[Paper Review] Core-collapse, evaporation and tidal effects: the life story of a self-interacting dark matter subhalo

Zhichao Carton Zeng, Annika H. G. Peter+6 more|arXiv (Cornell University)|Oct 1, 2021

Cosmology and Gravitation TheoriesPhysics and Astronomy180 references70 citations

TL;DR

This paper develops a hybrid semi-analytical and N-body method to simulate the evolution of self-interacting dark matter (SIDM) subhalos from core formation to core-collapse in lens-mass host halos. It finds that evaporation from host-subhalo interactions strongly delays or prevents core-collapse in subhalos with constant cross sections, implying that any observed ultra-compact dark substructures would require velocity-dependent cross sections or additional physics.

ABSTRACT

Self-interacting dark matter (SIDM) cosmologies admit an enormous diversity of dark matter (DM) halo density profiles, from low-density cores to high-density core-collapsed cusps. The possibility of the growth of high central density in low-mass halos, accelerated if halos are subhalos of larger systems, has intriguing consequences for small-halo searches with substructure lensing. However, following the evolution of $\lesssim 10^8 M_\odot$ subhalos in lens-mass systems ($\sim 10^{13}M_\odot$) is computationally expensive with traditional N-body simulations. In this work, we develop a new hybrid semi-analytical + N-body method to study the evolution of SIDM subhalos with high fidelity, from core formation to core-collapse, in staged simulations. Our method works best for small subhalos ($\lesssim 1/1000$ host mass), for which the error caused by dynamical friction is minimal. We are able to capture the evaporation of subhalo particles by interactions with host halo particles, an effect that has not yet been fully explored in the context of subhalo core-collapse. We find three main processes drive subhalo evolution: subhalo internal heat outflow, host-subhalo evaporation, and tidal effects. The subhalo central density grows only when the heat outflow outweighs the energy gain from evaporation and tidal heating. Thus, evaporation delays or even disrupts subhalo core-collapse. We map out the parameter space for subhalos to core-collapse, finding that it is nearly impossible to drive core-collapse in subhalos in SIDM models with constant cross sections. Any discovery of ultra-compact dark substructures with future substructure lensing observations favors additional degrees of freedom, such as velocity-dependence, in the cross section.

Motivation & Objective

• To study the full evolutionary lifecycle of SIDM subhalos—from core formation to core-collapse—within lens-mass host halos.
• To address the computational challenge of simulating low-mass subhalos (≲10⁸ M⊙) in massive host systems (∼10¹³ M⊙) using traditional N-body methods.
• To quantify the roles of internal heat outflow, host-subhalo evaporation, and tidal effects in subhalo evolution.
• To determine the parameter space under which core-collapse can occur in SIDM subhalos with constant cross sections.
• To assess the implications for future substructure lensing observations in distinguishing SIDM models.

Proposed method

• Develops a hybrid simulation framework combining semi-analytical modeling with high-fidelity N-body simulations to model subhalo evolution in stages.
• Applies a core-collapse criterion based on the Knudsen number (𝐾𝑛 ≈ 1), where the mean free path becomes comparable to the gravitational scale height.
• Uses a softened gravity force law with a fixed softening length, validated to introduce minimal numerical error near core-collapse.
• Tracks subhalo evolution through multiple pericentric passages, resolving particle evaporation and tidal stripping effects.
• Employs a staged simulation approach that focuses computational resources on critical phases of subhalo evolution.
• Validated the method on subhalos with mass ≤ 1/1000 of the host halo mass, where dynamical friction errors are minimized.

Experimental results

Research questions

• RQ1 What drives the evolution of SIDM subhalos from core formation to core-collapse in lens-mass host halos?
• RQ2 How do evaporation and tidal effects influence the timing and likelihood of core-collapse in SIDM subhalos?
• RQ3 Is core-collapse feasible in SIDM subhalos with constant cross sections under realistic cosmological concentrations?
• RQ4 What constraints do observed ultra-compact dark substructures place on the velocity dependence of the SIDM cross section?
• RQ5 How do competing processes—heat outflow, evaporation, and tidal heating—affect the net central density evolution of subhalos?

Key findings

• Evaporation of subhalo particles due to interactions with host halo particles is a dominant process that can delay or prevent core-collapse in SIDM subhalos.
• Core-collapse in subhalos with constant cross sections is nearly impossible under realistic cosmological concentrations, as evaporation and tidal heating typically outweigh internal heat outflow.
• The central density of a subhalo grows only when internal heat outflow exceeds energy gain from evaporation and tidal heating.
• Subhalos with [𝑀sub = 10¹⁰.⁵ M⊙, 𝜎T/𝑚 = 6 cm²/g, 𝑟peri:𝑟apo = 1/10, 𝑐 = 75] show a near-critical balance between cooling and heating, making core-collapse highly sensitive to timing and initial conditions.
• Any future detection of ultra-compact dark substructures via substructure lensing would strongly disfavor constant cross-section SIDM models and favor velocity-dependent cross sections or additional degrees of freedom.
• The method successfully captures the transition to the short-mean-free-path regime near core-collapse, justifying termination of N-body simulations at that point and suggesting a need for hydrodynamic or analytical follow-up.