Structure of axion miniclusters

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 Navarro-Frenk-White 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 meV less than or similar to ma less than or similar to 3 meV. Higher resolution simulations resolving the inner structure and axion star formation are necessary to explore this possibility further.

Automated summary

The peak-patch algorithm is used to identify the densest minicluster seeds in the initial axion density field. Tracking the subsequent gravitational collapse, it is found that the late-time miniclusters can be described by Navarro-Frenk-White profiles or a power-law density profile. Assuming that all miniclusters with an unresolved scale radius follow a power-law plus axion star density profile, a significant number of miniclusters dense enough for gravitational microlensing are identified. Higher resolution simulations are needed to explore the inner structure and axion star formation further.