Nested solitons in two-field fuzzy dark matter

TL;DR

This paper investigates the properties of two-field fuzzy dark matter haloes, revealing nested soliton structures and differences in small-scale structure formation compared to one-field models through numerical simulations.

Contribution

The study introduces the first detailed numerical analysis of nested solitons in two-field fuzzy dark matter and explores their formation and impact on cosmological structures.

Findings

Nested solitons form when the mass ratio is large.

Two-field models show distinct small-scale structures from one-field models.

Soliton formation depends on density fraction and mass ratio.

Abstract

Dark matter as scalar particles consisting of multiple species is well motivated in string theory where axion fields are ubiquitous. A two-field fuzzy dark matter (FDM) model features two species of ultralight axion particles with different masses, $m_1 \neq m_2$, which is extended from the standard one-field model with $m_a \sim 10^{-22}\,{\rm eV}$. Here we perform numerical simulations to explore the properties of two-field FDM haloes. We find that the central soliton has a nested structure when $m_2 \gg m_1$, which is distinguishable from the generic flat-core soliton in one-field haloes. However, the formation of this nested soliton is subject to many factors, including the density fraction and mass ratio of the two fields. Finally, we study non-linear structure formation in two-field cosmological simulations with self-consistent initial conditions and find that the small-scale structure in two-field cosmology is also distinct from the one-field model in terms of DM halo counts and soliton formation time.