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

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.

The highest-speed local dark matter particles come from the Large Magellanic Cloud

“…In particular, the kinematics and distribution of Gaia-Sausage stars are best matched by the disruption of a system on a radial orbit, resulting in a strongly radially anisotropic velocity distribution of stars in the solar neighborhood, which may have implications for directional direct-detection experiments [14]. The Gaia-Sausage and other debris-flow material may contribute at the ∼ 10% level to the Solar Neighborhood DM density, but the velocity distribution of stars associated with the Gaia-Sausage populate speeds of v min <500 km/s [13,14,40], much lower than the expected LMC debris flow.…”

“…In current models, the highest-speed DM particles are spatially homogenized remnants of recently accreted and disrupted subhalos [13,31,53]. For low-mass WIMPs, these highspeed particles can dominate the signal, since only the fastest particles are energetic enough to reach the experimental threshold.…”

Implications of the Gaia sausage for dark matter nuclear interactions

“…Our first step is to investigate the effect of uncertainty in the DM substructure fraction η sub , estimated by ref. [32] to be η sub = 0.42 +0.26 −0.22 . We show, in the right panel of Fig.…”

“…In fact, as illustrated in ref. [32] (see top panel of Fig. 7 for example), stellar streams turn out to be poor tracers of the DM velocity, since the tidal debris in the stream hasn't had enough time to completely mix with the halo.…”

Dark matter capture by the Sun: revisiting velocity distribution uncertainties

“…Different assumptions can be inferred from cosmological simulations, galactic dynamics and other approaches (see e.g [33][34][35][36][37][38]). Furthermore, DM only [39,40] or detailed hydrodynamical simulations of Milky Way size halos inhabited by a disc galaxy often show departure from the standard Maxwellian description of the DM halo velocities distribution [41][42][43][44][45][46].…”