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How common is the Milky Way-satellite system alignment?

Libeskind, N.I.; Frenk, C.S.; Cole, S.; Jenkins, A.; Helly, J.C.


N.I. Libeskind

J.C. Helly


The highly flattened distribution of satellite galaxies in the Milky Way (MW) presents a number of puzzles. First, its polar alignment stands out from the planar alignments commonly found in other galaxies. Secondly, recent proper-motion measurements reveal that the orbital angular momentum of at least three, and possibly as many as eight, of the MW's satellites points (within 30°) along the axis of their flattened configuration, suggesting some form of coherent motion. In this paper, we use a high-resolution cosmological simulation to investigate whether this pattern conflicts with the expectations of the cold dark matter model of structure formation. We find that this seemingly unlikely setup occurs often: approximately 35 per cent of the time, we find systems in which the angular momentum of three individual satellites points along, or close to, the short axis of the satellite distribution. In addition, in 30 per cent of the systems we find that the net angular momentum of the six best-aligned satellites lies within 35° of the short axis of the satellite distribution, as observed for the MW.


Libeskind, N., Frenk, C., Cole, S., Jenkins, A., & Helly, J. (2009). How common is the Milky Way-satellite system alignment?. Monthly Notices of the Royal Astronomical Society, 399(2), 550-558.

Journal Article Type Article
Publication Date 2009-10
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 399
Issue 2
Pages 550-558
Keywords galaxies: haloes; galaxies: kinematics and dynamics; Local Group; cosmology: theory; large-scale structure of UniverseDWARF SPHEROIDAL GALAXIES; DARK-MATTER HALOES; ABSOLUTE PROPER MOTION; GALACTIC SATELLITES; MILLENNIUM SIMULATION; LUMINOSITY FUNCTION; R