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Velocity anti-correlation of diametrically opposed galaxy satellites in the low-redshift Universe


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 structures1,2,3,4,5,6,7. 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 (>7σ 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.

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Figure 1: Satellite correlation test.
Figure 2: Anti-correlated satellites in the SDSS.
Figure 3: Correlation with environment.


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Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England. The Millennium-II simulation databases used in this paper and the web application providing online access to them were constructed as part of the activities of the German Astrophysical Virtual Observatory.

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All authors assisted in the development and writing of the paper. N.G.I. primarily contributed to the development of the test for planar alignments, and R.A.I. implemented this test on the SDSS galaxy catalogue.

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Correspondence to Neil G. Ibata.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Adopted velocity envelope relation.

Dots mark the distance–velocity distribution of satellites in the MS2 simulation that surround isolated host galaxies of similar luminosity and mass to the Milky Way32. The empirical envelope relation shown in red (300exp[−(300 kpc/R)0.8] km s−1) is used in our analysis as a means to reduce contamination from velocity outliers.

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Ibata, N., Ibata, R., Famaey, B. et al. Velocity anti-correlation of diametrically opposed galaxy satellites in the low-redshift Universe. Nature 511, 563–566 (2014).

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