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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.

We discuss some of the key open questions regarding the formation and evolution of globular clusters (GCs) during galaxy formation and assembly within a cosmological framework. The current state of the art for both observations and simulations is described, and we briefly mention directions for future research. The oldest GCs have ages greater than or equal to 12.5 Gyr and formed around the time of reionization. Resolved colour-magnitude diagrams of Milky Way GCs and direct imaging of lensed proto-GCs at z∼6 with the James Webb Space Telescope (JWST) promise further insight. GCs are known to host multiple populations of stars with variations in their chemical abundances. Recently, such multiple populations have been detected in ∼2 Gyr old compact, massive star clusters. This suggests a common, single pathway for the formation of GCs at high and low redshift. The shape of the initial mass function for GCs remains unknown; however, for massive galaxies a power-law mass function is favoured. Significant progress has been made recently modelling GC formation in the context of galaxy formation, with success in reproducing many of the observed GC-galaxy scaling relations.

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.

Recent observations have revealed streams of gas and stars in the halo of the Milky Way 1 , 2 , 3 that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies 2 , 4 , and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory 5 . Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes.

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.

A growing number of studies are revealing that many Milky Way globular clusters possess extended stellar structures beyond their traditional boundaries. Just how ubiquitous these structures are, and how they originate, are key questions to explore. In this contribution, we present a Bayesian technique that we have developed to separate probable members of globular clusters from the dominant Milky Way fore/background at large clustercentric radii and hence facilitate quantitative analyses of these intriguing structures. We demonstrate the promise of our method by showing how it recovers the known extended features around Palomar 5 and NGC 7089.

We present the results from the state-of-the-art wide-field survey of the M81 galaxy group that we are conducting with Hyper Suprime-Cam on Subaru Telescope. Our photometry reaches about 2 mag below the tip of the red giant branch (RGB) and reveals the spatial distribution of both old and young stars over an area of 5°2 around the M81. The young main-sequence (MS) stars closely follow the HI distribution and can be found in a stellar stream between M81 and NGC 3077 and in numerous outlying stellar associations. Our survey also reveals for the first time the very extended (>2 × R25) halos of RGB stars around M81, M82, and NGC 3077, as well as faint tidal streams that link these systems. The gravitational interactions between M81, M82 and NGC 3077 galaxies induced star formation in tidally stripped gas, and also significantly perturbed the older stellar components leading to disturbed halo morphologies.

Tidal features are a key observable prediction of the hierarchical model of galaxy formation and contain a wealth of information about the properties and history of a galaxy. Modern wide-field surveys such as LSST and Euclid will revolutionise the study of tidal features. However, the volume of data will prohibit visual inspection to identify features, thereby motivating a need to develop automated detection methods. This paper presents a visual classification of \\(\\sim2,000\\) galaxies from the DECaLS survey into different tidal feature categories: arms, streams, shells, and diffuse. We trained a Convolutional Neural Network (CNN) to reproduce the assigned visual classifications using these labels. Evaluated on a testing set where galaxies with tidal features were outnumbered \\(\\sim1:10\\), our network performed very well and retrieved a median \\(98.7\\pm0.3\\), \\(99.1\\pm0.5\\), \\(97.0\\pm0.8\\), and \\(99.4^{+0.2}_{-0.6}\\) per cent of the actual instances of arm, stream, shell, and diffuse features respectively for just 20 per cent contamination. A modified version that identified galaxies with any feature against those without achieved scores of \\(0.981^{+0.001}_{-0.003}\\), \\(0.834^{+0.014}_{-0.026}\\), \\(0.974^{+0.008}_{-0.004}\\), and \\(0.900^{+0.073}_{-0.015}\\) for the accuracy, precision, recall, and F1 metrics, respectively. We used a Gradient-weighted Class Activation Mapping analysis to highlight important regions on images for a given classification to verify the network was classifying the galaxies correctly. This is the first demonstration of using CNNs to classify tidal features into sub-categories, and it will pave the way for the identification of different categories of tidal features in the vast samples of galaxies that forthcoming wide-field surveys will deliver.

Many clues about the galaxy assembly process lurk in the faint outer regions of galaxies. Although quantitative study of these parts has been severely limited in the past, breakthroughs are now being made thanks to the combination of wide-area star counts, deep HST imagery and 8-m class spectroscopy. I highlight here some recent progress made on deciphering the fossil record encoded in the outskirts of our nearest large neighbours, M31 and M33.

We present a study of the structural properties and metallicity distribution of the nearby peculiar galaxy NGC3077. Using data from our survey of the M81 Group with the Hyper Suprime-Cam on the Subaru Telescope, we construct deep color-magnitude diagrams that are used to probe the old red giant branch population of NGC3077. We map these stars out to and beyond the nominal tidal radius, which allows us to derive the structural properties and stellar content of the peripheral regions. We show that NGC3077 has an extended stellar halo and pronounced ``S-shaped\" tidal tails that diverge from the radial profile of the inner region. The average metallicity of the old population in NGC3077 is estimated from individual RGBs to be \\(\\rm{[M/H]}=-0.98 \\pm 0.26\\), which decreases with the distance from the galaxy center as \\(\\rm{[M/H]}=-0.17\\) dex \\(\\rm{R_{h}}^{-1}\\). The metallicity of the S-shaped structure is similar to that of the regions lying at \\(r\\sim4\\times\\rm{R_{h}} (\\sim 30\\)~kpc), indicating that the stellar constituents of the tidal tails have come from the outer envelope of NGC3077. These results suggest that this peculiar galaxy was probably a rather normal dwarf elliptical galaxy before the tidal interaction with M81 and M82. We also examine the evidence in our dataset for the six recently-reported ultra-faint dwarf candidates around NGC3077. We recover a spatial overdensity of sources coinciding with only one of these.