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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.

The abundance of interstellar 7 Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun’s metallicity, is nearly equal to the Big Bang nucleosynthesis predictions. The search for local cosmological lithium-7 The predicted primordial abundance of the lithium-7 isotope in the primordial Universe is four times greater than that measured in the atmospheres of Galactic halo stars, but it is hard to trace this isotope in the Milky Way because it has most probably been burnt up. This paper reports the detection of interstellar lithium beyond the Milky Way, in the low-metallicity gas of the nearby Small Magellanic Cloud galaxy. Present-day lithium-7 abundance in this galaxy is nearly equal to the predictions of the standard theory of Big Bang nucleosynthesis — although the data can also be reconciled with non-standard models. The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background 1 . The predicted 2 , 3 , 4 primordial 7 Li abundance is four times that measured in the atmospheres of Galactic halo stars 5 , 6 , 7 . This discrepancy could be caused by modification of surface lithium abundances during the stars’ lifetimes 8 or by physics beyond the Standard Model that affects early nucleosynthesis 9 , 10 . The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium 11 that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar 7 Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun’s metallicity. The present-day 7 Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.

Dwarf galaxies pose substantial challenges for cosmological models. In particular, current models predict a dark-matter density that is divergent at the center, which is in sharp contrast with observations that indicate a core of roughly constant density. Energy feedback, from supernova explosions and stellar winds, has been proposed as a major factor shaping the evolution of dwarf galaxies. We present detailed cosmological simulations with sufficient resolution both to model the relevant physical processes and to directly assess the impact of stellar feedback on observable properties of dwarf galaxies. We show that feedback drives large-scale, bulk motions of the interstellar gas, resulting in substantial gravitational potential fluctuations and a consequent reduction in the central matter density, bringing the theoretical predictions in agreement with observations.

A dwarf galaxy companion to NGC 4449 is reported, and is found to be in a transient stage of tidal disruption. Aftermath of an intergalactic collision The nearby starburst galaxy NGC 4449 is surrounded by a neutral HI cloud complex with a variety of features including rings, shells and a counter-rotating core. This complexity is suggestive of interactions with another galaxy, possibly the dwarf irregular galaxy DDO 125, at a distance of 40 kiloparsecs. But new observations of the region around NGC 4449 reveal a previously unknown companion, a tidally distorted smaller galaxy, named NGC 4449B. Its properties, such as its S-shaped morphology (resembling the Sagittarius dwarf near the Milky Way), suggest that it has had a close encounter with NGC 4449. NGC 4449 is a nearby Magellanic irregular starburst galaxy 1 with a B-band absolute magnitude of −18 and a prominent, massive, intermediate-age nucleus 2 at a distance from Earth of 3.8 megaparsecs (ref. 3 ). It is wreathed in an extraordinary neutral hydrogen (H  i ) complex, which includes rings, shells and a counter-rotating core, spanning ∼90 kiloparsecs (kpc; refs 1 , 4 ). NGC 4449 is relatively isolated 5 , although an interaction with its nearest known companion—the galaxy DDO 125, some 40 kpc to the south—has been proposed as being responsible for the complexity of its H  i structure 6 . Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of −13.4 and a half-light radius of 2.7 kpc, with a full extent of around 8 kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf 7 near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies 7 , 8 but has hitherto been seen only in a dissolving globular cluster 9 . We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20 kpc from the galaxy’s centre.

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.

The known galaxies most dominated by dark matter (Draco, Ursa Minor and Andromeda IX) are satellites of the Milky Way and the Andromeda galaxies. They are members of a class of faint galaxies, devoid of gas, known as dwarf spheroidals, and have by far the highest ratio of dark to luminous matter. None of the models proposed to unravel their origin can simultaneously explain their exceptional dark matter content and their proximity to a much larger galaxy. Here we report simulations showing that the progenitors of these galaxies were probably gas-dominated dwarf galaxies that became satellites of a larger galaxy earlier than the other dwarf spheroidals. We find that a combination of tidal shocks and ram pressure swept away the entire gas content of such progenitors about ten billion years ago because heating by the cosmic ultraviolet background kept the gas loosely bound: a tiny stellar component embedded in a relatively massive dark halo survived until today. All luminous galaxies should be surrounded by a few extremely dark-matter-dominated dwarf spheroidal satellites, and these should have the shortest orbital periods among dwarf spheroidals because they were accreted early.

