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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 |$^{-1}$| with a velocity dispersion =16.4 |$^{+5.6}_{-3.8}$| km s |$^{-1}$| that is in keeping with its progenitor being a dwarf galaxy. We find no detectable velocity gradient along the stream. We determine |$-1.3\pm0.1$| |$\le$| |$\langle[\mathrm{ Fe/H}]_{\rm spec}\rangle$| |$\le$| |$-1.2\pm0.8$| 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, and 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. [ABSTRACT FROM AUTHOR]

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 six fields associated with the upper segment of the North West Stream together with eight fields and five 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 the upper segment (lower segment) finding |${\mu ^{*}_{\alpha }}$|  = 0.078 |$^{+0.015}_{-0.012}$| (0.085 |$^{+0.001}_{-0.002}$|⁠) mas yr−1 and |${\mu _{\delta }}$|  = |$-0.05^{+0.008}_{-0.009}$| (⁠|$-0.095^{+0.003}_{-0.005}$|⁠) mas yr−1. 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'. [ABSTRACT FROM AUTHOR]

Some ultra diffuse galaxies (UDGs) reveal many more globular clusters (GCs) than classical dwarf galaxies of the same stellar mass. These UDGs, with a mass in their GC system (⁠|$M_{\rm GC}$|⁠) approaching 10 per cent of their host galaxy stellar mass (⁠|$M_{\ast }$|⁠), are also inferred to have high halo mass to stellar mass ratios (⁠|$M_{\rm halo}/M_{\ast }$|⁠). They have been dubbed Failed Galaxies. It is unknown what role high GC formation efficiencies and/or low destruction rates play in determining the high |$M_{\rm GC}/M_{\ast }$| ratios of some UDGs. Here we present a simple model, which is informed by recent JWST observations of lensed galaxies and by a simulation in the literature of GC mass loss and tidal disruption in dwarf galaxies. With this simple model, we aim to constrain the effects of GC efficiency/destruction on the observed GC richness of UDGs and their variation with the integrated stellar populations of UDGs. We assume no ongoing star formation (i.e. quenching at early times) and that the disrupted GCs contribute their stars to those of the host galaxy. We find that UDGs, with high |$M_{\rm GC}/M_{\ast }$| ratios today, are most likely the result of very high GC formation efficiencies combined with modest rates of GC destruction. The current data loosely follow the model that ranges from the mean stellar population of classical dwarfs to that of metal-poor GCs as |$M_{\rm GC}/M_{\ast }$| increases. As more data becomes available for UDGs, our simple model can be refined and tested further. [ABSTRACT FROM AUTHOR]

We present ∼300 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 and K narrowband 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 ∼15 increase over literature metallicities. For And XVI, we measure 〈 [Fe/H] 〉 = − 2.17 − 0.05 + 0.05 , σ [Fe/H] = 0.33 − 0.07 + 0.07 , and ∇[Fe/H] = −0.23 ± 0.15 dex R e − 1 . We find that And XVI is more metal-rich than Milky Way ultrafaint dwarf galaxies of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure 〈 [Fe/H] 〉 = − 1.95 − 0.04 + 0.04 , σ [Fe/H] = 0.34 − 0.05 + 0.05 , and ∇[Fe/H]= −0.46 ± 0.10 dex R e − 1 , placing it on the dwarf galaxy mass–metallicity relation. Neither galaxy has a metallicity distribution function (MDF) 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 ≲ 106 L ⊙) 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 ([Fe/H] < –3.0) star candidates, which make up 7% of And XVI's MDF and 6% of And XXVIII's. [ABSTRACT FROM AUTHOR]

With a luminosity similar to that of Milky Way dwarf spheroidal systems like Sextans, but a spatial extent similar to that of ultra-diffuse galaxies, Andromeda (And) XIX is an unusual satellite of M31. To investigate the origin of this galaxy, we measure chemical abundances for And XIX derived from medium-resolution (R ∼ 6000) spectra from the Deep Extragalactic Imaging Multi-Object Spectrograph on the Keck II telescope. We coadd 79 red giant branch stars, grouped by photometric metallicity, in order to obtain a sufficiently high signal-to-noise ratio to measure 20 [Fe/H] and [ α /Fe] abundances via spectral synthesis. The latter are the first such measurements for And XIX. The mean metallicity we derive for And XIX places it ∼2 σ 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 And XIX, this suggests tidal interactions, rather than galaxy mergers, are strong contenders for its formation. [ABSTRACT FROM AUTHOR]

