Galactic forces rule dynamics of Milky Way dwarf galaxies

TL;DR

This paper shows that the observed velocity dispersions of Milky Way dwarf galaxies are significantly influenced by tidal forces from the Milky Way, challenging previous dark matter estimates and suggesting many dwarfs are dissolving.

Contribution

It demonstrates that Galactic tidal forces can account for dwarf galaxy velocity dispersions, questioning dark matter estimates and providing a new method to estimate the Milky Way's mass profile.

Findings

Velocity dispersions are induced by Galactic tidal forces.

Dark matter estimates based on velocity dispersions are unreliable.

Many dwarfs are likely dissolving into the Galactic halo.

Abstract

Dwarf galaxies populating the Galactic halo are assumed to host the largest fractions of dark matter, as calculated from their velocity dispersions. Their major axes are preferentially aligned with the Vast Polar Structure (VPOS) that is perpendicular to the Galactic disk, and we find their velocity gradients aligned as well. It suggests that tidal forces exerted by the Milky Way are distorting dwarf galaxies. Here we demonstrate on the basis of the impulse approximation that the Galactic gravitational acceleration induces the dwarf line-of-sight velocity dispersion, which is also evidenced by strong dependences between both quantities. Since this result is valid for any dwarf mass value, it implies that dark matter estimate in Milky Way dwarfs cannot be deduced from the product of their radius to the square of their line-of-sight velocity dispersion. This questions the high dark-matter fractions reported for these evanescent systems, and the universally adopted total-to-stellar mass relationship in the dwarf regime. It suggests that many dwarfs are at their first passage and are dissolving into the Galactic halo. It opens a promising way to estimate the Milky Way total mass profile at large distances.