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About half of the satellites in the Andromeda galaxy (M 31), all with the same sense of rotation about their host, form a planar subgroup that is extremely wide but also very thin. The Andromeda galaxy's orbiting companions Giant spiral galaxies are assembled from smaller systems through a process known as hierarchical clustering. In orbit around these giants are dwarf galaxies, which are presumably remnants of the galactic progenitors. Recent studies of the dwarf galaxies of the Milky Way have led some astronomers to suspect that their orbits are not randomly distributed. This suspicion, which challenges current theories of galaxy formation, is now bolstered by the discovery of a plane of dwarf galaxies corotating as a coherent pancake-like structure around the Andromeda galaxy, the Milky Way's close neighbour and in many respects its 'twin'. The structure is extremely thin yet contains about half of the dwarf galaxies in the Andromeda system. The authors report that 13 of the 15 satellites in the plane share the same sense of rotation. Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way 1 . It has previously been suspected 2 that dwarf galaxies may not be isotropically distributed around our Galaxy, because several are correlated with streams of H  i emission, and may form coplanar groups 3 . These suspicions are supported by recent analyses 4 , 5 , 6 , 7 . It has been claimed 7 that the apparently planar distribution of satellites is not predicted within standard cosmology 8 , and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion 9 , 10 , 11 . Here we report the existence of a planar subgroup of satellites in the Andromeda galaxy (M 31), comprising about half of the population. The structure is at least 400 kiloparsecs in diameter, but also extremely thin, with a perpendicular scatter of less than 14.1 kiloparsecs. Radial velocity measurements 12 , 13 , 14 , 15 reveal that the satellites in this structure have the same sense of rotation about their host. This shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way’s disk and with the vector between the Milky Way and Andromeda.

Globular clusters are some of the oldest bound stellar structures observed in the Universe 1 . They are ubiquitous in large galaxies and are believed to trace intense star-formation events and the hierarchical build-up of structure 2 , 3 . Observations of globular clusters in the Milky Way, and a wide variety of other galaxies, have found evidence for a ‘ metallicity floor ’ , whereby no globular clusters are found with chemical (metal) abundances below approximately 0.3 to 0.4 per cent of that of the Sun 4 – 6 . The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to form—both critical components for understanding the build-up of mass in the Universe 7 . Here we report measurements from the Southern Stellar Streams Spectroscopic Survey of the spatially thin, dynamically cold Phoenix stellar stream in the halo of the Milky Way. The properties of the Phoenix stream are consistent with it being the tidally disrupted remains of a globular cluster. However, its metal abundance ([Fe/H] = −2.7) is substantially below the empirical metallicity floor. The Phoenix stream thus represents the debris of the most metal-poor globular clusters discovered so far, and its progenitor is distinct from the present-day globular cluster population in the local Universe. Its existence implies that globular clusters below the metallicity floor have probably existed, but were destroyed during Galactic evolution. The Phoenix stream in the Milky Way halo is shown to be a tidally disrupted remnant of an unusually metal-poor globular cluster, which was possibly destroyed during Galactic evolution.

Galactic detritus around M31 A panoramic survey of the region around our nearest galactic neighbour, the well known Andromeda galaxy M31, has detected stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by M31's tidal field. The brightest companion, the Triangulum galaxy (M33), is surrounded by a previously unknown prominent stellar structure that provides evidence for a recent encounter with M31. This new view of galactic structures is consistent with hierarchical cosmological models in which galaxies grow in mass by the accretion of smaller ones. 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 and distant stars surrounding the galaxy. A panoramic survey of the Andromeda galaxy (M31) now reveals stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. In hierarchical cosmological models 1 , 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 2 , 3 . The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history 4 , 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 M v = -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.

Large galaxies grow through the accumulation of dwarf galaxies 1 , 2 . In principle it is possible to trace this growth history via the properties of a galaxy’s stellar halo 3 – 5 . Previous investigations of the galaxy Messier 31 (M31, Andromeda) have shown that outside a galactocentric radius of 25 kiloparsecs the population of halo globular clusters is rotating in alignment with the stellar disk 6 , 7 , as are more centrally located clusters 8 , 9 . The M31 halo also contains coherent stellar substructures, along with a smoothly distributed stellar component 10 – 12 . Many of the globular clusters outside a radius of 25 kiloparsecs are associated with the most prominent substructures, but some are part of the smooth halo 13 . Here we report an analysis of the kinematics of these globular clusters. We find two distinct populations rotating perpendicular to each other. The rotation axis for the population associated with the smooth halo is aligned with the rotation axis for the plane of dwarf galaxies 14 that encircles M31. We interpret these separate cluster populations as arising from two major accretion epochs, probably separated by billions of years. Stellar substructures from the first epoch are gone, but those from the more recent second epoch still remain. There are two distinct kinematic populations of globular clusters in Messier 31 (M31, the Andromeda galaxy) with rotation axes perpendicular to each other, suggesting that they arose from merger events separated by billions of years.

