Axion miniclusters made easy

We use a modified version of the peak patch excursion set formalism to compute the mass and size distribution of QCD axion miniclusters from a fully non-Gaussian initial density field obtained from numerical simulations of axion string decay. We find strong agreement with N-body simulations at significantly lower computational cost. We employ a spherical collapse model, and provide fitting functions for the modified barrier in the radiation era. The halo mass function at z = 629 has a power-law distribution M-0.6 for masses within the range 10(-15) less than or similar to M-circle dot less than or similar to 10(-10) M-circle dot, with all masses scaling as (m(a)/50 mu eV)(-0.5). We construct merger trees to estimate the collapse redshift and concentration mass relation, C(M), which is well described using analytical results from the initial power spectrum and linear growth. Using the calibrated analytic results to extrapolate to z = 0, our method predicts a mean concentration C similar to O(few) x 10(4). The low computational cost of our method makes future investigation of the statistics of rare, dense miniclusters easy to achieve.

Automated summary

The study presents a modified peak patch excursion set formalism for computing the mass and size distribution of QCD axion miniclusters, based on a fully non-Gaussian initial density field obtained from numerical simulations of axion string decay. Strong agreement with N-body simulations is found at significantly lower computational cost. The method employs a spherical collapse model and provides fitting functions for the modified barrier in the radiation era.