Structure of Axion Miniclusters

TL;DR

This paper investigates the internal structure of axion miniclusters using simulations, finding they are mostly well described by NFW profiles or power-laws, and identifies potential for gravitational microlensing signals.

Contribution

It introduces a method to identify dense axion minicluster seeds and analyzes their density profiles and potential observability, advancing understanding of axion dark matter structures.

Findings

Most miniclusters follow NFW profiles at late times

Approximately 80% fit a $r^{-2.9}$ power-law density profile

Some miniclusters could cause gravitational microlensing if axion mass is in the 0.2-3 meV range

Abstract

The Peak-Patch algorithm is used to identify the densest minicluster seeds in the initial axion density field simulated from string decay. The fate of these dense seeds is found by tracking the subsequent gravitational collapse in cosmological $N$-body simulations. We find that miniclusters at late times are well described by NFW profiles, although for around 80\% of simulated miniclusters a single power-law density profile of $r^{-2.9}$ is an equally good fit due to the unresolved scale radius. Under the assumption that all miniclusters with an unresolved scale radius are described by a power-law plus axion star density profile, we identify a significant number of miniclusters that might be dense enough to give rise to gravitational microlensing if the axion mass is $0.2 \,\mathrm{meV}\lesssim m_a \lesssim 3\,\mathrm{meV}$. Higher resolution simulations resolving the inner structure and axion star formation are necessary to explore this possibility further.