Structure Formation and Microlensing with Axion Miniclusters

TL;DR

This paper investigates the formation, properties, and microlensing signatures of axion miniclusters, providing new methods to constrain their abundance and opening avenues for axion detection through gravitational microlensing.

Contribution

It introduces a comprehensive framework for modeling axion miniclusters, including their mass function, structure, and microlensing signals, and derives the first observational constraints on their abundance.

Findings

Miniclusters can grow up to 10^6 times their initial mass.

Microlensing signals depend strongly on the fraction of dark matter in miniclusters.

Constraints on axion scenarios are significantly affected by minicluster abundance.

Abstract

If the symmetry breaking responsible for axion dark matter production occurs during the radiation-dominated epoch in the early Universe, then this produces large amplitude perturbations that collapse into dense objects known as axion miniclusters. The characteristic minicluster mass, $M_0$, is set by the mass inside the horizon when axion oscillations begin. For the QCD axion $M_0\sim 10^{-10}M_\odot$, however for an axion-like particle $M_0$ can approach $M_\odot$ or higher. Using the Press-Schechter formalism we compute the mass function of halos formed by hierarchical structure formation from these seeds. We compute the concentrations and collapse times of these halos and show that they can grow to be as massive as $10^6M_0$. Within the halos, miniclusters likely remain tightly bound, and we compute their gravitational microlensing signal taking the fraction of axion dark matter collapsed into miniclusters, $f_{\rm MC}$, as a free parameter. A large value of $f_{\rm MC}$ severely weakens constraints on axion scenarios from direct detection experiments. We take into account the non-Gaussian distribution of sizes of miniclusters and determine how this effects the number of microlensing events. We develop the tools to consider microlensing by an extended mass function of non-point-like objects, and use microlensing data to place the first observational constraints on $f_{\rm MC}$. This opens a new window for the potential discovery of the axion.