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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.

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.

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.

At the lowest galactic mass scales, evidence of a merger between two galaxies is provided by the kinematic detection of a stellar stream -- indicative of an accretion event -- in the dwarf spheroidal galaxy Andromeda II, one of the satellite galaxies of Andromeda.

At the lowest galactic mass scales, evidence of a merger between two galaxies is provided by the kinematic detection of a stellar stream — indicative of an accretion event — in the dwarf spheroidal galaxy Andromeda II, one of the satellite galaxies of Andromeda. Dwarf galaxies merge in Andromeda II Galaxy formation theory predicts mergers between lower mass galaxies. But although the accretion of small systems onto large ones such as the Milky Way has been observed indirectly, no mergers between low-mass galaxies — those of total mass less than that of a billion Sun-like stars — have been identified. Nicola Amorisco et al . now report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal Andromeda II. They conclude that they are observing the remnant of a merger between two dwarf galaxies. This illustrates the scale-free character of galaxy formation, down to the lowest galactic mass scales. Driven by gravity, massive structures like galaxies and clusters of galaxies are believed to grow continuously through hierarchical merging and accretion of smaller systems. Observational evidence of accretion events is provided by the coherent stellar streams crossing the outer haloes of massive galaxies, such as the Milky Way 1 or Andromeda 2 . At similar mass scales, around 10 11 solar masses in stars, further evidence of merging activity is also ample 3 , 4 , 5 . Mergers of lower-mass galaxies are expected within the hierarchical process of galaxy formation 6 , but have hitherto not been seen for galaxies with less than about 10 9 solar masses in stars 7 , 8 . Here we report the kinematic detection of a stellar stream in one of the satellite galaxies of Andromeda, the dwarf spheroidal Andromeda II, which has a mass of only 10 7 solar masses in stars 9 . The properties of the stream show that we are observing the remnant of a merger between two dwarf galaxies. This had a drastic influence on the dynamics of the remnant, which is now rotating around its projected major axis 10 . The stellar stream in Andromeda II illustrates the scale-free character of the formation of galaxies, down to the lowest galactic mass scales.

The total mass of a galaxy group, such as the Milky Way (MW) and the Andromeda Galaxy (M 31), is typically determined from the kinematics of satellites within their virial zones. Bahcall and Tremaine (1981) proposed the v2r estimator as an alternative to the virial theorem. In this work, we extend their approach by incorporating the three-dimensional spatial distribution of satellites within the system to improve the reliability and accuracy of galaxy mass estimates. Applying this method to a comprehensive dataset of local group satellites based on recent, high-precision distance measurements, we estimate the total mass of the MW to be (7.9±2.3)×1011 M⊙ and that of M 31 to be (15.5±3.4)×1011 M⊙. The effectiveness of the method is constrained by the precision of distance measurements, making it particularly well suited for the local group, but challenging to apply to more distant systems.

For decades cosmologists have known that the Milky Way and its companion galaxy Andromeda are traveling through space at about 1.4 million miles per hour relative to the rest of the expanding universe, and now they may know why.

Dark matter is a popular candidate to a new source of primary-charged particles, especially positrons in cosmic rays, which are proposed to account for observable anomalies. While this hypothesis of decaying or annihilating DM is mostly applied for our Galaxy, it could possibly lead to some interesting phenomena when applied for the other ones. In this work, we look into the hypothetical asymmetry in gamma radiation from the upper and lower hemisphere of the dark matter halo of the Andromeda galaxy due to inverse Compton scattering of starlight on the DM-produced electrons and positrons. While our 2D toy model raises expectations for the possible effect, a more complex approach gives negligible effect for the dark halo case, but shows some prospects for a dark disk model.