arXiv subjects

Michelle L. M. Collins

Publications and source records attributed to Michelle L. M. Collins.

At least 19 recordsLinked to original sources

arXiv2026

It's Not Just Star Formation: A trend of low dark matter densities in the Andromeda dwarf galaxy system

Dynamical mass modeling of Andromeda (M31) dwarf spheroidal (dSph) galaxies has revealed a growing trend of lower central dark matter (DM) densities than predicted by pure DM structure formation in Lambda Cold Dark Matter ($Λ$CDM) cosmology simulations and lower than most Milky Way (MW) satellites. So far, however, only four of the 35 confirmed M31 dSphs have been successfully mass modeled. In this second paper of a series, we aim to better understand growing Local Group (LG) dSph patterns by mass modeling seven more M31 dSphs: Andromeda I, III, V, VII, IX, XXXI, and XXXII. We update the kinematics of each dwarf and estimate their central dark matter densities at 150 pc using the dynamical Jeans modeling tool, GravSphere. We also update their DM halo mass, $M_{\rm{200}}$, via abundance matching. We find Andromeda III and V to have central DM densities in line with $Λ$CDM expectations, resembling dSphs around the Milky Way. The remaining five dwarfs have anomalously low central densities, continuing a growing trend seen for M31 satellites. We investigate each dwarf's star formation history and find that star formation-induced `DM heating' is disfavored as the sole explanation of these lower central densities. We consider the effect of tides and halo concentration scatter on these systems and predict that they should be on more plunging orbits than their denser counterparts. If this prediction is misaligned with the data, it could necessitate new physics beyond the Standard Cosmological Model.

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arXiv2026

Andromeda XXXVI: discovery of a new ultra-faint dwarf galaxy towards M31

We present deep imaging of Andromeda XXXVI (And XXXVI), a dwarf galaxy discovered through visual inspection of the Pan-Andromeda Archaeological Survey, using observations obtained with the OSIRIS+GTC instrument. The colour-magnitude diagram of And XXXVI shows a well-defined red giant branch (RGB). However, constraining a distance is challenging because the tip of the RGB is sparsely populated and no horizontal branch stars are found. The RGB is nevertheless well matched by an old (12.5 Gyr), metal-poor ([Fe/H]= - 2.5) isochrone shifted to the distance of Andromeda (776 kpc). With a projected distance of 119 kpc from M31, And XXXVI is therefore likely a satellite of Andromeda. With MV -5.9 +-0.1, half-light radius rh 64 +30-19 pc, and an ellipticity 0.015 +0.032-0.012, And XXXVI is one of the faintest of ultra-faint M31 dwarfs discovered to date. The discovery of And XXXVI adds to the faint end of M31's satellite luminosity function, suggesting the presence of an even larger population of very faint satellites. Deeper space-based imaging and/or spectroscopic observations are needed to better constrain its position within M31's halo. Combined with a detailed star formation history, such data would help determine whether its old, metal-poor stellar population indicates early quenching, similar to the trends seen in Milky Way satellites, and whether And XXXVI could be considered a reionisation fossil.

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arXiv2025

Is M31 at the center of its satellite system ?

The arrangement of M31's dwarf galaxies exhibits anisotropy, with the majority located in the hemisphere between the Milky Way and the host galaxy. This study aims to investigate whether M31's present location is aligned with the center of its distribution of dwarf galaxies. We use forward modeling to infer the center of the M31 satellite 3D spatial distribution, folding in the completeness of dwarf galaxy searches. We observe a displacement of the center of the satellite distribution, relative to the center of M31, of approximately 10--50 kpc towards the Milky Way. Nonetheless, the center of M31 remains compatible with the center of the dwarf galaxy distribution given the broad constraints on its position, with the significance of the shift ranging from $\leq 1σ$ to $1.9σ$, depending on the assumed form of the volumetric spatial distribution of satellites. If M31 is truly offset from its satellite system, a quadrupling of the number of known satellites would be necessary to infer a significant ($3σ$) offset. Hence, expanding the number of known dwarf galaxies is crucial to deepen our understanding of the distribution of M31 satellites and further shed on its peculiar structure.

