Inferred Evidence For Dark Matter Kinematic Substructure with SDSS-Gaia

TL;DR

This paper demonstrates that a significant portion of the local dark matter halo exhibits kinematic substructure, likely originating from disrupted satellites, challenging standard models and impacting dark matter detection strategies.

Contribution

It provides evidence that dark matter in the Milky Way contains substructure correlated with stellar debris, using SDSS-Gaia data and mixture likelihood analysis.

Findings

Dark matter shows kinematic substructure consistent with tidal debris.

A disrupted satellite's debris dominates the anisotropic stellar population.

Results challenge the Standard Halo Model used in dark matter detection.

Abstract

We use the distribution of accreted stars in SDSS-Gaia DR2 to demonstrate that a non-trivial fraction of the dark matter halo within Galactocentric radii of 7.5-10 kpc and $|z| > 2.5$ kpc is in substructure, and thus may not be in equilibrium. Using a mixture likelihood analysis, we separate the contributions of an old, isotropic stellar halo and a younger anisotropic population. The latter dominates and is uniform within the region studied. It can be explained as the tidal debris of a disrupted massive satellite on a highly radial orbit, and is consistent with mounting evidence from recent studies. Simulations that track the tidal debris from such mergers find that the dark matter traces the kinematics of its stellar counterpart. If so, our results indicate that a component of the nearby dark matter halo that is sourced by luminous satellites is in kinematic substructure referred to as debris flow. These results challenge the Standard Halo Model, which is discrepant with the distribution recovered from the stellar data, and have important ramifications for the interpretation of direct detection experiments.