Solitons and halos for self-interacting scalar dark matter

TL;DR

This study investigates the formation and evolution of solitons in scalar-field dark matter halos with self-interactions, revealing conditions for rapid formation, coexistence of multiple density spikes, and the impact of halo size and density profiles.

Contribution

The paper introduces numerical simulations and a kinetic theory to analyze soliton formation in self-interacting scalar dark matter halos, highlighting the dependence on halo size and density structure.

Findings

Solitons form quickly when halo size is near the Jeans length.

Large halos with flat cores develop solitons more slowly.

Multiple density spikes can coexist within a single halo.

Abstract

We study the formation and evolution of solitons supported by repulsive self-interactions inside extended halos, for scalar-field dark matter scenarios. We focus on the semiclassical regime where the quantum pressure is typically much smaller than the self-interactions. We present numerical simulations, with initial conditions where the halo is described by the WKB approximation for its eigenfunction coefficients. We find that when the size of the system is of the order of the Jeans length associated with the self-interactions, a central soliton quickly forms and makes about 50% of the total mass. However, if the halo is ten times greater than this self-interaction scale, a soliton only quickly forms in cuspy halos where the central density is large enough to trigger the self-interactions. If the halo has a flat core, it takes a longer time for a soliton to appear, after small random fluctuations on the de Broglie wavelength size build up to reach a large enough density. In some cases, we observe the co-existence of several narrow density spikes inside the larger self-interaction-supported soliton. All solitons appear robust and slowly grow, unless they already make up 40% of the total mass. We develop a kinetic theory, valid for an inhomogeneous background, to estimate the soliton growth rate for low masses. It explains the fast falloff of the growth rate as resonances between the ground state and halo excited states disappear. Our results suggest that cosmological halos would show a large scatter for their soliton mass, depending on their assembly history.