A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy

Journal name:
Nature
Volume:
493,
Pages:
62–65
Date published:
(03 January 2013)
DOI:
doi:10.1038/nature11717
Received
Accepted
Published online

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 Hi 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 (M31), comprising about half of the population. The structure is at least 400kiloparsecs in diameter, but also extremely thin, with a perpendicular scatter of less than 14.1kiloparsecs. 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

  1. Figure 1: Map of the Andromeda satellite system.
    Map of the Andromeda satellite system.

    The homogeneous PAndAS survey (irregular polygon) provides the source catalogue for the detection and distance measurements of the 27 satellite galaxies20 (filled circles) used in this study. Near M31 (blue ellipse), the high background hampers the detection of new satellites and precludes reliable distance measurements for M32 and NGC205 (labelled black open circles); we therefore exclude the region inside 2.5° (dashed circle) from the analysis. The seven satellites known outside the PandAS area (green circles and arrows) constitute a heterogeneous sample, discovered in various surveys with non-uniform spatial coverage, and their distances are not measured in the same homogeneous way. A reliable spatial analysis requires a data set with homogeneous selection criteria, so we do not include these objects in the sample either. The analysis shows that the satellites marked red are confined to a highly planar structure. We note that this structure is approximately perpendicular to lines of constant Galactic latitude, so it is therefore aligned approximately perpendicular to the Milky Way’s disk (the grid squares are 4°×4°).

  2. Figure 2: Satellite galaxy positions as viewed from Andromeda.
    Satellite galaxy positions as viewed from Andromeda.

    The Aitoff–Hammer projection shows the sample of 27 satellites20 (filled circles from Fig. 1) as they would be seen from the centre of the Andromeda galaxy. In these coordinates the disk of Andromeda lies along the equator. ‘M31-centric galactic latitude’ means what a fictitous observer in the M31 galaxy would call ‘galactic latitude’. The background image represents the probability density function of the poles derived from 105 iterations of resampling the 27 satellites from their distance probability density functions, and finding the plane of lowest root mean square from a subsample of 15 (the colour scale on the right shows the relative probability of the poles, and is dimensionless). A clear narrow peak at (lM31 = 100.9°±0.9°, bM31 = 38.2°±1.4°) highlights the small uncertainty in the best-fit plane. The solid red line, which passes within less than 1° of the position of the Milky Way (yellow circle labelled ‘MW’), represents the plane corresponding to this best pole location.

  3. Figure 3: Three-dimensional view (online only in the PDF version) or two-dimensional screenshot (in the print version) of the planar, rotating structure.
    Three-dimensional view (online only in the PDF version) or two-dimensional screenshot (in the print version) of the planar, rotating structure.

    The coordinate system is such that the z direction is parallel to the vector pointing from the Milky Way to M31, x increases eastwards and y northwards. Only the radial component of the velocity of each satellite is measured, and these velocities are shown as vectors pointing either towards or away from the Milky Way. As in Figs 1 and 2, red spheres mark the planar satellites, and blue spheres represent the ‘normal’ population. The coherent kinematic behaviour of the spatially very thin structure (red) is clearly apparent viewed from the yz plane. With the exception of AndXIII and AndXXVII, the satellites in the planar structure that lie to the north of M31 recede from us, whereas those to the South approach us; this property strongly suggests rotation. Our velocity measurements15 (supplemented by values from the literature14), have very small uncertainties, typically <5kms−1. The irregular green polygon (visible only in the xy plane of the three-dimensional online version) shows the PAndAS survey area, the white circle (visible only in the xy plane of the three-dimensional online version) indicates a projected radius of 150kpc at the distance of M31, and the white arrow (visible only in the three-dimensional online version) marks a velocity scale of 100kms−1. (AndXXVII is not shown in this diagram because its most likely distance is 476kpc behind M31). This figure is three-dimensionally interactive in the online version (allowing the reader to change the magnification and viewing angle), and was constructed with the S2PLOT programming library26.