Galaxies go head-to-head The Andromeda galaxy (Messier 31) is big in galaxy evolution studies, since it is the nearest giant spiral galaxy, and one of the largest in the Local Group. It also looks odd, and its appearance has long puzzled astronomers. It has a well known outer ring offset from the galactic nucleus, and the outer disk is warped. Now an offset inner dust ring has been discovered in infrared images from the Spitzer Space Telescope. Numerical simulations indicate that both rings formed when a companion galaxy plunged almost head-on through the disk of M31. The most likely interloper is the dwarf galaxy M32. This paper reports the presence of a second, inner ring offset by half a kiloparsec from the centre of the Andromeda galaxy. Numerical simulations indicate that both rings result from a companion galaxy plunging head-on through the centre of the disk of M31. The unusual morphology of the Andromeda galaxy (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M31 has a well-known 1 , 2 , 3 , 4 , 5 , 6 , 7 outer ring of star formation at a radius of ten kiloparsecs whose centre is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped, as seen at both optical 8 and radio 9 wavelengths. The halo contains numerous loops and ripples. Here we report the presence of a second, inner dust ring with projected dimensions of 1.5 × 1 kiloparsecs and offset by about half a kiloparsec from the centre of the galaxy (based upon an analysis of previously-obtained data 10 ). The two rings appear to be density waves propagating in the disk. Numerical simulations indicate that both rings result from a companion galaxy plunging through the centre of the disk of M31. The most likely interloper is M32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.

The stellar Initial Mass Function (IMF) suggests that stars with sub-solar mass form in very large numbers. Most attractive places for catching low-mass star formation in the act are young stellar clusters and associations, still (half-)embedded in star-forming regions. The low-mass stars in such regions are still in their pre–main-sequence (PMS) evolutionary phase, i.e., they have not started their lives on the main-sequence yet. The peculiar nature of these objects and the contamination of their samples by the fore- and background evolved populations of the Galactic disk impose demanding observational techniques, such as X-ray surveying and optical spectroscopy of large samples for the detection of complete numbers of PMS stars in the Milky Way. The Magellanic Clouds, the metal-poor companion galaxies to our own, demonstrate an exceptional star formation activity. The low extinction and stellar field contamination in star-forming regions of these galaxies imply a more efficient detection of low-mass PMS stars than in the Milky Way, but their distance from us make the application of the above techniques unfeasible. Nonetheless, imaging with the Hubble Space Telescope within the last five years yield the discovery of solar and sub-solar PMS stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of such objects are identified as the low-mass stellar content of star-forming regions in these galaxies, changing completely our picture of young stellar systems outside the Milky Way, and extending the extragalactic stellar IMF below the persisting threshold of a few solar masses. This review presents the recent developments in the investigation of the PMS stellar content of the Magellanic Clouds, with special focus on the limitations by single-epoch photometry that can only be circumvented by the detailed study of the observable behavior of these stars in the color-magnitude diagram. The achieved characterization of the low-mass PMS stars in the Magellanic Clouds allowed thus a more comprehensive understanding of the star formation process in our neighboring galaxies.

Interactions between galaxies are common, and influence physical properties such as the global morphology and star-formation rate 1 (Hubble type). Galaxies can interact in many different ways: they can merge together; they can pass through each other, with gas being stripped from the smaller of the two and compressed in the larger; and they can interact gravitationally 2 (including, for example, tides in clusters). The relative importance of these mechanisms is often not clear, as the strength of each depends on poorly known parameters such as the density, extent and nature of the dark-matter haloes that surround galaxies 3 . A nearby example of a galaxy interaction where the mechanism is controversial is that between our Galaxy and two of its neighbours, the Magellanic Clouds. Here we present the results of an atomic-hydrogen survey that help to elucidate this mechanism. Our data reveal a new stream of gas that lies in the opposite direction to the trailing Magellanic Stream and leads the motion of the Clouds. The existence of both leading and trailing streams supports a gravitational interaction whereby the streams are torn from the bodies of the Magellanic Clouds by tidal forces.

The oldest and most metal-poor Milky Way stars form a kinematically hot halo, which motivates the two major formation scenarios for our galaxy: extended hierarchical accretion and rapid collapse. RR Lyrae stars are excellent tracers of old and metal-poor populations. We measured the kinematics of 43 RR Lyrae stars in the inner regions of the nearby Large Magellanic Cloud (LMC) galaxy. The velocity dispersion equals 53 ± 10 kilometers per second, which indicates that a kinematically hot metal-poor old halo also exists in the LMC. This result suggests that our galaxy and smaller late-type galaxies such as the LMC have similar early formation histories.