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 broad-band filters. Our GC candidates have luminosities ranging from −5.9 ≥ MV ≥ −10.4 and half-light radii of 1.4 ≤ r h ≤ 11.5 pc. We find the total number of GCs (N GC) = 27 ± 5 after applying completeness corrections, which implies a specific frequency of S N = 4.0 ± 0.8, consistent with expectations based on its luminosity. The GC system appears to have a bimodal colour distribution, with 30 percent of the GC candidates having redder colours. We also find five objects with extremely blue colours that could be young star clusters linked to an intense star formation episode that occurred in IC 2574 ∼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 ± 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. [ABSTRACT FROM AUTHOR]

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, MV  = −6.0 ± 0.3, and |$L=2.2^{+0.7}_{-0.5}\times 10^4\, {\rm L}_\odot$|⁠. 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 ∼1.5 Gyr. If these are truly members of the galaxy, it would transform our understanding of how reionization 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. [ABSTRACT FROM AUTHOR]

We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD; M V > − 7.0, 4.9 < log 10 (M * (z = 0) / M ⊙) < 5.5) satellite galaxies of M31 based on deep color–magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main-sequence turnoff of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass by z = 5 (12.6 Gyr ago), similar to known UFDs around the MW, but that 10%–40% of their stellar mass formed at later times. We uncover one remarkable UFD, And xiii, which formed only 10% of its stellar mass by z = 5, and 75% in a rapid burst at z ∼ 2–3, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This "young" UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least-massive MW UFDs (M *(z = 5) ≲ 5 × 104 M ⊙) are likely quenched by reionization, whereas more-massive M31 UFDs (M *(z = 5) ≳ 105 M ⊙) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs. [ABSTRACT FROM AUTHOR]

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 nearly 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 νr = −107.7 ± 1.0 kms−1 and a velocity dispersion of |$\sigma _{\rm \nu }=3.7^{+1.2}_{-1.1}$|  kms−1. With this updated velocity dispersion and a new literature measurement of the radial surface brightness profile, we derive a mass contained within the half-light radius of |$M(r\lt r_{\rm h})=4.7^{+3.0}_{-2.9}\times 10^6$|  M⊙. This mass corresponds to a mass-to-light ratio of |$\text{[M/L]}_{\rm r_{h}}=25^{+17}_{-16}$|  M⊙/L⊙, demonstrating that And XXV is most-likely dark matter dominated. We also measure the metallicity of And XXV to be |$\rm [Fe/H]=-1.9\pm 0.1$|  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, ρDM(150 pc) =  |$2.3^{+1.4}_{-1.1}\times 10^7$|  M⊙ kpc−3, which makes And XXV a clear outlier when compared to other local group 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. [ABSTRACT FROM AUTHOR]

We report the discovery of an ultrafaint dwarf in the constellation of Pegasus. Pegasus V (Peg V)/Andromeda XXXIV 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 overdensity 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 MV  = −6.3 ± 0.2 and a half-light radius of r half = 89 ± 41 pc. It is located ∼260 kpc from Andromeda in the outskirts of its halo. The RGB is well fitted 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 reionization fossil. This is the first detection of an ultrafaint dwarf outside the deep Pan-Andromeda Archaeological Survey area, and points to a rich, faint satellite population in the outskirts of our nearest neighbour. [ABSTRACT FROM AUTHOR]