Main

We undertook the Pan-Andromeda Archaeological Survey16 (PAndAS) to obtain a large-scale panorama of the halo of the Andromeda galaxy (M31), a view that is not available to us for the Milky Way. This Canada–France–Hawaii Telescope survey imaged about 400 square degrees around M31, 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 150kiloparsecs (kpc) from M31, and about 50kpc from M33, the most massive satellite of M31. 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 20–50kpc) to the 27 dwarf galaxies (filled circles in Fig. 1) visible within the PAndAS survey area20 that lie beyond the central 2.5°.

Figure 1: Map of the Andromeda satellite system.
Map of the Andromeda satellite system.

The homogeneous PAndAS survey (irregular polygon) provides the source catalogue for the detection and distance measurements of the 27 satellite galaxies20 (filled circles) used in this study. Near M31 (blue ellipse), the high background hampers the detection of new satellites and precludes reliable distance measurements for M32 and NGC205 (labelled black open circles); we therefore exclude the region inside 2.5° (dashed circle) from the analysis. The seven satellites known outside the PandAS area (green circles and arrows) constitute a heterogeneous sample, discovered in various surveys with non-uniform spatial coverage, and their distances are not measured in the same homogeneous way. A reliable spatial analysis requires a data set with homogeneous selection criteria, so we do not include these objects in the sample either. The analysis shows that the satellites marked red are confined to a highly planar structure. We note that this structure is approximately perpendicular to lines of constant Galactic latitude, so it is therefore aligned approximately perpendicular to the Milky Way’s disk (the grid squares are 4°×4°).

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 M31, 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).

Figure 2: Satellite galaxy positions as viewed from Andromeda.
Satellite galaxy positions as viewed from Andromeda.

The Aitoff–Hammer projection shows the sample of 27 satellites20 (filled circles from Fig. 1) as they would be seen from the centre of the Andromeda galaxy. In these coordinates the disk of Andromeda lies along the equator. ‘M31-centric galactic latitude’ means what a fictitous observer in the M31 galaxy would call ‘galactic latitude’. The background image represents the probability density function of the poles derived from 105 iterations of resampling the 27 satellites from their distance probability density functions, and finding the plane of lowest root mean square from a subsample of 15 (the colour scale on the right shows the relative probability of the poles, and is dimensionless). A clear narrow peak at (lM31 = 100.9°±0.9°, bM31 = 38.2°±1.4°) highlights the small uncertainty in the best-fit plane. The solid red line, which passes within less than 1° of the position of the Milky Way (yellow circle labelled ‘MW’), represents the plane corresponding to this best pole location.

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 M31, while the northern satellites recede away from us with respect to their host galaxy.

Figure 3: Three-dimensional view (online only in the PDF version) or two-dimensional screenshot (in the print version) of the planar, rotating structure.
Three-dimensional view (online only in the PDF version) or two-dimensional screenshot (in the print version) of the planar, rotating structure.

The coordinate system is such that the z direction is parallel to the vector pointing from the Milky Way to M31, x increases eastwards and y northwards. Only the radial component of the velocity of each satellite is measured, and these velocities are shown as vectors pointing either towards or away from the Milky Way. As in Figs 1 and 2, red spheres mark the planar satellites, and blue spheres represent the ‘normal’ population. The coherent kinematic behaviour of the spatially very thin structure (red) is clearly apparent viewed from the yz plane. With the exception of AndXIII and AndXXVII, the satellites in the planar structure that lie to the north of M31 recede from us, whereas those to the South approach us; this property strongly suggests rotation. Our velocity measurements15 (supplemented by values from the literature14), have very small uncertainties, typically <5kms−1. The irregular green polygon (visible only in the xy plane of the three-dimensional online version) shows the PAndAS survey area, the white circle (visible only in the xy plane of the three-dimensional online version) indicates a projected radius of 150kpc at the distance of M31, and the white arrow (visible only in the three-dimensional online version) marks a velocity scale of 100kms−1. (AndXXVII is not shown in this diagram because its most likely distance is 476kpc behind M31). This figure is three-dimensionally interactive in the online version (allowing the reader to change the magnification and viewing angle), and was constructed with the S2PLOT programming library26.

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±0.6kpc (<14.1kpc at 99% confidence), that corotate around M31 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 400kpc along the line of sight and nearly 300kpc north-to-south. Indeed, since AndromedaXIV and CassiopeiaII 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 M31 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 M31 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 5Gyr for satellites at 150kpc). 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 (<14.1kpc), 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 1,000kms1, 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 M31 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.

