First Simulations of Axion Minicluster Halos

TL;DR

This paper uses N-body simulations to study the formation and properties of axion miniclusters, revealing their mass distribution, density profiles, and fraction of dark matter in these structures.

Contribution

First simulation-based analysis of axion minicluster formation, confirming theoretical predictions and detailing their mass distribution and density profiles.

Findings

Miniclusters form with masses up to 10^{-12} M_sun

Density profiles are approximately NFW-like

About 75% of axion dark matter is in miniclusters at redshift 100

Abstract

We study the gravitational collapse of axion dark matter fluctuations in the post-inflationary scenario, so-called axion miniclusters, with N-body simulations. Largely confirming theoretical expectations, overdensities begin to collapse in the radiation-dominated epoch and form an early distribution of miniclusters with masses up to $10^{-12}\,M_\odot$. After matter-radiation equality, ongoing mergers give rise to a steep power-law distribution of minicluster halo masses. The density profiles of well-resolved halos are NFW-like to good approximation. The fraction of axion dark matter in these bound structures is $\sim 0.75$ at redshift $z=100$.