 kpc reveal metal-rich populations ([Fe/H] > −1.0), not dissimilar to what has been observed for the haloes of spiral galaxies. The deepest CMD to date of this elliptical is presented by Rejkuba et al (2011) for the HST field at 40 kpc: this study concludes that the vast majority of Cen A's halo population is old (∼ 12 Gyr), with a younger (∼ 2−4 Gyr) component accounting for ∼ 20% of the total population.
The first wide-field study of Cen A was performed with the ground-based VLT/VIMOS imager, reaching out to ∼ 85 kpc along both minor and major axes (Crnojević et al 2013). Cen A's halo population seems to extend all the way out to this large radius. This study confirms the relatively high metallicity for halo populations found by the HST studies, although with a considerable presence of metal-poor stars at all radii; the authors also highlight the absence of a strong metallicity gradient from a ∼ 30 kpc radius out to the most distant regions probed. This study suggests that the outer regions of Cen A's halo show an increase in ellipticity as a function of radius, which could, however, be interpreted as the presence of substructure contaminating the observed fields. A subsequent study exploits additional HST pointings out to a remarkably large radius of ∼ 150 kpc: the edge of Cen A's halo is not reached even by this study (Rejkuba et al 2014). This dataset, analysed together with the previous HST pointings, confirms that a very mild metallicity gradient is present, with median metallicities remaining high out to the largest distances probed. Rejkuba et al (2014), however, also detect a significant pointing-to-pointing variation in both the RGB star counts and the median metallicity, which is likely indicative of non-mixed accreted populations.
Recently, the PISCeS survey (see previous Section) has sketched a PAndAS-like picture of Cen A's halo out to ∼ 150 kpc: the RGB stellar density map derived from a mosaic of Magellan/Megacam images is presented in Fig. 7. This map, very much like the ones obtained for M31 and NGC 891, uncovers a plethora of faint substructures, both in the inner regions of the target galaxy and in its outskirts. The morphological variety of these features is reminiscent of that observed in PAndAS, with shells, plumes, an extended cloud and long tidal streams. In particular, one of the newly discovered dwarf satellites of Cen A is clearly in the process of being disrupted, with ∼ 2 deg long tails: taking into account the stellar content of these tails, this galaxy's pre-disruption luminosity could have been similar to that of Sagittarius in the LG. This survey also led to the discovery of nine (confirmed) dwarf satellites down to MV ∼ −7. Their properties are consistent with those of faint LG satellites, but some of them lie at the faint/diffuse end of the LG luminosity/surface brightness/radius distribution: this indicates that we might be looking at previously unexplored physical regimes for these faintest satellites, which opens new exciting perspectives for future studies.
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Figure 7. Surface density map of RGB stars in the halo of Cen A, obtained with Magellan/Megacam as part of the PISCeS survey. The map extends out to a radius of 150 kpc in the north and east directions (physical and density scales are reported). Several tidal features are easily recognized, including a stunning disrupting dwarf with tails 2 deg long in the outer halo, an extended sparse cloud to the south of the galaxy, as well as arcs and plumes around the inner regions, tracing both ongoing and past accretion events. Fig. 3 from Crnojević et al (2016), reproduced by permission of the AAS |