We report deep imaging observations with DOLoRes@TNG of an ultra-faint dwarf satellite candidate of the Triangulum galaxy (M33) found by visual inspection of the public imaging data release of the DESI Legacy Imaging Surveys. Pisces VII/Triangulum (Tri) III is found at a projected distance of |$72\, {\rm kpc}$| from M33, and using the tip of the red giant branch method, we estimate a distance of |$D=1.0^{+0.3}_{-0.2}\, {\rm Mpc}$|⁠ , meaning the galaxy could either be an isolated ultra-faint or the second known satellite of M33. We estimate an absolute magnitude of MV  = −6.1 ± 0.2 if Pisces VII/Tri II is at the distance of M33, or as bright as MV  = −6.8 ± 0.2 if the galaxy is isolated. At the isolated distance, it has a physical half-light radius of |$r_{\rm h}=131\pm 61\, {\rm pc}$| consistent with similarly faint galaxies around the Milky Way. As the tip of the red giant branch is sparsely populated, constraining a precision distance is not possible, but if Pisces VII/Tri III can be confirmed as a true satellite of M33, it is a significant finding. With only one potential satellite detected around M33 previously (Andromeda XXII/Tri I), it lacks a significant satellite population in stark contrast to the similarly massive Large Magellanic Cloud. The detection of more satellites in the outskirts of M33 could help to better illuminate if this discrepancy between expectation and observations is due to a poor understanding of the galaxy formation process, or if it is due to the low luminosity and surface brightness of the M33 satellite population which has thus far fallen below the detection limits of previous surveys. If it is truly isolated, it would be the faintest known field dwarf detected to date. [ABSTRACT FROM AUTHOR]

Andromeda XXI (And XXI) has been proposed as a dwarf spheroidal galaxy with a central dark matter density that is lower than expected in the standard |$\Lambda$| cold dark matter (⁠|$\Lambda$| CDM) cosmology. In this work, we present dynamical observations for 77 member stars in this system, more than doubling previous studies to determine whether this galaxy is truly a low-density outlier. We measure a systemic velocity of |$v_r=-363.4\pm 1.0{\rm \, km\, s^{-1}}$| and a velocity dispersion of |$\sigma _v=6.1^{+1.0}_{-0.9}{\rm \, km\, s^{-1}}$|⁠ , consistent with previous work and within |$1\sigma$| of predictions made using the modified Newtonian dynamics framework. We also measure the metallicity of our member stars from their spectra, finding a mean value of |${\rm [Fe/H]}=-1.7\pm 0.1$|  dex. We model the dark matter density profile of And XXI using an improved version of gravsphere , finding a central density of |$\rho _{\rm DM}({\rm 150 pc})=2.6_{-1.5}^{+2.4} \times 10^7 \, {\rm M_\odot \, kpc^{-3}}$| at 68 per cent confidence, and a density at two half-light radii of |$\rho _{\rm DM}({\rm 1.75 kpc})=0.9_{-0.2}^{+0.3} \times 10^6 \, {\rm M_\odot \, kpc^{-3}}$| at 68 per cent confidence. These are both a factor of |${\sim }3\!-\!5$| lower than the densities expected from abundance matching in |$\Lambda$| CDM. We show that this cannot be explained by 'dark matter heating' since And XXI had too little star formation to significantly lower its inner dark matter density, while dark matter heating only acts on the profile inside the half-light radius. However, And XXI's low density can be accommodated within |$\Lambda$| CDM if it experienced extreme tidal stripping (losing |${\gt}95{{\ \rm per\ cent}}$| of its mass), or if it inhabits a low-concentration halo on a plunging orbit that experienced repeated tidal shocks. [ABSTRACT FROM AUTHOR]

We present new chemo-kinematics of the Hercules dwarf galaxy based on Keck II-DEIMOS spectroscopy. Our 21 confirmed members, including 9 newly confirmed members, have a systemic velocity of v Herc = 46.4 ± 1.3 km s−1 and a velocity dispersion |$\sigma _{v,\mathrm{Herc}}=4.4^{+1.4}_{-1.2}$| km s−1, consistent with previous studies. From the strength of the Ca  ii triplet, we obtain a metallicity of [Fe/H] = −2.48 ± 0.19 dex and dispersion of |$\sigma _{\rm {[Fe/H]}}= 0.63^{+0.18}_{-0.13}$| dex. This result is within 1σ of previous measurements, and makes Hercules a particularly metal-poor galaxy, placing it slightly below the standard mass–metallicity relation. Previous photometric and spectroscopic evidence suggests that Hercules is tidally disrupting and may be on a highly radial orbit. From our identified members, we measure no significant velocity gradient. By cross-matching with the second Gaia data release, we determine an uncertainty-weighted mean proper motion of |$\mu _{\alpha }^*=\mu _{\alpha }\cos (\delta)=-0.153\pm {0.074}$| mas yr−1, μδ = −0.397 ± 0.063 mas yr−1. This proper motion is slightly misaligned with the elongation of Hercules, in contrast to models which suggest that any tidal debris should be well aligned with the orbital path. Future observations may resolve this tension. [ABSTRACT FROM AUTHOR]

