Dark matter spiral arms in Milky Way-like halos

Context . The coupling between the dark matter (DM) halo and the stellar disc is a key factor in galactic evolution. While the interaction between structures like the Galactic bar and DM halos has been explored (e.g. slowing down of the bar due to dynamical friction), the effect of spiral arms on the DM halo distribution has received limited attention. Aims . We aim to detect and characterize the interaction between the stellar spiral arms and the DM halo. Methods . We analysed a suite of simulations featuring strong stellar spiral arms, ranging in complexity from test-particle models to fully cosmological hydrodynamical simulations. Using Fourier transforms, we mapped the phase and amplitude of the stellar spirals at different times and radii. We then applied the same methodology to DM particles near the stellar disc and compared trends in Fourier coefficients and phases between the two components. Results . We detect a clear spiral arm signal in the DM distribution, correlated with the stellar spirals, confirming the reaction of the halo. The strength of the DM spirals consistently measures around 10% of that of the stellar spiral arms. In the N -body simulation, the DM spiral persistently trails the stellar spiral arm by approximately 10 ∘ . A strong spiral signal of a few kilometres per second appears in the radial, azimuthal, and vertical velocities of halo particles, distinct from the stellar kinematic signature. In a test-particle simulation with an analytical spiral potential (omitting self-gravity), we reproduce a similar density and kinematic response, showing that the test-particle halo responds in the same way as the N -body halo. This similarity confirms that we are observing the forced response of the halo to the stellar spiral arms potential. Finally, we also find the presence of DM spiral arms in a pure N -body simulation with an external perturber, and isolated and cosmological hydrodynamical simulations, indicating that the dynamical signatures of the forced response in the DM halo are independent of the dynamical origin of the stellar spiral arms. Conclusions . We reveal the ubiquitous presence of DM spiral arms in Milky Way-like galaxies, driven by a forced response to the stellar spiral potential.