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arXiv2025

The emergence of globular clusters and globular-cluster-like dwarfs

Globular clusters (GCs) are among the oldest and densest stellar systems in the Universe, yet how they form remains a mystery. Here we present a suite of cosmological simulations in which both dark-matter-free GCs and dark-matter-rich dwarf galaxies naturally emerge in the Standard Cosmology. We show that these objects inhabit distinct locations in the size-luminosity plane and that they have similar ages, age spread, metallicity and metallicity spread to globulars and dwarfs in the nearby Universe. About half of our simulated globulars form by means of regular star formation near the centres of their host dwarf, with the rest forming further out, triggered by mergers. The latter are more tidally isolated and more likely to survive to the present day. Finally, our simulations predict the existence of a new class of object that we call 'globular-cluster-like dwarfs' (GCDs). These form from a single, self-quenching, star-formation event in low-mass dark-matter halos at high redshift and have observational properties intermediate between globulars and dwarfs. We identify several dwarfs in our Galaxy, such as Reticulum II (refs. 2-4), that could be in this new class. If so, they promise unprecedented constraints on dark-matter models and new sites to search for metal-free stars.

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arXiv2025

One Stream or Two -- Exploring Andromeda's North West Stream

We present results of our dynamical stream modelling for the North West Stream in the outer halo of the Andromeda galaxy (M31). Comprising two main segments, the North West Stream was thought to be a single structured arching around M31. However, recent evidence suggests that it is two separate, unrelated, streams. To test this hypothesis we use observational data from 6 fields associated with the upper segment of the North West Stream together with 8 fields and 5 globular clusters associated with the lower segment to constrain model orbits. We fit both segments of the stream using a fixed potential model for M31 and an orbit integrator to compare orbits with the observed streams. We measure the central tracks and predict proper motions for for the upper segment (lower segment) finding ${μ^*_α}$ = 0.078$^{+0.015}_{-0.012}$ (0.085$^{+0.001}_{-0.002}$) mas/yr and ${μ_δ}$ = $-$0.05$^{+0.008}_{-0.009}$ ($-$0.095$^{+0.003}_{-0.005}$) mas/yr. Our results support the hypothesis that the dwarf spheroidal galaxy Andromeda XXVII is the progenitor of the upper segment of the North West Stream and that the upper and lower segments do not comprise a single structure. We propose that the upper segment, which appears to be on an infall trajectory with M31, be renamed the "Andromeda XXVII Stream" and the lower segment, also apparently infalling towards M31, retain the name "North West Stream".

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arXiv2025

Properties of the Lower Segment of M31's North West Stream

We present a kinematic and spectroscopic analysis of 40 red giant branch stars, in 9 fields, exquisitely delineating the lower segment of the North West Stream (NW-K2), which extends for $\sim$80 kpc from the centre of the Andromeda galaxy. We measure the stream's systemic velocity as -439.3$^{+4.1}_{-3.8}$ km/s with a velocity dispersion = 16.4$^{+5.6}_{-3.8}$ km/s that is in keeping with its progenitor being a dwarf galaxy. We find no detectable velocity gradient along the stream. We determine $-$1.3$\pm$0.1 $\le$ <[Fe/H]$_{\rm spec}$> $\le$ $-$1.2$\pm$0.8 but find no metallicity gradient along the stream. We are able to plausibly associate NW-K2 with the globular clusters PandAS-04, PandAS-09, PAndAS-10, PAndAS-11, PandAS-12 but not with PandAS-13 or PandAS-15 which we find to be superimposed on the stream but not kinematically associated with it.