The dwarf galaxy distribution surrounding M31 is significantly anisotropic in nature. Of the 30 dwarf galaxies in this distribution, 15 form a disc-like structure and 23 are contained within the hemisphere facing the Milky Way. Using a realistic local potential, we analyse the conditions required to produce and maintain these asymmetries. We find that some dwarf galaxies are required to have highly eccentric orbits in order to preserve the presence of the hemispherical asymmetry with an appropriately large radial dispersion. Under the assumption that the dwarf galaxies originate from a single association or accretion event, we find that the initial size and specific energy of that association must both be relatively large in order to produce the observed hemispherical asymmetry. However if the association was large in physical size, the very high-energy required would enable several dwarf galaxies to escape from the M31 and be captured by the Milky Way. Furthermore, we find that associations that result in this structure have total specific energies concentrated around |$E = V_{\rm esc}^{2} - V_{\rm init}^{2} \sim 200^2$| – |$300^2\ \rm {km^2\ s^{-2}}$|⁠ , implying that the initial velocity and initial position needed to produce the structure are strongly correlated. The overlap of initial conditions required to produce the radial dispersion, angular dispersion, and the planar structure is small and suggests that either they did not originate from a single accretion event, or that these asymmetric structures are short-lived. [ABSTRACT FROM AUTHOR]

With a central surface brightness of μ0 = 29.3 mag arcsec−2, and half-light radius of |$r_{\rm half}=3.1^{+0.9}_{-1.1}$|  kpc, Andromeda XIX (And XIX) is an extremely diffuse satellite of Andromeda. We present spectra for ∼100 red giant branch stars in this galaxy, plus 16 stars in a nearby stellar stream. With this exquisite data set, we re-derive the properties of And XIX, measuring a systemic velocity of 〈 v r 〉 = −109.0 ± 1.6 km s−1 and a velocity dispersion of |$\sigma _{vr} = 7.8^{+1.7}_{-1.5}$|  km s−1 (higher than derived in our previous work). We marginally detect a velocity gradient along the major axis of |$\frac{{\rm d} v}{{\rm d}\chi }= -2.1\pm 1.8~{\rm km\, s}^{-1}$|  kpc−1. We find its mass-to-light ratio is higher than galaxies of comparable stellar mass (⁠|${}[M/L]_{\rm half} = 278^{+146}_{-198}\, \mathrm{M}_\odot /\mathrm{L}_\odot$|⁠), but its dynamics place it in a halo with a similar total mass to these galaxies. This could suggest that And XIX is a 'puffed up' dwarf galaxy, whose properties have been altered by tidal processes, similar to its Milky Way counterpart, Antlia II. For the nearby stream, we measure |$v_r=-279.2\pm 3.7~{\rm km\, s}^{-1}$|⁠ , and |$\sigma _v=13.8^{+3.5}_{-2.6}~{\rm km\, s}^{-1}$|⁠. We measure its metallicity, and find it to be more metal rich than And XIX, implying that the two features are unrelated. Finally, And XIX's dynamical and structural properties imply it is a local analogue to ultradiffuse galaxies (UDGs). Its complex dynamics suggest that the masses of distant UDGs measured from velocity dispersions alone should be carefully interpreted. [ABSTRACT FROM AUTHOR]

