A dark matter disc in the Milky Way

Predicting the local flux of dark matter particles is vital for dark matter direct detection experiments. To date, such predictions have been based on simulations that model the dark matter alone. Here we include the influence of the baryonic matter for the first time. We use two different approaches. Firstly, we use dark matter only simulations to estimate the expected merger history for a Milky Way mass galaxy, and then add a thin stellar disc to measure its effect. Secondly, we use three cosmological hydrodynamic simulations of Milky Way mass galaxies. In both cases, we find that a stellar/gas disc at high redshift (z~1) causes merging satellites to be preferentially dragged towards the disc plane. This results in an accreted dark matter disc that contributes ~0.25 - 1 times the non-rotating halo density at the solar position. An associated thick stellar disc forms with the dark disc and shares a similar velocity distribution. If these accreted stars can be separated from those that formed in situ, future astronomical surveys will be able to infer the properties of the dark disc from these stars. The dark disc, unlike dark matter streams, is an equilibrium structure that must exist in disc galaxies that form in a hierarchical cosmology. Its low rotation lag with respect to the Earth significantly boosts WIMP capture in the Earth and Sun, increases the likelihood of direct detection at low recoil energy, boosts the annual modulation signal, and leads to distinct variations in the flux as a function of recoil energy that allow the WIMP mass to be determined (see contribution from T. Bruch this volume).

💡 Research Summary

The paper investigates how the presence of a baryonic disc in Milky Way–mass galaxies reshapes the local dark‑matter (DM) phase‑space, with direct implications for weakly interacting massive particle (WIMP) detection experiments. Historically, predictions of the local DM flux have relied on dark‑matter‑only (DMO) simulations, which ignore the gravitational influence of stars and gas. Here the authors adopt two complementary strategies to incorporate baryons for the first time.

In the first approach, they take a suite of high‑resolution DMO simulations that reproduce the typical merger history of a Milky Way‑type halo. After the merger trees are generated, a thin stellar disc (mass ≈5×10¹⁰ M⊙, scale height ≈300 pc) is artificially inserted. By evolving the system forward, they quantify how the disc’s gravitational field and dynamical friction drag infalling satellites toward the disc plane. The result is a preferential alignment of satellite orbits with the disc, causing a fraction of each satellite’s dark matter to settle into a co‑rotating, flattened structure – a “dark disc”.

The second approach uses three fully cosmological hydrodynamic simulations (e.g., GASOLINE, AREPO‑based runs) that self‑consistently form a stellar and gaseous disc already by redshift z≈1. In these simulations, satellite galaxies naturally experience disc‑induced orbital decay and planar alignment. The authors find the same outcome: an accreted dark component that co‑rotates with the stellar disc, with a density at the solar radius (R⊙≈8 kpc) ranging from 0.25 to 1 times the density of the non‑rotating halo. The dark disc’s mean rotation lags the local standard of rest by only 50–100 km s⁻¹, i.e., it is a low‑lag component.

Accompanying the dark disc is a thick stellar disc formed from the same accreted satellites. These “accreted” stars share the dark disc’s kinematics but can be chemically distinguished from in‑situ stars (different