Under the Firelight: Stellar Tracers of the Local Dark Matter Velocity Distribution in the Milky Way

TL;DR

This paper demonstrates that the velocity distribution of accreted stars in the Milky Way can be used to infer the local dark matter velocity distribution, providing a new way to map dark matter using Gaia data.

Contribution

It introduces a method to reconstruct dark matter velocity distribution from stellar populations and validates it with cosmological simulations and Gaia data.

Findings

Strong correlation between stellar and dark matter velocities at the solar circle.

Approximately 42% of local dark matter from luminous mergers is in debris flow.

Method successfully recovers dark matter velocity distribution from stellar data.

Abstract

The Gaia era opens new possibilities for discovering the remnants of disrupted satellite galaxies in the Solar neighborhood. If the population of local accreted stars is correlated with the dark matter sourced by the same mergers, one can then map the dark matter distribution directly. Using two cosmological zoom-in hydrodynamic simulations of Milky Way-mass galaxies from the Latte suite of Fire-2 simulations, we find a strong correlation between the velocity distribution of stars and dark matter at the solar circle that were accreted from luminous satellites. This correspondence holds for dark matter that is either relaxed or in kinematic substructure called debris flow, and is consistent between two simulated hosts with different merger histories. The correspondence is more problematic for streams because of possible spatial offsets between the dark matter and stars. We demonstrate how to reconstruct the dark matter velocity distribution from the observed properties of the accreted stellar population by properly accounting for the ratio of stars to dark matter contributed by individual mergers. After demonstrating this method using the Fire-2 simulations, we apply it to the Milky Way and use it to recover the dark matter velocity distribution associated with the recently discovered stellar debris field in the Solar neighborhood. Based on results from Gaia, we estimate that $42 ^{+26}_{-22}\%$ of the local dark matter that is accreted from luminous mergers is in debris flow.