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The dipole repeller

Nature Astronomy volume 1, Article number: 0036 (2017) Cite this article

Abstract

Our Local Group of galaxies is moving with respect to the cosmic microwave background (CMB) with a velocity1 of VCMB = 631 ± 20 km s−1 and participates in a bulk flow that extends out to distances of ~20,000 km s−1 or more2–4. There has been an implicit assumption that overabundances of galaxies induce the Local Group motion5–7. Yet underdense regions push as much as overdensities attract8, but they are deficient in light and consequently difficult to chart. It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s−1 away contributes significantly to the observed flow9. We show here that repulsion from an underdensity is important and that the dominant influences causing the observed flow are a single attractor — associated with the Shapley concentration — and a single previously unidentified repeller, which contribute roughly equally to the CMB dipole. The bulk flow is closely anti-aligned with the repeller out to 16,000 ± 4,500 km s−1. This ‘dipole repeller’ is predicted to be associated with a void in the distribution of galaxies.

The large-scale structure of the Universe is encoded in the flow field of galaxies. A detailed analysis of the flow uncovers the rich structure manifested by the distribution of galaxies, such as the prominent nearby clusters10–13, the Laniakea supercluster14 and the Arrowhead mini-supercluster15. A close correspondence between the observed density field, derived from redshift surveys, and the reconstructed three-dimensional (3D) flow field has been established out to beyond 100 megaparsecs and down to a resolution of a few megaparsecs16. Yet the flow contains more information on distant structures, as found from tides and from continuity across the zone obscured by the Galactic disk, the ‘zone of avoidance’11. The Cosmicflows-2 dataset of galaxy distances16 provides reasonably dense coverage to R ≈ 10,000 km s−1 (distances are expressed in terms of their equivalent Hubble velocity).

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Figure 1: A face-on view of a slice 6,000 km s−1 thick, normal to the direction of the pointing vector r ˆ = ( 0.604,0.720 , − 0.342 ) .
Figure 2: A 3D view of the velocity field.
Figure 3: Aitoff projection in galactic coordinates of the principal structures and directions that characterize the flow.

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Acknowledgements

We thank J. Sorce and S. Gottloeber for discussions and A. Dupuy for her help in preparing Fig. 3. We thank K. Bowles and S. Thompson for the narration in the Supplementary Video. Support has been provided by the Israel Science Foundation (1013/12), the Institut Universitaire de France, the US National Science Foundation, Space Telescope Science Institute for observations with Hubble Space Telescope, the Jet Propulsion Lab for observations with Spitzer Space Telescope and NASA for analysis of data from the Wide-field Infrared Survey Explorer.

Author information

Authors and Affiliations

  1. Racah Institute of Physics, Hebrew University, Jerusalem, 91904, Israel

    Yehuda Hoffman

  2. Institut de Recherche sur les Lois Fondamentales de l'Univers, CEA, Université Paris-Saclay, Gif-sur-Yvette, 91191, France

    Daniel Pomarède

  3. Institute for Astronomy (IFA), University of Hawaii, 2680 Woodlawn Drive, Honolulu, 96822, Hawaii, USA

    R. Brent Tully

  4. IPN Lyon, UCB Lyon 1/CNRS/IN2P3, University of Lyon, Villeurbanne, 69622, France

    Hélène M. Courtois

Authors
  1. Yehuda Hoffman
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  2. Daniel Pomarède
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  3. R. Brent Tully
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  4. Hélène M. Courtois
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Contributions

R.B.T. and H.M.C. carried out the observations and data analysis; D.P. contributed graphics and visualization; Y.H. carried out the numerical and theoretical analysis. All co-authors contributed to the writing of the paper, led by Y.H.

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Correspondence to Yehuda Hoffman.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Figures 1–3, Supplementary Table 1. (PDF 1220 kb)

Supplementary Video

The dipole repeller. (MP4 83651 kb)

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Hoffman, Y., Pomarède, D., Tully, R. et al. The dipole repeller. Nat Astron 1, 0036 (2017). https://doi.org/10.1038/s41550-016-0036

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