Flux of Extragalactic Dark Matter in Direct Detection Experiments

TL;DR

This work demonstrates that a substantial extragalactic dark matter component from the Local Group contributes non-negligibly to the Solar neighborhood, imprinting a strong directional dependence on the local velocity distribution. By modeling directional Galactic escape velocities with a dipole plus quadrupole structure and defining an extragalactic criterion, the authors quantify the Local Group DM fraction (≈$26 ext{--}27 ext%$ by number) and its substantial flux share (≈$38 ext%$), with anisotropy peaking in specific sky directions. They show that the extragalactic high-velocity tail elevates the recoil-rate at high $E_R$ and amplifies annual modulation, particularly for low-mass WIMPs, and that directional detectors could isolate multiple DM components (Galactic, extragalactic, and substructure) via their distinct angular and velocity signatures. The results imply revised expectations for direct-detection experiments and motivate the development of directional sensitivity to exploit the multi-component DM environment.

Abstract

We calculate the contribution of extragalactic dark matter to the local dark matter density and flux in the Milky Way. By analyzing the Galactic escape velocity as a function of direction, we establish the criterion for separating dark matter particles bound to the Milky Way and those originating from the Local Group environment. Our analysis reveals that approximately 25% of dark matter particles in the Solar neighborhood have an extragalactic origin, contributing nearly 38% of the total mass flux. The directional dependence of this extragalactic component shows significant anisotropy across the sky, with implications for direct detection experiments. We provide quantitative predictions for detection rates and signatures that could help identify the extragalactic dark matter component in current and future experiments.

Figures (11)

• Figure 1: Minimum escape velocity as a function of direction in a spherical projection of Galactic coordinates within our model. The map displays significant angular variation, with the highest escape velocities ($>700$ km/s, yellow region) concentrated around galactic coordinates $(90^{\circ}, 0^{\circ})$ in the eastern hemisphere, while the lowest escape velocities ($<400$ km/s, dark purple) appear in the opposite direction. This anisotropy reflects the LSR motion and the asymmetric gravitational potential of the Milky Way.
• Figure 2: Total mass flux of dark matter ($\rho {\bf v}$) in GeV/cm$^2$/s. The map in galactic coordinates shows a higher flux (yellow) in the western galactic hemisphere.
• Figure 3: The extragalactic dark matter fraction shows a strong dipole asymmetry with significantly higher extragalactic dark matter contribution (yellow/orange) of up to 70% in the western galactic hemisphere, contrasted with minimal contribution (dark purple) in the eastern hemisphere.
• Figure 4: The extragalactic dark matter flux in GeV/cm$^2$/s ($\times 10^{6}$), shows significant angular variation with the highest flux values ($\sim 2 \times 10^{7}$ GeV/cm$^2$/s, yellow regions) concentrated in the western galactic hemisphere and a pronounced minimum in the opposite direcction.
• Figure 5: Angular dependence of the dark-matter velocity distribution. Probability-density functions are shown for all particles (blue), the LSR direction (orange), and the anti-LSR direction (green). The vertical dashed red line indicates the average escape velocity of $520\ \mathrm{km\,s^{-1}}$.