Dark Shards: velocity substructure from Gaia and direct searches for dark matter

TL;DR

This paper identifies stellar substructures in the Milky Way's halo using Gaia data, linking them to dark matter streams that significantly impact direct detection experiments and their expected signals.

Contribution

It introduces the concept of Dark Shards, stellar streams associated with dark matter, and analyzes their effects on dark matter detection signals and directional detection strategies.

Findings

Discovery of multiple dark matter streams with distinct velocities.

Dark Shards influence the timing and shape of detection signals.

Enhanced dark matter flux towards Cygnus due to these streams.

Abstract

Data from the Gaia satellite show that the solar neighbourhood of the Milky Way's stellar halo is imprinted with substructure from several accretion events. Evidence of these events is found in "the Shards", stars clustering with high significance in both action space and metallicity. Stars in the Shards share a common origin, likely as ancient satellite galaxies of the Milky Way, so will be embedded in dark matter (DM) counterparts. These "Dark Shards" contain two substantial streams (S1 and S2), as well as several retrograde, prograde and lower energy substructures. The retrograde stream S1 has a very high Earth-frame speed of $\sim 550$ km s$^{-1}$ while S2 moves on a prograde, but highly polar orbit and enhances the peak of the speed distribution at around $300$ km s$^{-1}$. The presence of the Dark Shards locally leads to modifications of many to the fundamental properties of experimental DM signals. The~S2 stream in particular gives rise to an array of effects in searches for axions and in the time dependence of nuclear recoils: shifting the peak day, inducing nonsinusoidal distortions, and increasing the importance of the gravitational focusing of DM by the Sun. Dark Shards additionally bring new features for directional signals, while also enhancing the DM flux towards Cygnus.