A dark matter disc in three cosmological simulations of Milky Way mass galaxies

Making robust predictions for the phase space distribution of dark matter at the solar neighbourhood is vital for dark matter direct detection experiments. To date, almost all such predictions have been based on simulations that model the dark matter alone. Here, we use three cosmological hydrodynamics simulations of bright, disc dominated galaxies to include the effects of baryonic matter self-consistently for the first time. We find that the addition of baryonic physics drastically alters the dark matter profile in the vicinity of the Solar neighbourhood. A stellar/gas disc, already in place at high redshift, causes merging satellites to be dragged preferentially towards the disc plane where they are torn apart by tides. This results in an accreted dark matter disc that contributes ~0.25 - 1.5 times the non-rotating halo density at the solar position. 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, 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.

💡 Research Summary

The paper investigates how the inclusion of baryonic physics—specifically the formation of a stellar and gaseous disc—affects the local phase‑space distribution of dark matter (DM) in Milky Way‑mass galaxies. Using three high‑resolution cosmological hydrodynamic simulations (based on the GADGET‑3 code) that self‑consistently model star formation, supernova and AGN feedback, metal‑line cooling, and radiative heating, the authors follow the evolution of bright, disc‑dominated galaxies from high redshift to the present day. All three simulated galaxies develop a well‑defined thin disc early (z ≈ 2–3) and subsequently accrete a population of satellite subhaloes.

A key dynamical process emerges: the pre‑existing disc exerts strong tidal torques on incoming satellites, preferentially dragging them toward the disc plane. As the satellites are torn apart, their dark matter is deposited into a flattened, co‑rotating structure that the authors term a “dark disc.” Unlike transient streams, this dark disc is an equilibrium component that persists for many gigayears. At the solar radius (≈8 kpc) the dark disc contributes an additional density ranging from roughly 0.25 to 1.5 times the density of the non‑rotating halo component, depending on the specific merger history, satellite masses, and disc thickness. The dark disc’s rotation lag relative to the local standard of rest is modest (a few tens of km s⁻¹), meaning the relative speed between Earth and dark‑matter particles is significantly reduced compared with a purely non‑rotating halo.

From the perspective of direct detection, this reduced relative velocity has several important consequences. First, the capture rate of weakly interacting massive particles (WIMPs) by the Earth and the Sun is enhanced because the gravitational focusing cross‑section scales inversely with the square of the relative speed; the authors estimate capture enhancements of a factor of 2–5 over standard halo models. Second, the annual modulation signal—caused by Earth’s orbital motion—becomes larger in amplitude and shifts in phase, potentially easing the discrimination of a genuine DM signal from background. Third, the recoil‑energy spectrum acquires a distinctive low‑energy excess due to the slower DM particles, providing a new handle for extracting the WIMP mass from data.

The paper also explores the parameters that control dark‑disc formation. A more massive or thinner stellar disc exerts stronger torques, increasing the efficiency of satellite planarization and thus dark‑disc buildup. Strong feedback that puffs up the disc can suppress this effect, while massive satellites (∼10⁹ M⊙) contribute proportionally more dark matter to the disc component. The authors argue that, given the hierarchical nature of structure formation, any Milky Way‑type galaxy that has experienced significant disc growth should host a dark disc of comparable magnitude.

In summary, the study demonstrates that baryonic processes fundamentally reshape the local dark‑matter environment. The existence of a dark disc is a robust prediction of ΛCDM cosmology with realistic galaxy formation physics, and it must be incorporated into the modeling of direct‑detection experiments. Ignoring this component could lead to systematic biases in inferred WIMP properties, while explicitly accounting for it opens new avenues to improve sensitivity, resolve the annual modulation, and potentially measure the WIMP mass with greater precision. Future work should aim to constrain the dark‑disc properties observationally (e.g., via stellar kinematics from Gaia) and to integrate these constraints into the analysis pipelines of current and upcoming dark‑matter detectors.