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

Area of Science:

• Cosmology
• Astrophysics
• Particle Physics

Background:

• Self-interacting dark matter (SIDM) is a leading candidate for dark matter.
• Gravothermal collapse is a key process in the evolution of dark matter halos.
• The conducting fluid model is commonly used to describe gravothermal evolution but assumes local thermodynamic equilibrium.

Purpose of the Study:

• To investigate deviations from local thermodynamic equilibrium during the gravothermal collapse of SIDM halos.
• To assess the validity of the conducting fluid model in describing SIDM halo evolution.
• To develop and apply a novel kinetic solver for simulating gravothermal collapse.

Main Methods:

• Development and application of a novel kinetic solver, kiss-sidm, based on the direct simulation Monte Carlo framework.
• Tracing the gravothermal evolution of SIDM halos, including both long and short mean free path regimes.
• Application to a canonical case with isotropic, velocity-independent scattering.

Main Results:

• Gravothermal collapse of SIDM halos can deviate from local thermodynamic equilibrium.
• The commonly adopted conducting fluid model's prediction of self-similar evolution can be altered or broken.
• Departures from local thermodynamic equilibrium occur in the intermediate mean free path region, modifying late-time evolution.

Conclusions:

• The conducting fluid model is insufficient for fully describing SIDM halo gravothermal collapse.
• Kinetic effects become important in the intermediate and late stages of collapse.
• The kiss-sidm code provides a viable alternative to fluid models, enabling a fully kinetic treatment.