Simulations of Solitonic Core Mergers in Ultra-Light Axion Dark Matter Cosmologies
The paper "Simulations of Solitonic Core Mergers in Ultra-Light Axion Dark Matter Cosmologies" by Schwabe, Niemeyer, and Engels offers an in-depth analysis of the dynamics involved in the merger of solitonic cores predicted to form within ultra-light axion dark matter halos. The study deploys three-dimensional simulations utilizing the Schrödinger-Poisson (SP) framework, which is integral to understanding the trajectory and final state of self-gravitating scalar fields in the nonrelativistic regime.
Core Merger Dynamics
These solitonic cores are characterized as ground state configurations of boson stars, distinguished by solitonic central regions enveloped by halos with Navarro-Frenk-White (NFW)-like density profiles. The principal aim of the study is to unravel the mass loss mechanics of newly formed cores as they undergo gravitational cooling, a key process in the FDM (Fuzzy Dark Matter) scenario where the cores radiate away excess mass and energy essential for settling into relaxed states.
The simulations span a variety of initial conditions including differing mass ratios, relative phases, orbital angular momenta, and separations. Some pivotal findings include:
- Independence from Initial Conditions: The mass of the final core is largely invariant to initial phase difference and angular momentum, depending solely on the mass ratio, total initial mass, and system energy. This indicates a level of robustness in the merger dynamics, simplifying theoretical modeling of hierarchical structure formation.
- Rotating Ellipsoids: In cases of non-zero angular momentum, solitonic cores transition from spherical symmetry to rotating ellipsoid configurations, consistent with previous analyses by Rindler-Daller and Shapiro on BEC (Bose-Einstein Condensate) dark matter models.
Unbound Versus Bound Scenarios
Two regimes are considered: unbound and bound mergers. For unbound mergers where the total energy is positive, solitonic cores behave similar to solitons, superposing without significant disruption or permanent interaction. This behavior validates theoretical soliton dynamics where such wave-like configurations maintain their integrity during collisions.
In contrast, bound systems (negative energy scenarios) exhibit rapid coalescence into a new core. Notably, in cases of perfect phase opposition and equal mass merge events, repulsive interaction—termed 'bouncing'—can manifest due to destructive interference. However, this phenomenon disappears unless conditions are finely tuned, suggesting limited cosmological implications regarding dark matter halo cores unless specific criteria are met.
Multiple Core Mergers
Extending the analysis to multimerger scenarios with multiple solitonic cores, the simulations reinforce the bipartite density profile finding. Despite variations in initial conditions, such systems result in a robust solitonic core surrounded by NFW-like halos, aligning closely with prior cosmological SP simulations.
Implications and Future Research Directions
The implications for cosmology and galaxy formation modeling are manifold with consistent solitonic core behavior leading to potential simplification of hierarchical structure formation models within FDM cosmologies. Future research should focus on integrating baryonic physics into these simulations and extending the cosmological scale, overcoming current computational limitations in spatial resolution.
Overall, the work contributes to a nuanced understanding of dark matter halo structures within FDM frameworks, propelling theoretical and computational astrophysics towards more sophisticated models of early universe dynamics and galaxy evolution. It also invites deeper exploration into how such solitonic structures interact with visible baryonic matter and influence observable cosmic phenomena.
