Simulations of solitonic core mergers in ultralight axion dark matter cosmologies

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 Schrödinger-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.

Core-halo mass relation of ultralight axion dark matter from merger history

“…Comparing the initial and final masses of merging cores, [23] find a universal behavior of the core mass loss in mergers that depends nearly entirely on the mass ratio. Implementing this relation in a semianalytic model (SAM) for galaxy formation, [27] studies the effects of the core on the substructure of Milky way-sized FDM halos.…”

“…Removing any residual effects of the scaling symmetry by constructing and analyzing scale invariant quantities, [23] were unable to reproduce this relation in simulations of solitonic core mergers. Furthermore, the model of [11] does not account for the combined evolution of M c and M h by halo mergers after the initial collapse of density perturbations which is known to be an important ingredient in hierarchical structure formation.…”

“…If self-interactions can be neglected, this type of dark matter candidate is often referred to as fuzzy dark matter (FDM) [4,5]. Unlike CDM which produces cuspy halo profiles, FDM produces flat halo cores [10,23,24] on scales smaller than the de Broglie wavelength of particles with the halo's virial velocity, the so-called quantum Jeans length [5,25]. Below this scale, quantum effects suppress gravitational collapse.…”

“…Simulations of cosmological structure formation [10] and merging solitonic solutions [11,23] based on the Schrödinger-Poisson (SP) equations indicate that FDM halos contain distinct cores surrounded by NavarroFrenk-White-like profiles [34]. [10] find that the mass of these cores, M c , is related to the halo mass, M h , by a power law relation, M c ∝ M 1/3 h .…”

Brief History of Ultra-light Scalar Dark Matter Models

“…It is suggested that the SFDM can explain the contradictory behaviors of DM in collisions of galaxy clusters [90][91][92][93][94]. For a fast collision as observed in the Bullet cluster two dark matter halos pass each other in a solitonlike way, while for a slow collision as observed in the Abell 520 the halos merge due to the wave nature of the SFDM.…”