Small-scale structure of fuzzy and axion-like dark matter

TL;DR

This paper reviews the small-scale structures and unique behaviors of fuzzy and axion-like dark matter, including solitonic cores, miniclusters, and axion stars, highlighting their differences from traditional cold dark matter.

Contribution

It provides an overview of current research topics on the formation and properties of small-scale structures in axion-like dark matter models.

Findings

FDM halos feature solitonic core formation.

Suppressed small-scale perturbations are characteristic of FDM.

Axion miniclusters and axion stars are potential bound objects.

Abstract

Axion-like particle (ALP) dark matter shows distinctive behavior on scales where wavelike effects dominate over self-gravity. Ultralight axions are candidates for fuzzy dark matter (FDM) whose de Broglie wavelength in virialized halos reaches scales of kiloparsecs. Important features of FDM scenarios are the formation of solitonic halo cores, suppressed small-scale perturbations, and enhanced gravitational relaxation. More massive ALPs, including the QCD axion, behave like CDM on galactic scales but may be clumped into axion miniclusters if they were produced after inflation. Just as FDM halos, axion miniclusters may host the formation of coherent bound objects (axion stars) by Bose-Einstein condensation. This article presents a selection of topics in this field that are currently under active investigation.