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arXiv2025

Mass Modeling the Andromeda Dwarf Galaxies: Andromeda VI and Andromeda XXIII

Accurately mapping the mass profiles of low mass dwarf spheroidal (dSph) galaxies allows us to test predictions made by dark matter (DM) models. To date, such analyses have primarily been performed on Milky Way (MW) satellites. Meanwhile, the Andromeda Galaxy (M31) is home to 35 known dwarf galaxies, yet only two have been successfully mass-modeled so far. A more comprehensive study of Local Group dwarfs is necessary to better understand the nature of dark matter. In this study, we have undertaken a dynamical study of two higher-luminosity Andromeda dwarf galaxies: Andromeda VI (And VI) and Andromeda XXIII (And XXIII). We infer an enclosed mass for And VI of M(r $<$ r$_{h}$) = (4.9 $\pm$ 1.5) $\times$ 10$^{7}$ M$_{\odot}$, corresponding to a mass-to-light ratio of $[M/L]_{r_{\rm{h}}}$ = (27.1 $\pm$ 8.2) M$_{\odot}$/L$_{\odot}$. We infer an enclosed mass for And XXIII of M(r $<$ r$_{h}$) = (3.1 $\pm$ 1.9) $\times$ 10$^{7}$ M$_{\odot}$, corresponding to a mass-to-light ratio of $[M/L]_{r_{\rm{h}}}$ = (90.2 $\pm$ 53.9) M$_{\odot}$/L$_{\odot}$. Using the dynamical Jeans modeling tool, \gravsphere, we determine And VI and And XXIII's dark matter density at 150 pc, finding $ρ_{\rm{DM,VI}}$(150 pc) = (1.4 $\pm$ 0.5) $\times 10^{8}$ M$_{\odot}$ kpc$^{-3}$ and $ρ_{\rm{DM,XXIII}}$(150 pc) = 0.5$\substack{+0.4 \\ -0.3} \times 10^{8}$ M$_{\odot}$ kpc$^{-3}$. Our results make And VI the first mass-modeled M31 satellite to fall into the cuspy regime. And XXIII has a lower density, implying either a more cored central dark matter density, or a lowering of the density through tides. This adds And XXIII to a growing list of M31 dwarfs with a central density lower than most MW dwarfs and lower than expected for isolated dwarfs in the Standard Cosmology. This could be explained by the M31 dwarfs having experienced stronger tides than their MW counterparts.

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arXiv2025

The satellite galaxies of the Milky Way and Andromeda

The satellite galaxies of the Local Group provide us with an important probe of galaxy formation, evolution, and cosmology. The two large spirals that dominate this group -- the Milky Way and Andromeda -- are each host to tens of satellites, ranging in stellar mass from $M_*=3\times10^9\,{\rm M_\odot}$ down to as little as $M_*\sim1000\,{\rm M_\odot}$. In this review, we (1) provide an overview of the known satellite population of the Milky Way and Andromeda, including how they are discovered and their observed properties; (2) discuss their importance in understanding the nature of dark matter, star formation in the early Universe, the assembly histories of their massive hosts, and the impact of reionisation on the lowest mass galaxies; and (3) highlight the coming revolution and challenges of this field as new observatories and facilities come online. In the coming decades, the study of Local Group satellites should allow us to place competitive constraints on both dark matter and galaxy evolution.

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arXiv2024

Elemental Abundances in And XIX From Coadded Spectra

With a luminosity similar to that of Milky Way dwarf spheroidal (dSph) systems like Sextans, but a spatial extent similar to that of ultradiffuse galaxies (UDGs), Andromeda (And) XIX is an unusual satellite of M31. To investigate the origin of this galaxy, we measure chemical abundances for AndXIX derived from medium-resolution (R$\sim$6000) spectra from Keck II/DEIMOS. We coadd 79 red giant branch stars, grouped by photometric metallicity, in order to obtain a sufficiently high signal-to-noise ratio (S/N) to measure 20 [Fe/H] and [$α$/Fe] abundances via spectral synthesis. The latter are the first such measurements for AndXIX. The mean metallicity we derive for AndXIX places it $\sim2σ$ higher than the present-day stellar mass-metallicity relation for Local Group dwarf galaxies, potentially indicating it has experienced tidal stripping. A loss of gas and associated quenching during such a process, which prevents the extended star formation necessary to produce shallow [$α$/Fe]--[Fe/H] gradients in massive systems, is also consistent with the steeply decreasing [$α$/Fe]--[Fe/H] trend we observe. In combination with the diffuse structure and disturbed kinematic properties of AndXIX, this suggests tidal interactions, rather than galaxy mergers, are strong contenders for its formation.

