Core Collapse Beyond the Fluid Approximation: The Late Evolution of Self-Interacting Dark Matter Halos

TL;DR

This work develops KiSS-SIDM, a kinetic, DSMC-based solver for the gravothermal evolution of self-interacting dark matter halos that can cover long-, intermediate-, and short-mean-free-path regimes without calibration. It shows that while LTE deep in the short-MFP core validates the conducting-fluid model, the intermediate-MFP region near the core boundary remains out of equilibrium, with anisotropic velocity distributions and viscous dissipation altering late-time evolution. These non-equilibrium effects modify core and halo structure in ways that are not captured by traditional fluid treatments, though the fluid description remains applicable in the SMFP core. The method provides an efficient, publicly available benchmark for SIDM collapse and cautions against overreliance on local-thermodynamic-equilibrium assumptions in the IMFP regime, with implications for black hole formation scenarios and subgrid modeling in cosmological runs.

Abstract

We show that the gravothermal collapse of self-interacting dark matter (SIDM) halos can deviate from local thermodynamic equilibrium. As a consequence, the self-similar evolution predicted by the commonly adopted conducting fluid model can be altered or broken. Our results are obtained using a novel, efficient kinetic solver called KiSS-SIDM for tracing the gravothermal evolution based on the Direct Simulation Monte Carlo (DSMC) framework. In the long mean free path stage, the code is a viable alternative to the fluid model, yet requires no calibration parameters. Further, this method enables a fully kinetic treatment well into the late, short mean free path, stage of the collapse. We apply the method to a canonical case with isotropic, velocity independent scattering. We find that although a fluid treatment is appropriate deep in the short mean free path core, departures from local thermodynamic equilibrium develop in the intermediate mean free path region bounding the core, which modify the late-time evolution. KiSS-SIDM is publicly available at https://gitlab.com/Socob/KiSS-SIDM.

Figures (5)

• Figure 1: Left: The density profiles in the simulation at different times (solid) and at a matched central density (at the final time) in the fluid model (dotted). The MFP at different densities is also plotted (length of the dashed lines). Right: The velocity dispersion profiles for the same cases, 3D (solid) and triple the radial velocity dispersion (dashed). In the fluid model, the velocity dispersion is isotropic.
• Figure 2: The Lagrangian derivative of the energy (solid), the mass-integrated, temperature-weighted time derivative of the specific entropy (dashed), and the theoretical conductive luminosity (dash-dotted) at three points in the evolution, corresponding to \ref{['fig:profiles']}. The late evolution is shown on a separate panel due to the differing scales of interest.
• Figure 3: The velocity distribution function at a late time ($t/t_0 = 437$) in the IMFP region.
• Figure 4: The DSMC radial velocity dispersion (solid) and fluid 1D velocity dispersions (dashed) at a sequence of central densities after the core enters the SMFP regime.
• Figure 5: The time evolution of the mean density in a central region with differing numbers of particles.