We present an extensive study of the Sagittarius II (Sgr II) stellar system using MegaCam g and i photometry, narrow-band, metallicity-sensitive calcium H&K doublet photometry and Keck II/DEIMOS multiobject spectroscopy. We derive and refine the Sgr II structural and stellar properties inferred at the time of its discovery. The colour–magnitude diagram implies Sgr II is old (12.0 ± 0.5 Gyr) and metal poor. The CaHK photometry confirms the metal-poor nature of the satellite ([Fe/H] CaHK = −2.32 ± 0.04 dex) and suggests that Sgr II hosts more than one single stellar population (⁠|$\sigma _\mathrm{[FeH]}^\mathrm{CaHK} = 0.11^{+0.05}_{-0.03}$| dex). Using the Ca infrared triplet measured from our highest signal-to-noise spectra, we confirm the metallicity and dispersion inferred from the Pristine photometric metallicities ([Fe/H]spectro = −2.23 ± 0.05 dex, |$\sigma _\mathrm{[Fe/H]}^\mathrm{spectro} = 0.10 ^{+0.06}_{-0.04}$| dex). The velocity dispersion of the system is found to be |$\sigma _{v} = 2.7^{+1.3}_{-1.0} {\rm \, km \,\, s^{-1}}$| after excluding two potential binary stars. Sgr II's metallicity and absolute magnitude (MV  = −5.7 ± 0.1 mag) place the system on the luminosity–metallicity relation of the Milky Way dwarf galaxies despite its small size. The low but resolved metallicity and velocity dispersions paint the picture of a slightly dark-matter-dominated satellite (⁠|$M/L = 23.0^{+32.8}_{-23.0}$| M⊙ L |$^{-1}_{\odot }$|⁠). Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocentre of |$118.4 ^{+28.4}_{-23.7} {\rm \, kpc}$| and a pericentre of |$54.8 ^{+3.3}_{-6.1} {\rm \, kpc}$|⁠. The orbit of Sgr II is consistent with the trailing arm of the Sgr stream and indicates that it is possibly a satellite of the Sgr dSph that was tidally stripped from the dwarf's influence. [ABSTRACT FROM AUTHOR]

We present a kinematic and spectroscopic analysis of 38 red giant branch stars, in seven fields, spanning the dwarf spheroidal galaxy Andromeda XXVII and the upper segment of the North West Stream. Both features are located in the outer halo of the Andromeda galaxy at a projected radius of 50–80 kpc, with the stream extending for ∼3° on the sky. Our data are obtained as part of the PAndAS survey and enables us to confirm that Andromeda XXVII's heliocentric distance is 827 ± 47 kpc and spectroscopic metallicity is |$-2.1^{+0.4}_{-0.5}$|⁠. We also re-derive Andromeda XXVII's kinematic properties, measuring a systemic velocity  =  |$-526.1^{+10.0}_{-11.0}$|  km s−1 and a velocity dispersion that we find to be non-Gaussian but for which we derive a formal value of 27.0 |$^{+2.2}_{-3.9}$|  km s−1. In the upper segment of the North West Stream we measure mean values for the metallicity = −1.8 ± 0.4, systemic velocity  = −519.4 ± 4.0 km s−1, and velocity dispersion  = 10.0 ± 4.0 km s−1. We also detect a velocity gradient of 1.7 ± 0.3 km s−1 kpc−1 on an infall trajectory towards M31. With a similar gradient, acting in the same direction, in the lower segment we suggest that the North West Stream is not a single structure. As the properties of the upper segment of the North West Stream and Andromeda XXVII are consistent within 90 per cent confidence limits, it is likely that the two are related and plausible that Andromeda XXVII is the progenitor of this stream. [ABSTRACT FROM AUTHOR]

We present new horizontal branch (HB) distance measurements to 17 of the faintest known M31 satellites (−6 ≲  M V ≲ −13) based on deep Hubble Space Telescope (HST) imaging. The colour–magnitude diagrams extend ∼1–2 mag below the HB, which provides for well-defined HBs, even for faint galaxies in which the tip of the red giant branch (TRGB) is sparsely populated. We determine distances across the sample to an average precision of 4 per cent (∼30 kpc at 800 kpc). We find that the majority of these galaxies are in good agreement, though slightly farther (0.1–0.2 mag) when compared to recent ground-based TRGB distances. Two galaxies (And IX and And XVII) have discrepant HST and ground-based distances by ∼0.3 mag (∼150 kpc), which may be due to contamination from Milky Way foreground stars and/or M31 halo stars in sparsely populated TRGB regions. We use the new distances to update the luminosities and structural parameters for these 17 M31 satellites. The new distances do not substantially change the spatial configuration of the M31 satellite system. We comment on future prospects for precise and accurate HB distances for faint galaxies in the Local Group and beyond. [ABSTRACT FROM AUTHOR]

