N-body simulations of dark matter–baryon interactions

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
² Excellence Cluster ORIGINS, Boltzmannstrasse 2, 85748 Garching, Germany
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁴ Physik Department T31, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany
⁵ Department of Physics and Astronomy, University of Southern California, Los Angeles, CA 90089, USA
⁶ Department of Astronomy & Astrophysics, University of California, San Diego, La Jolla, CA 92093, USA

^★ Corresponding author: mfischer@usm.lmu.de

Received: 1 April 2025
Accepted: 28 June 2025

Abstract

Context. Dark matter (DM) particles can interact with particles characterised by the standard model. Although there are a number of constraints derived from direct and indirect detection experiments, the dynamical evolution of astrophysical objects could offer a promising probe for such interactions. Obtaining astrophysical predictions is challenging and primarily limited by our ability to simulate scattering between DM and baryonic particles within N-body and hydrodynamics simulations.

Aims. We have developed the first scheme allowing for the simulation of these interacting dark matter (IDM) models, accurately accounting for their angular and velocity dependence, as well as the mass ratio between the DM and baryonic scattering partners.

Methods. To describe DM-baryon interactions, we used an N-body code together with its implementation of smoothed-particle hydrodynamics (SPH) and meshless finite mass. The interaction itself was realised in a pairwise fashion by creating a virtual scattering partner from the baryonic particle and allowing it to interact with a DM particle using a scattering routine initially developed for self-interacting dark matter (SIDM). After the interaction, the virtual particle is rejoined with the baryonic particle, fulfilling the requirements of energy and momentum conservation.

Results. Through several test problems, we demonstrated that we are able to reproduce the analytic solutions with our IDM scheme. This includes a test for scattering with a physical mass ratio of 1:1000, which is beyond the limits of current SIDM simulations. We comment on various numerical aspects and challenges, and we describe the limitations of our numerical scheme. Furthermore, we study the impact of IDM on halo formation with a collapsing over-density.

Conclusions. We find that it is possible to accurately model IDM within N-body and hydrodynamics simulations commonly used in astrophysics. Finally, our scheme allows for novel predictions to be made and new constraints on DM-baryon scattering to be set.