Most dwarf spheroidal galaxies surrounding the Milky Way cannot be dark-matter dominated satellites

TL;DR

Most Milky Way dwarf spheroidal galaxies are unlikely to be dark-matter dominated satellites; instead, their high velocity dispersions are probably caused by tidal shocks during close orbital passages.

Contribution

This study challenges the assumption that dwarf spheroidal galaxies are dark matter dominated by proposing tidal shocks as an alternative explanation for their kinematic properties.

Findings

High probability that dSphs are affected by tidal shocks.

Velocity dispersions can be explained without dark matter.

Dark matter properties correlate with distance to the Milky Way.

Abstract

Milky Way dwarf spheroidal galaxies are the tiniest observed galaxies and are currently associated with the largest fractions of dark matter, which is revealed by their too large velocity dispersions. However, most of them are found near their orbital pericenters. This leads to a very low probability, P = 2 $10^{-7}$, that they could be long-lived satellites such as sub-halos predicted by cosmological simulations. Their proximity to their pericenters suggests instead that they are affected by tidal shocks, which provide sufficient kinematic energy to explain their high velocity dispersions. Dependency of the dark matter properties to their distance to the Milky Way appears to favor tidally shocked and out of equilibrium dSphs instead of self-equilibrium systems dominated by dark matter.