Soliton Merger Rates and Enhanced Axion Dark Matter Decay

TL;DR

This paper models the formation, merger rates, and decay of axion dark matter solitons, predicting their significant contribution to early universe energy injection and potential observational signatures.

Contribution

It introduces a new calculation of soliton merger rates and links soliton instability to enhanced dark matter decay, providing novel insights into axion dark matter phenomenology.

Findings

Soliton mass function derived using extended Press-Schechter formalism.

Calculated soliton major merger rates for axion dark matter.

Predicted energy injection from soliton decay exceeds supernova contributions at high redshift.

Abstract

Solitons are observed to form in simulations of dark matter (DM) halos consisting of bosonic fields. We use the extended Press-Schechter formalism to compute the mass function of solitons, assuming various forms for the relationship between halo mass and soliton mass. We further provide a new calculation of the rate of soliton major mergers. Solitons composed of axion DM are unstable above a critical mass, and decay to either relativistic axions or photons, depending on the values of the coupling constants. We use the computed soliton major merger rate to predict the enhanced DM decay rate due to soliton instability. For certain values of currently allowed axion parameters, the energy injection into the intergalactic medium from soliton decays to photons is comparable to or larger than the energy injection due to core collapse supernovae at $z>10$. A companion paper explores the phenomenology of such an energy injection.