Set the Night on FIRE: Building an Empirical Local Dark Matter Velocity Distribution

Abstract

The majority of terrestrial direct detection experiments for Dark Matter (DM) rely on the Standard Halo Model (SHM), which assumes the local DM velocity distribution follows a Maxwell-Boltzmann distribution. However, galaxy mergers can deposit DM that remains kinematically clustered today, inducing deviations from the smooth SHM prediction. Previous studies have suggested that the local stellar velocity distribution may serve as a tracer for DM populations originating from the same progenitor systems. In this work, we systematically investigate how merger mass and accretion time affect the correlation between local stellar and DM velocity distributions in Milky Way-like galaxies from the FIRE-2 simulations. We find a strong correlation between traceable DM components and their stellar counterparts, with the tightest correspondence arising from lower-mass mergers accreted at earlier cosmic times. For the remaining DM that lacks an identifiable stellar counterpart, which dominate the full DM fraction, we find that its velocity distribution is well described by a component-wise generalized Gaussian. Combining these two ingredients, we reconstruct the full local DM velocity distribution. This framework captures merger-induced features-such as co-rotation of accreted material with the galactic disk-that are entirely absent in the SHM. Finally, we propagate uncertainties through the reconstruction and show that they are dominated by the stellar mass-halo mass relation, which is unlikely to improve substantially in the near term. We therefore argue that this framework approaches the current limit of our ability to characterize the local DM velocity distribution.

Figures (24)

• Figure 1: Edge-on stellar surface density maps at $z=0$ for the seven Milky Way--mass simulations: m12i, m12f, m12w, m12m, m12b, m12c, and m12r. Stellar particles are shown within $|x| \le 30$ kpc and $|z| \le 30$ kpc in the principal-axis frame of the host galaxy. White regions correspond to higher stellar surface density. The figure highlights morphological diversity in disk structure among the simulated Milky Way analogs.
• Figure 2: Example accumulated fraction of the stellar and different DM components as a function of redshift for m12i. The red curve represents all accreted stars. The green curve represents DM accreted from major mergers heavier than $10^9~M_{\odot}$ and the blue curve shows the DM accreted either smoothly into the main halo or with unresolved small subhalos ($<10^9M_{\odot}$)
• Figure 3: Fractional contributions of the individual components to the local DM population. The diffuse component is generally the dominant contributor across the m12 sample. By contrast, the DM contributions from the top mergers—ranked by their associated stellar mass in the solar neighborhood—vary substantially, and in some cases can be negligible relative to the other components. The exact numerical values of all fractions are listed in Table. \ref{['tab:mergerfractionnew']}.
• Figure 4: Example velocity distribution for the largest m12i major merger in radial component $v_r$, transverse component $v_t$ and speed $|\vec{v}|$. It can be noticed that the stellar velocity distributions roughly follow the DM counterpart.
• Figure 5: EMD for $v_r$, $v_{\theta}$, and $v_{\phi}$ between the stellar and DM velocity distributions with respect to the host disk for the top four mergers of all six of our m12 galaxies as a function of redshift. Each marker shape stands for a different m12 and is colored by the merger mass. The horizontal dashed lines at EMD=0.2 and EMD=0.05 mark the thresholds for meaningfully different and nearly identical distributions, respectively. It can be seen that the stellar and DM radial velocities are similar to each other due to violent relaxation. The difference in speed is thus mainly driven by the transverse component. The transverse EMD decreases with redshift, reflecting the phase mixing of earlier mergers.