The tangential velocity excess of the Milky Way satellites

TL;DR

This paper finds that the Milky Way's satellite galaxies exhibit unusually high tangential velocities compared to predictions from the standard b2CDM cosmological model, challenging current galaxy formation theories.

Contribution

It provides the first detailed comparison of Milky Way satellite kinematics with b2CDM predictions, highlighting significant discrepancies in velocity anisotropy and orbital energy distribution.

Findings

Milky Way satellites have a velocity anisotropy b2=-2.2b14, in the 2.9% tail of b2CDM predictions.

Most satellites have low radial velocities, with 9 out of 10 having less than 20% of kinetic energy in radial motion.

The tangential velocity excess is not related to the Galactic 'disc of satellites'.

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.