References

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  2. Lynden-Bell, D. Dwarf galaxies and globular clusters in high velocity hydrogen streams. Mon. Not. R. Astron. Soc. 174, 695710 (1976)
  3. Lynden-Bell, D. & Lynden-Bell, R. M. Ghostly streams from the formation of the Galaxy’s halo. Mon. Not. R. Astron. Soc. 275, 429442 (1995)
  4. Metz, M., Kroupa, P. & Jerjen, H. The spatial distribution of the Milky Way and Andromeda satellite galaxies. Mon. Not. R. Astron. Soc. 374, 11251145 (2007)
  5. Metz, M., Kroupa, P. & Libeskind, N. I. The orbital poles of Milky Way satellite galaxies: a rotationally supported disk of satellites. Astrophys. J. 680, 287294 (2008)
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  7. Pawlowski, M. S., Pflamm-Altenburg, J. & Kroupa, P. The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way. Mon. Not. R. Astron. Soc. 423, 11091126 (2012)
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  13. Tollerud, E. J. et al. The SPLASH survey: spectroscopy of 15 M31 dwarf spheroidal satellite galaxies. Astrophys. J. 752, 45 (2012)
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  16. McConnachie, A. W. et al. The remnants of galaxy formation from a panoramic survey of the region around M31. Nature 461, 6669 (2009)
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Acknowledgements

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.

Author information

Affiliations

  1. Observatoire Astronomique de Strasbourg, 11 rue de l’Université, F-67000 Strasbourg, France

    • Rodrigo A. Ibata &
    • Nicolas F. Martin
  2. Sydney Institute for Astronomy, School of Physics, A28, The University of Sydney, New South Wales 2006, Australia

    • Geraint F. Lewis
  3. Department of Physics and Astronomy, Macquarie University, New South Wales 2109, Australia

    • Anthony R. Conn
  4. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

    • Michael J. Irwin
  5. NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, British Columbia, V9E 2E7, Canada

    • Alan W. McConnachie
  6. Department of Physics and Atmospheric Science, Dalhousie University, 6310 Coburg Road, Halifax, Nova Scotia, B3H 4R2, Canada

    • Scott C. Chapman
  7. Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

    • Michelle L. Collins &
    • Nicolas F. Martin
  8. University of Massachusetts, Department of Astronomy, LGRT 619-E, 710 North Pleasant Street, Amherst, Massachusetts 01003-9305, USA

    • Mark Fardal
  9. Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

    • Annette M. N. Ferguson
  10. Lycée International des Pontonniers, 1 rue des Pontonniers, F-67000 Strasbourg, France

    • Neil G. Ibata
  11. The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek, Australian Capital Province 2611, Australia

    • A. Dougal Mackey
  12. Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia, V8P 5C2, Canada

    • Julio Navarro
  13. Department of Physics and Astronomy, University of California, Los Angeles, PAB, 430 Portola Plaza, Los Angeles, California 90095-1547, USA

    • R. Michael Rich
  14. LERMA, UMR CNRS 8112, Observatoire de Paris, 61 Avenue de l’Observatoire, 75014 Paris, France

    • David Valls-Gabaud
  15. Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, Ontario, K7L 3N, Canada

    • Lawrence M. Widrow

Contributions

All authors assisted in the development and writing of the paper. In addition, the structural and kinematic properties of the dwarf population, and the significance of the Andromeda plane were determined by R.A.I., G.F.L. and A.R.C., based on distances determined by the same group (as part of the PhD research of A.R.C.). In addition, A.W.M. is the Principal Investigator of PAndAS; M.J.I. and R.A.I. led the data processing effort; R.A.I. was the Principal Investigator of an earlier CFHT MegaPrime/MegaCam survey, which PAndAS builds on (which included S.C.C., A.M.N.F., M.J.I., G.F.L., N.F.M. and A.W.M.). R.M.R. is Principal Investigator of the spectroscopic follow-up with the Keck Telescope. M.L.C. and S.C.C. led the analysis of the kinematic determination of the dwarf population, and N.F.M. led the detection of the dwarf population from PAndAS data. N.G.I. performed the initial analysis of the satellite kinematics.

Competing financial interests

The authors declare no competing financial interests.

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