We present new FLAMES+GIRAFFE spectroscopy of 36 member stars in the isolated Local Group dwarf spheroidal galaxy Tucana. We measure a systemic velocity for the system of |$v_{\mathrm{Tuc}} = 216.7_{-2.8}^{+2.9}$|  km s−1, and a velocity dispersion of |$\sigma _{\mathrm{v,Tuc}} = 14.4_{-2.3}^{+2.8}$|  km s−1. We also detect a rotation gradient of |$\frac{{\rm d}v_{r}}{{\rm d}\chi } = 7.6^{+4.2}_{-4.3}$| km s−1 kpc−1, which reduces the systemic velocity to |$v_{\mathrm{Tuc}} = 215.2_{-2.7}^{+2.8}$| km s−1 and the velocity dispersion to |$\sigma _{v,\mathrm{Tuc}} = 13.3_{-2.3}^{+2.7}$| km s−1. We perform Jeans modelling of the density profile of Tucana, using the line-of-sight velocities of the member stars. We find that it favours a high central density consistent with 'pristine' subhaloes in Λ cold dark matter, and a massive dark matter halo (∼1010 M⊙) consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, making it an ideal laboratory for testing dark matter models. [ABSTRACT FROM AUTHOR]

High-mass galaxies, with halo masses |$M_{200}\ge 10^{10}\, {\rm M}_{\odot }$|⁠, reveal a remarkable near-linear relation between their globular cluster (GC) system mass and their host galaxy halo mass. Extending this relation to the mass range of dwarf galaxies has been problematic due to the difficulty in measuring independent halo masses. Here we derive new halo masses based on stellar and H i gas kinematics for a sample of nearby dwarf galaxies with GC systems. We find that the GC system mass–halo mass relation for galaxies populated by GCs holds from halo masses of M 200∼ 1014M |$\odot$| down to below M 200∼109M |$\odot$| , although there is a substantial increase in scatter towards low masses. In particular, three well-studied ultradiffuse galaxies, with dwarf-like stellar masses, reveal a wide range in their GC-to-halo mass ratios. We compare our GC system–halo mass relation to the recent model of El Badry et al. finding that their fiducial model does not reproduce our data in the low-mass regime. This may suggest that GC formation needs to be more efficient than assumed in their model, or it may be due to the onset of stochastic GC occupation in low-mass haloes. Finally, we briefly discuss the stellar mass–halo mass relation for our low-mass galaxies with GCs, and we suggest some nearby dwarf galaxies for which searches for GCs may be fruitful. [ABSTRACT FROM AUTHOR]

Triangulum (M33) is the most massive satellite galaxy of Andromeda (M31), with a stellar mass of about 3 × 109  M $$\odot$$ . Based on abundance matching techniques, M33's total mass at infall is estimated to be of order 1011  M $$\odot$$ . ΛCDM simulations predict that M33-mass halos host several of their own low-mass satellite companions, yet only one candidate M33 satellite has been detected in deep photometric surveys to date. This 'satellites of satellites' hierarchy has recently been explored in the context of the dwarf galaxies discovered around the Milky Way's Magellanic Clouds in the Dark Energy Survey. Here, we discuss the number of satellite galaxies predicted to reside within the virial radius (∼160 kpc) of M33 based on ΛCDM simulations. We also calculate the expected number of satellite detections in N fields of data using various ground-based imagers. Finally, we discuss how these satellite population predictions change as a function of M33's past orbital history. If M33 is on its first infall into M31's halo, its proposed satellites are expected to remain bound to M33 today. However, if M33 experienced a recent tidal interaction with M31, the number of surviving satellites depends strongly on the distance achieved at pericenter due to the effects of tidal stripping. We conclude that a survey extending to ∼100 kpc around M33 would be sufficient to constrain its orbital history and a majority of its satellite population. In the era of WFIRST, surveys of this size will be considered small observing programs. [ABSTRACT FROM AUTHOR]

We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the 'ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ∼45 pc from its centre. Using a grid of collisional N -body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results, therefore, favour a DM core. This implies that either a cold DM cusp was 'heated up' at the centre of Eri II by bursty star formation or we are seeing an evidence for physics beyond cold DM. [ABSTRACT FROM AUTHOR]

