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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.

Measurements of the velocities of pairs of diametrically opposed satellite galaxies of host galaxies in the local Universe show that satellite pairs out to a distance of 150 kiloparsecs from their hosts are anti-correlated in their velocities and that galaxies in the larger-scale environment are strongly clumped along the axis joining the inner satellite pair. Plane truth about satellite galaxies Both the Milky Way and the Andromeda galaxies are associated with a number of dwarf satellite galaxies apparently co-rotating in the same plane. This paper suggests that such co-rotating planes of satellites may be ubiquitous. Rodrigo Ibata and colleagues measured the velocities of pairs of diametrically opposed satellite galaxies in the local Universe and found that out to a distance of 150 kiloparsecs they are preferentially anti-correlated; in the larger-scale environment, out to about 2 megaparsecs, galaxies are distributed mainly in clumps along the axis joining the inner satellite pair. Recent work has shown that the Milky Way and the Andromeda galaxies both possess the unexpected property that their dwarf satellite galaxies are aligned in thin and kinematically coherent planar structures 1 , 2 , 3 , 4 , 5 , 6 , 7 . It is interesting to evaluate the incidence of such planar structures in the larger galactic population, because the Local Group may not be a representative environment. Here we report measurements of the velocities of pairs of diametrically opposed satellite galaxies. In the local Universe (redshift z  < 0.05), we find that satellite pairs out to a distance of 150 kiloparsecs from the galactic centre are preferentially anti-correlated in their velocities (99.994 per cent confidence level), and that the distribution of galaxies in the larger-scale environment (out to distances of about 2 megaparsecs) is strongly clumped along the axis joining the inner satellite pair (>7 σ confidence). This may indicate that planes of co-rotating satellites, similar to those seen around the Andromeda galaxy, are ubiquitous, and their coherent motion suggests that they represent a substantial repository of angular momentum on scales of about 100 kiloparsecs.

Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to formgiant galaxies like the Milky Way^sup 1^. It has previously been suspected^sup 2^ that dwarf galaxies may not be isotropically distributed around our Galaxy, because several are correlated with streams ofHI emission, andmay form coplanar groups^sup 3^. These suspicions are supported by recent analyses^sup 4-7^. It has been claimed7 that the apparently planar distribution of satellites is not predicted within standard cosmology^sup 8^, and cannot simply represent amemory of past coherent accretion. However, other studies dispute this conclusion^sup 9-11^. 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^sup 12-15^ 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. [PUBLICATION ABSTRACT]

We have recently shown (Ibata et al. 2014) that pairs of satellite galaxies located diametrically opposite each other around their host possess predominantly anti-correlated velocities. This is consistent with a scenario in which \\(\\sim 50\\)% of satellite galaxies belong to kinematically-coherent rotating planar structures, similar to those detected around the giant galaxies of the Local Group. Here we extend this analysis, examining the incidence of satellites of giant galaxies drawn from an SDSS photometric redshift catalog. We find that there is a \\(\\sim 17\\)% overabundance (\\(> 3 \\sigma\\) significance) of candidate satellites at positions diametrically opposite a spectroscopically confirmed satellite. We show that cosmological simulations do not possess this property when the contamination is included, and that there are in fact, after subtracting contamination, 2 to 3 times more satellites diametrically opposed to a spectroscopically confirmed satellite than at \\(90\\deg\\) from it. We also examine the correlation between the satellite pair positions and the orientation of the host galaxy major axis. We find that those satellite pairs with anti-correlated velocities have a strong preference (\\(\\sim 3:1\\)) to align with the major axis of the host whereas those with correlated velocities display the opposite behavior. This correlation of the satellite alignments appears to be stronger than the well-documented preference of satellites to be located close to the major axis of their host. We finally show that repeating a similar analysis to Ibata et al. (2014) with same-side satellites is generally hard to interpret, but is not inconsistent with our previous results when strong quality-cuts are applied on the sample. All these unexpected correlations strongly suggest that a substantial fraction of satellite galaxies are causally-linked in their formation and evolution.

Recent work has shown that both the Milky Way and the Andromeda galaxies possess the unexpected property that their dwarf satellite galaxies are aligned in thin and kinematically coherent planar structures. It is now important to evaluate the incidence of such planar structures in the larger galactic population, since the Local Group may not be a sufficiently representative environment. Here we report that the measurement of the velocity of pairs of diametrically opposed galaxy satellites provides a means to determine statistically the prevalence of kinematically coherent planar alignments. In the local universe (redshift \\(z<0.05\\)), we find that such satellite pairs out to a galactocentric distance of \\(150\\) kpc are preferentially anti-correlated in their velocities (99.994% confidence level), and that the distribution of galaxies in the larger scale environment (beyond 150 kpc and up to \\(\\approx 2\\) Mpc) is strongly elongated along the axis joining the inner satellite pair (\\(>7\\sigma\\) confidence). Our finding may indicate that co-rotating planes of satellites, similar to that seen around the Andromeda galaxy, are ubiquitous in nature, while their coherent motion also suggests that they are a significant repository of angular momentum on \\(\\sim 100\\) kpc scales.

In a recent contribution, Bahl \\& Baumgardt investigated the incidence of planar alignments of satellite galaxies in the Millennium-II simulation, and concluded that vast thin planes of dwarf galaxies, similar to that observed in the Andromeda galaxy (M31), occur frequently by chance in \\(\\Lambda\\)-Cold Dark Matter cosmology. However, their analysis did not capture the essential fact that the observed alignment is simultaneously radially extended, yet thin, and kinematically unusual. With the caveat that the Millennium-II simulation may not have sufficient mass resolution to identify confidently simulacra of low-luminosity dwarf galaxies, we re-examine that simulation for planar structures, using the same method as employed by Ibata et al. (2013) on the real M31 satellites. We find that 0.04\\% of host galaxies display satellite alignments that are at least as extreme as the observations, when we consider their extent, thickness and number of members rotating in the same sense. We further investigate the angular momentum properties of the co-planar satellites, and find that the median of the specific angular momentum derived from the line of sight velocities in the real M31 structure (\\(1.3\\times10^4\\) km/s kpc) is very high compared to systems drawn from the simulations. This analysis confirms that it is highly unlikely that the observed structure around the Andromeda galaxy is due to a chance occurrence. Interestingly, the few extreme systems that are similar to M31 arise from the accretion of a massive sub-halo with its own spatially-concentrated entourage of orphan satellites.

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. An early analysis noted that dwarf galaxies may not be isotropically distributed around our Galaxy, as several are correlated with streams of HI emission, and possibly form co-planar groups. These suspicions are supported by recent analyses, and it has been claimed that the apparently planar distribution of satellites is not predicted within standard cosmology, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion. Here we report the existence (99.998% significance) of a planar sub-group of satellites in the Andromeda galaxy, comprising approximately 50% of the population. The structure is vast: at least 400 kpc in diameter, but also extremely thin, with a perpendicular scatter <14.1 kpc (99% confidence). Radial velocity measurements reveal that the satellites in this structure have the same sense of rotation about their host. This finding shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum, a new insight for our understanding of the origin of these most dark matter dominated of galaxies. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way's disk and is co-planar with the Milky Way to Andromeda position vector. The existence of such extensive coherent kinematic structures within the halos of massive galaxies is a fact that must be explained within the framework of galaxy formation and cosmology.