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arXiv2024

Stellar Metallicities and Gradients in the Faint M31 Satellites Andromeda XVI and Andromeda XXVIII

We present $\sim300$ stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI ($M_V = -7.5$) and Andromeda XXVIII ($M_V = -8.8$) derived using metallicity-sensitive Calcium H & K narrow-band Hubble Space Telescope imaging. These are the first individual stellar metallicities in And~XVI (95 stars). Our And~XXVIII sample (191 stars) is a factor of $\sim15$ increase over literature metallicities. For And~XVI, we measure $\langle \mbox{[Fe/H]}\rangle = -2.17^{+0.05}_{-0.05}$, $σ_{\mbox{[Fe/H]}}=0.33^{+0.07}_{-0.07}$, and $\nabla_{\mbox{[Fe/H]}} = -0.23\pm0.15$ dex $R_e^{-1}$. We find that And XVI is more metal-rich than MW UFDs of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure $\langle \mbox{[Fe/H]}\rangle = -1.95^{+0.04}_{-0.04}$, $σ_{\mbox{[Fe/H]}}=0.34^{+0.07}_{-0.07}$, and $\nabla_{\mbox{[Fe/H]}} = -0.46 \pm 0.10$~dex~$R_e^{-1}$, placing it on the dwarf galaxy mass-metallicity relation. Neither galaxy has a metallicity distribution function with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint ($L \lesssim 10^6~L_{\odot}$) galaxies outside the Milky Way halo. Both galaxies' gradients are consistent with predictions from the FIRE simulations, where an age-gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal poor ($\mbox{[Fe/H]} < -3.0$) star candidates, which make up 7% of And~XVI's MDF and 6% of And~XXVIII's.

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arXiv2024

Discovery of Globular Cluster Candidates in the Dwarf Irregular Galaxy IC 2574 Using HST/ACS Imaging

We report the discovery of 23 globular cluster (GC) candidates around the relatively isolated dwarf galaxy IC 2574 within the Messier 81 (M81) group, at a distance of 3.86 Mpc. We use observations from the HST Advanced Camera for Surveys (ACS) to analyse the imaging in the F814W and F555W broadband filters. Our GC candidates have luminosities ranging from $-5.9 \geq M_V \geq -10.4$ and half-light radii of $1.4 \leq r_h \leq 11.5$ pc. We find the total number of GCs ($N_{\mathrm{GC}})=27\pm5$ after applying completeness corrections, which implies a specific frequency of $S_N = 4.0\pm0.8$, consistent with expectations based on its luminosity. The GC system appears to have a bimodal colour distribution, with 30% of the GC candidates having redder colours. We also find 5 objects with extremely blue colours that could be young star clusters linked to an intense star formation episode that occurred in IC 2574 $\sim$1 Gyr ago. We make an independent measurement of the halo mass of IC 2574 from its kinematic data, which is rare for low mass galaxies, and find log $M_{200} = 10.93 \pm 0.08$. We place the galaxy on the well-known GC system mass-halo mass relation and find that it agrees well with the observed near-linear relation. IC 2574 has a rich GC population for a dwarf galaxy, which includes an unusually bright $ω$ Cen-like GC, making it an exciting nearby laboratory for probing the peculiar efficiency of forming massive GCs in dwarf galaxies.