We present a chemodynamical analysis of the Leo V dwarf galaxy, based on the Keck II DEIMOS spectra of eight member stars. We find a systemic velocity for the system of (vr) = 170.9-1.9+2.1 kms-1 and barely resolve a velocity dispersion for the system, with σvr = 2.3-1.6+3.2 kms-1, consistent with previous studies of Leo V. The poorly resolved dispersion means we are unable to adequately constrain the dark-matter content of Leo V. We find an average metallicity for the dwarf of [Fe/H] =-2.48 ± 0.21 and measure a significant spread in the iron abundance of its member stars, with -3.1 ≤ [Fe/H] ≤ -1.9 dex, which clearly identifies Leo V as a dwarf galaxy that has been able to self-enrich its stellar population through extended star formation. Owing to the tentative photometric evidence for the tidal substructure around Leo V, we also investigate whether there is any evidence for tidal stripping or shocking of the system within its dynamics. We measure a significant velocity gradient across the system, of dv/dx = -4.1-2.6+2.8 kms-1 arcmin-1 (or dv/dx = -71.9-45.6+50.8 kms-1 kpc-1), which points almost directly towards the Galactic Centre. We argue that Leo V is likely a dwarf on the brink of dissolution, having just barely survived a past encounter with the centre of the Milky Way. [ABSTRACT FROM AUTHOR]

We present MUSE observations of the debated ultrafaint stellar system Crater. We spectroscopically confirm 26 member stars of this system via radial velocity measurements. We derive the systematic instrumental velocity uncertainty of MUSE spectra to be 2.27 km s-1. This new data set increases the confirmed member stars of Crater by a factor of 3. One out of three bright blue stars and a fainter blue star just above the main-sequence turn-off are also found to be likely members of the system. The observations reveal that Crater has a systemic radial velocity of νsys = 148.18+1.08 -1.15 km s-1, whereas the most likely velocity dispersion of this system is sv = 2.04+2.19. The total dynamical mass of the system, assuming dynamical equilibrium is then M-1.06 km s-1. The total dynamical mass of the system, assuming dynamical equilibrium is then MtotM+4.9 implying a mass-to-light ratio of M/L-1.2 ? 105 M⊙/LV = 8.52+28.0 -6.5 M⊙/L⊙, which is consistent with a purely baryonic stellar population within its errors and no significant evidence for the presence of dark matter was found. We also find evidence for a velocity gradient in the radial velocity distribution. We conclude that our findings strongly support that Crater is a faint intermediate-age outer halo globular cluster and not a dwarf galaxy. [ABSTRACT FROM AUTHOR]

We investigate the claim that all dwarf spheroidal galaxies (dSphs) reside within halos that share a common, universal mass profile as has been derived for dSphs of the galaxy. By folding in kinematic information for 25 Andromeda dSphs, more than doubling the previous sample size, we find that a singular mass profile cannot be found to fit all of the observations well. Further, the best-fit dark matter density profile measured solely for the Milky Way dSphs is marginally discrepant with that of the Andromeda dSphs (at just beyond the 1σ level), where a profile with lower maximum circular velocity, and hence mass, is preferred. The agreement is significantly better when three extreme Andromeda outliers, And XIX, XXI, and XXV, all of which have large half-light radii (≳ 600 pc) and low-velocity dispersions (σv < 5 km s–1), are omitted from the sample. We argue that the unusual properties of these outliers are likely caused by tidal interactions with the host galaxy. [ABSTRACT FROM AUTHOR]

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies. [ABSTRACT FROM AUTHOR]

As part of the Z-PAndAS Keck II DEIMOS survey of resolved stars in our neighboring galaxy, Andromeda (M31), we have built up a unique data set of measured velocities and chemistries for thousands of stars in the Andromeda stellar halo, particularly probing its rich and complex substructure. In this contribution, we will discuss the structural, dynamical and chemical properties of Andromeda's dwarf spheroidal galaxies, and how there is no observational evidence for a difference in the evolutionary histories of those found on and off M31's vast plane of satellites. We will also discuss a possible extension to the most significant merger event in M31 - the Giant Southern Stream - and how we can use this feature to refine our understanding of M31's mass profile, and its complex evolution. [ABSTRACT FROM PUBLISHER]