The Fermi bubbles are giant, γ-ray-emitting lobes emanating from the nucleus of the Milky Way discovered in ~1–100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi bubbles that has been interpreted as a signature of collimated outflows from the Galaxy’s supermassive black hole. Here we show via a spatial template analysis that much of the γ-ray emission associated with the brightest region of substructure—the so-called cocoon—is probably due to the Sagittarius dwarf spheroidal galaxy (dSph). This large Milky Way satellite is viewed through the Fermi bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sagittarius dSph has no ongoing star formation, but we nevertheless demonstrate that the dwarf’s millisecond pulsar population can plausibly supply the γ-ray signal that our analysis associates with its stellar template. The measured spectrum is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron–positron pairs injected by millisecond pulsars belonging to the Sagittarius dSph, combined with these objects’ magnetospheric emission. This finding plausibly suggests that millisecond pulsars produce significant γ-ray emission among old stellar populations, potentially confounding indirect dark-matter searches in regions such as the Galactic Centre, the Andromeda galaxy and other massive Milky Way dSphs. A bright patch in the Fermi bubbles, previously attributed to a jet launched by the Galaxy’s central black hole, is actually due to gamma-ray emission by millisecond pulsars in a background, satellite galaxy of the Milky Way.

We built the most extended stellar density and/or surface brightness radial profiles for 13 old Large Magellanic Cloud (LMC) globular clusters (GCs). The studied GCs located farther than ~ 5 kpc from the LMC center would not seem to present any hint of extended stellar structures, while those closer than ~ 5 kpc do show extended structures. Such an excess of stars tightly depends on the position of the GCs, so that the closer the GC to the LMC center, the larger the excess of stars. Furthermore, the GC radii also show a remarkable trend with the position of the GC in the LMC disc. These outcomes can be fully interpreted in the light of the known GC radial velocity disc-like kinematics, from which GCs have been somehow mostly experiencing the influence of the LMC gravitational field at their respective mean distances from the LMC center.

We present the star formation histories (SFHs) of 20 faint M31 satellites (\\(-12 \\lesssim M_V \\lesssim -6\\)) that were measured by modeling sub-horizontal branch (HB) depth color-magnitude diagrams constructed from Hubble Space Telescope (HST) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration \\(3-6\\) Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent (\\(<3\\) Gyr) quenching times. We suggest that systematic differences in the quenching times of M31 and MW satellites may be a reflection of the varying accretion histories of M31 and the MW. This result implies that the formation histories of low-mass satellites may not be broadly representative of low-mass galaxies in general. Among the M31 satellite population we identify two distinct groups based on their SFHs: one with exponentially declining SFHs (\\(\\tau \\sim 2\\) Gyr) and one with rising SFHs with abrupt quenching. We speculate how these two groups could be related to scenarios for a recent major merger involving M31. The Cycle 27 HST Treasury survey of M31 satellites will provide well-constrained ancient SFHs to go along with the quenching times we measure here. The discovery and characterization of M31 satellites with \\(M_V \\gtrsim -6\\) would help quantify the relative contributions of reionization and environment to quenching of the lowest-mass satellites.

We present new horizontal branch (HB) distance measurements to 17 of the faintest known M31 satellites (\\(-6 \\lesssim M_{V} \\lesssim -13\\)) based on deep Hubble Space Telescope (HST) imaging. The color-magnitude diagrams extend \\(\\sim\\)1-2 magnitudes 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% (\\(\\sim 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 \\(\\sim 0.3\\) mag (\\(\\sim 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.

We present a panoramic survey of the isolated Local Group dwarf irregular galaxy NGC 6822. Our photometry reaches \\(\\sim2-3\\) magnitudes deeper than most previous studies and spans the widest area around the dwarf compared to any prior work. We observe no stellar over-densities in the outskirts of NGC 6822 to \\(V\\sim 30\\) mag\\(\\,\\)arcsec\\(^{-2}\\) and a projected radius of \\(16.5\\) kpc. This indicates that NGC 6822 has not experienced any recent interaction with a companion galaxy, despite previous suggestions to the contrary. Similarly, we find no evidence for any dwarf satellites of NGC 6822 to a limiting luminosity \\(M_V\\approx -5\\). NGC 6822 contains a disk of HI gas and young stars, oriented at \\(\\sim 60\\) degrees to an extended spheroid composed of old stellar populations. We observe no correlation between the distribution of young stars and spheroid members. Our imaging allows us to trace the spheroid to nearly \\(11\\) kpc along its major axis, commensurate with the extent of the NGC 6822 globular cluster system. We find that the spheroid becomes increasingly flattened at larger radii, and its position angle twists by up to \\(40\\) degrees. We use Gaia EDR3 astrometry to measure a proper motion for NGC 6822, and then sample its orbital parameter space. While this galaxy has spent the majority of its life in isolation, we find that it likely passed within the virial radius of the Milky Way \\(\\sim3-4\\) Gyr ago. This may explain the apparent flattening and twisting observed in the outskirts of its spheroid.

We present Gemini South GMOS g,i photometry of 14 intermediate-age Large Magellanic Cloud (LMC) star clusters, namely: NGC 2155, 2161, 2162, 2173, 2203, 2209, 2213, 2231, 2249, Hodge 6, SL 244, 505, 674, and 769, as part of a continuing project to investigate the extended Main Sequence Turnoff (EMSTO) phenomenon. Extensive artificial star tests were made over the observed field of view. These tests reveal the observed behaviour of photometric errors with magnitude and crowding. The cluster stellar density radial profiles were traced from star counts over the extent of the observed field. We adopt clus- ter radii and build colour-magnitude diagrams (CMDs) with cluster features clearly identified. We used the cluster (g,g-i) CMDs to estimate ages from the matching of theoretical isochrones. The studied LMC clusters are confirmed to be intermediate-age clusters, which range in age 9.10 < log(t) < 9.60. NGC 2162 and NGC 2249 look like new EMSTO candidates, in addition to NGC 2209, on the basis of having dual red clumps.