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arXiv · 1612.01529

The tangential velocity excess of the Milky Way satellites

Marius Cautun·Carlos S. Frenk
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Abstract

We estimate the systemic orbital kinematics of the Milky Way classical satellites and compare them with predictions from the \Lambda{} cold dark matter (\Lambda{}CDM) model derived from a semi-analytical galaxy formation model applied to high resolution cosmological N-body simulations. We find that the Galactic satellite system is atypical of \Lambda{}CDM systems. The subset of 10 Galactic satellites with proper motion measurements has a velocity anisotropy, \beta=-2.2$\pm$0.4, that lies in the 2.9% tail of the \Lambda{}CDM distribution. Individually, the Milky Way satellites have radial velocities that are lower than expected for their proper motions, with 9 out of the 10 having at most 20% of their orbital kinetic energy invested in radial motion. Such extreme values are expected in only 1.5% of \Lambda{}CDM satellites systems. This tangential motion excess is unrelated to the existence of a Galactic "disc of satellites". We present theoretical predictions for larger satellite samples that may become available as more proper motion measurements are obtained.

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Marius Cautun, Carlos S. Frenk. 2016-12-05. The tangential velocity excess of the Milky Way satellites. https://doi.org/10.1093/mnrasl/slx025

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