Simulations of solitonic core mergers in ultra-light axion dark matter cosmologies

TL;DR

This paper uses 3D simulations to study how solitonic cores in ultra-light axion dark matter merge, revealing how their final structures depend on initial conditions and confirming the formation of cores with NFW-like tails.

Contribution

It provides new insights into the dynamics of solitonic core mergers in axion dark matter, including effects of mass ratios, angular momentum, and phase differences, using detailed numerical simulations.

Findings

Final core mass depends on mass ratio, total mass, and energy.

Cores become rotating ellipsoids with non-zero angular momentum.

Final density profiles have solitonic cores with NFW-like tails.

Abstract

Using three-dimensional simulations, we study the dynamics and final structure of merging solitonic cores predicted to form in ultra-light axion dark matter halos. The classical, Newtonian equations of motion of a self-gravitating scalar field are described by the Schr\"odinger-Poisson equations. We investigate mergers of ground state (boson star) configurations with varying mass ratios, relative phases, orbital angular momenta and initial separation with the primary goal to understand the mass loss of the emerging core by gravitational cooling. Previous results showing that the final density profiles have solitonic cores and NFW-like tails are confirmed. In binary mergers, the final core mass does not depend on initial phase difference or angular momentum and only depends on mass ratio, total initial mass, and total energy of the system. For non-zero angular momenta, the otherwise spherical cores become rotating ellipsoids. The results for mergers of multiple cores are qualitatively identical.