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arXiv2024

Pisces VII/Triangulum III -- M33's second dwarf satellite galaxy

Pisces VII/Triangulum III (Pisc~VII) was discovered in the DESI Legacy Imaging Survey and was shown to be a Local Group dwarf galaxy with follow-up imaging from the 4-m Telescopio Nazionale Galileo. However, this imaging was unable to reach the horizontal branch of Pisc VII, preventing a precision distance measurement. The distance bound from the red giant branch population placed Pisc VII as either an isolated ultra-faint dwarf galaxy or the second known satellite galaxy of Triangulum (M33). Using deep imaging from Gemini GMOS-N, we have resolved the horizontal branch of Pisc VII, and measure a distance of $D=916^{+65}_{-53}$~kpc, making Pisc VII a likely satellite of M33. We also remeasure its size and luminosity from this deeper data, finding $r_{\rm half}=186^{+58}_{-32}$ pc, $M_V=-6.0\pm0.3$ and $L=2.2^{+0.7}_{-0.5}\times10^4\,{\rm L}_\odot$. Given its position in the M33 halo, we argue that Pisc VII could support the theory that M33 is on its first infall to the Andromeda system. We also discuss the presence of blue plume and helium burning stars in the colour-magnitude diagram of Pisc VII that are consistent with ages of $\sim1.5$~Gyr. If these are truly members of the galaxy, it would transform our understanding of how reionisation affects the faintest galaxies. Future deep imaging and dynamics could allow significant insight into both the stellar populations of Pisc VII and the evolution of M33

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arXiv2023

RomAndromeda: The Roman Survey of the Andromeda Halo

As our nearest large neighbor, the Andromeda Galaxy provides a unique laboratory for investigating galaxy formation and the distribution and substructure properties of dark matter in a Milky Way-like galaxy. Here, we propose an initial 2-epoch ($Δt\approx 5$yr), 2-band Roman survey of the entire halo of Andromeda, covering 500 square degrees, which will detect nearly every red giant star in the halo (10$σ$ detection in F146, F062 of 26.5, 26.1AB mag respectively) and yield proper motions to $\sim$25 microarcsec/year (i.e., $\sim$90 km/s) for all stars brighter than F146 $\approx 23.6$ AB mag (i.e., reaching the red clump stars in the Andromeda halo). This survey will yield (through averaging) high-fidelity proper motions for all satellites and compact substructures in the Andromeda halo and will enable statistical searches for clusters in chemo-dynamical space. Adding a third epoch during the extended mission will improve these proper motions by $\sim t^{-1.5}$, to $\approx 11$ km/s, but this requires obtaining the first epoch in Year 1 of Roman operations. In combination with ongoing and imminent spectroscopic campaigns with ground-based telescopes, this Roman survey has the potential to yield full 3-d space motions of $>$100,000 stars in the Andromeda halo, including (by combining individual measurements) robust space motions of its entire globular cluster and most of its dwarf galaxy satellite populations. It will also identify high-velocity stars in Andromeda, providing unique information on the processes that create this population. These data offer a unique opportunity to study the immigration history, halo formation, and underlying dark matter scaffolding of a galaxy other than our own.

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arXiv2022

Andromeda XXV -- a dwarf galaxy with a low central dark matter density

Andromeda (And) XXV has previously been reported as a dwarf spheroidal galaxy (dSph) with little-to-no dark matter. However, the uncertainties on this result were significant. In this study, we double the number of member stars and re-derive the kinematics and mass of And XXV. We find that And XXV has a systemic velocity of $ν_\mathrm{r}=-107.7\pm1.0 \mathrm{~km s}^{-1}$ and a velocity dispersion of $σ_ν=4.5\pm1.0\mathrm{~km s}^{-1}$. With this better constrained velocity dispersion, we derive a mass contained within the half-light radius of $M(r< r_\mathrm{h})=6.9^{+3.2}_{-2.8}\times10^6\mathrm{~M}_\odot$. This mass corresponds to a mass-to-light ratio of $\mathrm{[M/L]}_\mathrm{r_\mathrm{h}}=37^{+17}_{-15}\mathrm{~M}_\odot/\mathrm{L}_\odot$, demonstrating, for the first time, that And XXV has an unambiguous dark matter component. We also measure the metallicity of And XXV to be $\mathrm{[Fe/H]}=-1.9\pm0.1$$\mathrm{~}$dex, which is in agreement with previous results. Finally, we extend the analysis of And XXV to include mass modelling using GravSphere. We find that And XXV has a low central dark matter density, $ρ_\mathrm{DM}(150\mathrm{pc})= 2.7^{+1.8}_{-1.6}\times10^7\mathrm{~M}_\odot\mathrm{kpc}^{-3}$, making And XXV a clear outlier when compared to other Local Group (LG) dSphs of the similar stellar mass. In a companion paper, we will explore whether some combination of dark matter cusp-core transformations and/or tides can explain And XXV's low density.

