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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]

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 a detailed study of the faint Milky Way satellite Draco II (Dra II) from deep CFHT/MegaCam broad-band g and i photometry and narrow-band metallicity-sensitive CaHK observations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modelling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour–magnitude space implies Dra II is old (13.5 ± 0.5 Gyr), very metal-poor, very faint ($$L_V = 180 ^{+124}_{-72} {\rm \, L_\odot}$$), and at a distance $$d = 21.5 \pm 0.4 {\rm \, kpc}$$. The narrow-band, metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ([Fe/H] = −2.7 ± 0.1 dex). Even though our study benefits from a doubling of the spectroscopic sample size compared to previous investigations, the velocity dispersion of the system is still only marginally resolved ($$\sigma _{vr}\lt 5.9 {\rm \, km \,\, s^{-1}}$$ at the 95 per cent confidence level) and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity ($$\langle v_{r}\rangle = -342.5^{+1.1}_{-1.2}{\rm \, km \,\, s^{-1}}$$). We further show that the spectroscopically confirmed members of Dra II have a mean proper motion of $$(\mu _\alpha ^*,\mu _\delta)=(1.26 \pm 0.27,0.94 \pm 0.28) {\rm \, mas/yr}$$ in the Gaia DR2 data, which translates to an orbit with a pericentre and an apocentre of $$21.3 ^{+0.7}_{-1.0}$$ and $$153.8 ^{+56.7}_{-34.7} {\rm \, kpc}$$, respectively. Taken altogether, these properties favour the scenario of Dra II being a potentially disrupting dwarf galaxy. The low-significance extra-tidal features we map around the satellite tentatively support this scenario. [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 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]