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Planes of satellite galaxies : when exceptions are the rule.

Cautun, M. and Bose, S. and Frenk, C. S. and Guo, Q. and Han, J. and Hellwing, W. A. and Sawala, T. and Wang, W. (2015) 'Planes of satellite galaxies : when exceptions are the rule.', Monthly notices of the Royal Astronomical Society., 452 (4). pp. 3838-3852.


The detection of planar structures within the satellite systems of both the Milky Way (MW) and Andromeda (M31) has been reported as being in stark contradiction to the predictions of the standard cosmological model (Λ cold dark matter – ΛCDM). Given the ambiguity in defining a planar configuration, it is unclear how to interpret the low incidence of the MW and M31 planes in ΛCDM. We investigate the prevalence of satellite planes around galactic mass haloes identified in high-resolution cosmological simulations. We find that planar structures are very common, and that ∼10 per cent of ΛCDM haloes have even more prominent planes than those present in the Local Group. While ubiquitous, the planes of satellite galaxies show a large diversity in their properties. This precludes using one or two systems as small-scale probes of cosmology, since a large sample of satellite systems is needed to obtain a good measure of the object-to-object variation. This very diversity has been misinterpreted as a discrepancy between the satellite planes observed in the Local Group and ΛCDM predictions. In fact, ∼10 per cent of ΛCDM galactic haloes have planes of satellites that are as infrequent as the MW and M31 planes. The look-elsewhere effect plays an important role in assessing the detection significance of satellite planes and accounting for it leads to overestimating the significance level by a factor of 30 and 100 for the MW and M31 systems, respectively.

Item Type:Article
Keywords:Galaxies: abundances, Galaxies: haloes, Galaxies: statistics, Dark matter.
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Publisher statement:This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©: 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:09 July 2015
Date deposited:22 October 2015
Date of first online publication:October 2015
Date first made open access:No date available

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