Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way1. It has previously been suspected2 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 groups3. These suspicions are supported by recent analyses4, 5, 6, 7. It has been claimed7 that the apparently planar distribution of satellites is not predicted within standard cosmology8, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion9, 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 measurements12, 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.
At a glance
We undertook the Pan-Andromeda Archaeological Survey16 (PAndAS) to obtain a large-scale panorama of the halo of the Andromeda galaxy (M 31), a view that is not available to us for the Milky Way. This Canada–France–Hawaii Telescope survey imaged about 400 square degrees around M 31, which is the only giant galaxy in the Local Group besides our Milky Way. Stellar objects are detected out to a projected distance of about 150 kiloparsecs (kpc) from M 31, and about 50 kpc from M 33, the most massive satellite of M 31. The data reveal a substantial population of dwarf spheroidal galaxies that accompany Andromeda17.
The distances to the dwarf galaxies can be estimated by measuring the magnitude of the tip of the red-giant branch18. Improving on earlier methods, we have developed a Bayesian approach that yields the probability distribution function for the distance to each individual satellite19. In this way we now have access to homogeneous distance measurements (typical uncertainties 20–50 kpc) to the 27 dwarf galaxies (filled circles in Fig. 1) visible within the PAndAS survey area20 that lie beyond the central 2.5°.
In Fig. 2 these distance measurements are used to calculate the sky positions of the homogeneous sample of 27 dwarf galaxies as they would appear from the centre of the Andromeda galaxy. Visually, there appears to be a correlation close to a particular great circle (red line): this suggests that there is a plane, centred on M 31, around which a subsample of the satellites have very little scatter. This is confirmed by the Monte Carlo analysis presented in the Supplementary Information, where we show that the probability of the alignment of the subsample of nsub = 15 satellites marked red in Figs 1 and 2 occurring at random is 0.13% (see Supplementary Fig. 1).
Following this discovery, we sought to investigate whether the subsample displayed any kinematic coherence. The radial velocity of each satellite is shown in Fig. 3, corrected for the bulk motion of the Andromeda system towards us: what is immediately striking is that 13 out of the 15 satellites possess coherent rotational motion, such that the southern satellites are approaching us with respect to M 31, while the northern satellites recede away from us with respect to their host galaxy.
The probability that 13 or more out of 15 objects should share the same sense of rotation is 1.4% (allowing for right-handed or left-handed rotation). Thus the kinematic information confirms the spatial correlation initially suspected from a visual inspection of Fig. 2. The total significance of the planar structure is approximately 99.998%.
Thus we conclude that we have detected, with very high confidence, a coherent planar structure of 13 satellites with a root-mean-square thickness of 12.6 ± 0.6 kpc (<14.1 kpc at 99% confidence), that corotate around M 31 with a (right-handed) axis of rotation that points approximately east. The three-dimensional configuration can be assessed visually in Fig. 3. The extent of the structure is gigantic, over 400 kpc along the line of sight and nearly 300 kpc north-to-south. Indeed, since Andromeda XIV and Cassiopeia II lie at the southern and northern limits of the PAndAS survey, respectively, it is quite probable that additional (faint) satellites belonging to this structure are waiting to be found just outside the PAndAS footprint. Although huge in extent, the structure appears to be lopsided, with most of the satellites populating the side of the halo of M 31 that is nearest to the Milky Way. The completeness analysis we have undertaken shows that this configuration is not due to a lowered detection sensitivity at large distance, but reflects a true paucity of satellites in the more distant halo hemisphere21.
The existence of this structure had been hinted at in earlier work22, thanks to the planar alignment we report on here being viewed nearly edge-on, although the grouping could not be shown to be statistically significant from the information available at that time. Other previously claimed alignments do not match the present plane, although they share some of the member galaxies: the most significant alignment of ref. 23 has a pole 45° away from that found here, and the (tentative) poles of the configuration in ref. 4 are 23.4° away, whereas those of a later contribution6 have poles 34.1° and 25.4° distant from ours. Without the increased sample size, reliable three-dimensional positions and radial velocities, and most importantly, a spatially unbiased selection function resulting from the homogeneous panoramic coverage of PAndAS, the nature, properties and conclusive statistical significance of the present structure could not be inferred.
The present detection proves that in some giant galaxies, a significant fraction of the population of dwarf satellite galaxies, in this case around 50% (13 out of 27 over the homogeneously surveyed PAndAS area), are aligned in coherent planar structures, sharing the same direction of angular momentum. The Milky Way is the only other giant galaxy where we have access to high-quality three-dimensional positional data, and the existence of a similar structure around our Galaxy is strongly suggested by current data2, 5, 7. The implications for the origin and dynamical history of dwarf galaxies are profound. It also has a strong bearing on the analyses of dark matter in these darkest of galaxies, because one cannot now justifiably assume such objects to have evolved in dynamical isolation.
Intriguingly, the Milky Way lies within 1° of the plane reported here, the pole of the plane and the pole of the Milky Way’s disk are approximately perpendicular (81°), and furthermore this plane is approximately perpendicular to the plane of satellites that has been proposed to surround the Galaxy (given that its pole points approximately towards Andromeda7 within the uncertainties). Although these alignments may be chance occurrences, it is nevertheless essential information about the structure of the nearby Universe that must be taken into account in future simulations aimed at modelling the dynamical formation history of the Local Group.
The formation of this structure around M 31 poses a puzzle. For discussion, we envisage two broad classes of possible explanations: accretion or in situ formation. In either type of model, the small scatter of the satellites out of the plane is difficult to explain, even though the orbital timescales for the satellites are long (around 5 Gyr for satellites at 150 kpc). All the galaxies in the plane are known to have old, evolved, stellar populations, and so in situ formation would additionally imply that the structure is ancient.
In an accretion scenario, the dynamical coherence points to an origin in a single accretion of a group of dwarf galaxies. However, the spatial extent of the progenitor group would have to be broadly equal to or smaller than the current plane thickness (<14.1 kpc), yet no such groups are known. Interpreting the coherent rotation as a result of our viewing perspective24 requires a bulk tangential velocity for the in-falling group of the order of 1,000 km s−1, which seems unphysically high. A further possibility is that we are witnessing accretion along filamentary structures that are fortuitously aligned. In situ formation may be possible if the planar satellite galaxies formed like tidal-dwarf galaxies in an ancient gas-rich galaxy merger7, but then the dwarf galaxies should be essentially devoid of dark matter. If the planar M 31 dwarfs are dynamically relaxed, the absence of dark matter would be greatly at odds with inferences from detailed observations25 of Milky Way satellites, assuming the standard theory of gravity. An alternative possibility is that gas was accreted preferentially onto dark matter sub-halos that were already orbiting in this particular plane, but then the origin of the plane of sub-haloes would still require explanation. We conclude that it remains to be seen whether galaxy formation models within the current cosmological framework can explain the existence of this vast, thin, rotating structure of dwarf galaxies within the halo of our nearest giant galactic neighbour.
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We thank the staff of the Canada-France-Hawaii Telescope for taking the PAndAS data, and for their continued support throughout the project. We thank one of our referees, B. Tully, for pointing out that IC 1613 could also be associated to the planar structure. R.A.I. and D.V.G. gratefully acknowledge support from the Agence Nationale de la Recherche though the grant POMMME, and would like to thank B. Famaey for discussions. G.F.L. thanks the Australian Research Council for support through his Future Fellowship and Discovery Project. This work is based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope, which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. Some of the data presented here were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.
- Supplementary Information (126K)
This file contains Supplementary Text 1-2, Supplementary Figure 1 and a Supplementary Reference.