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arXiv2022

A detailed star formation history for the extremely diffuse Andromeda XIX dwarf galaxy

We present deep imaging of the ultra-diffuse Andromeda XIX dwarf galaxy from the Advance Camera for Surveys on the Hubble Space Telescope which resolves its stellar populations to below the oldest main sequence turn-off. We derive a full star formation history for the galaxy using MATCH, and find no evidence of star formation in the past 8 Gyr. We calculate a quenching time of $τ_{90}=9.7\pm0.2$~Gyr, suggesting Andromeda~XIX ceased forming stars very early on. This early quenching, combined with its extremely large half-light radius, low density dark matter halo and lower than expected metallicity make it a unique galaxy within the Local Group and raises questions about how it formed. The early quenching time allows us to rule out feedback from bursty star formation as a means to explain its diffuse stellar population and low density dark matter halo. We find that the extended stellar population, low density halo and star formation could be explained by either tidal interactions (such as tidal shocking) or by late dry mergers, with the latter also explaining its low metallicity. Proper motions and detailed abundances would allow us to distinguish between these two scenarios.

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arXiv2022

The Hubble Space Telescope Survey of M31 Satellite Galaxies I. RR Lyrae-based Distances and Refined 3D Geometric Structure

We measure homogeneous distances to M31 and 38 associated stellar systems ($-$16.8$\le M_V \le$ $-$6.0), using time-series observations of RR Lyrae stars taken as part of the Hubble Space Telescope Treasury Survey of M31 Satellites. From $>700$ orbits of new/archival ACS imaging, we identify $>4700$ RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 days and 0.04 mag. Based on Period-Wesenheit-Metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of $\sim20$ kpc (3%) and $\sim10$ kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that $\sim80$% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms $7-23$ kpc) planar "arc" of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess physical proximity of notable associations such as the NGC 147/185 pair and M33/AND XXII; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with $M_V > -9.5$, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system.

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arXiv2022

Observational constraints on stellar feedback in dwarf galaxies

Feedback to the interstellar medium (ISM) from ionising radiation, stellar winds and supernovae is central to regulating star formation in galaxies. Due to their low mass ($M_{*} < 10^{9}$\,M$_\odot$), dwarf galaxies are particularly susceptible to such processes, making them ideal sites to study the detailed physics of feedback. In this perspective, we summarise the latest observational evidences for feedback from star forming regions and how this drives the formation of 'superbubbles' and galaxy-wide winds. We discuss the important role of external ionising radiation -- 'reionisation' -- for the smallest galaxies. And, we discuss the observational evidences that this feedback directly impacts galaxy properties such as their star formation histories, metal content, colours, sizes, morphologies and even their inner dark matter densities. We conclude with a look to the future, summarising the key questions that remain unanswered and listing some of the outstanding challenges for galaxy formation theories.

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arXiv2022

Pegasus V -- a newly discovered ultra-faint dwarf galaxy on the outskirts of Andromeda

We report the discovery of an ultra-faint dwarf in the constellation of Pegasus. Pegasus~V (Peg~V) was initially identified in the public imaging data release of the DESI Legacy Imaging Surveys and confirmed with deep imaging from Gemini/GMOS-N. The colour-magnitude diagram shows a sparse red giant branch (RGB) population and a strong over-density of blue horizontal branch stars. We measure a distance to Peg~V of $D=692^{+33}_{-31}$~kpc, making it a distant satellite of Andromeda with $M_V=-6.3\pm0.2$ and a half-light radius of $r_{\rm half}=89\pm41$~pc. It is located $\sim260$~kpc from Andromeda in the outskirts of its halo. The RGB is well-fit by a metal-poor isochrone with [Fe/H]$=-3.2$, suggesting it is very metal poor. This, combined with its blue horizontal branch could imply that it is a reionisation fossil. This is the first detection of an ultra-faint dwarf outside the deep Pan-Andromeda Archaeological Survey area, and points to a rich, faint satellite population in the outskirts of our nearest neighbour.

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