Soliton Formation and the Core-Halo Mass Relation: An Eigenstate Perspective

TL;DR

This paper investigates the formation of solitonic cores in ultra-light dark matter halos through simulations of mergers, revealing that the core-halo mass relation is not universal and depends on merger history.

Contribution

It introduces an eigenstate-based method to isolate the solitonic core and compares it with fitting functions, providing new insights into the core-halo mass relation in ULDM halos.

Findings

Eigenstate approach yields core mass estimates up to 30% different from fitting methods.

Core-halo mass relation varies with merger history, with slopes around 1/3 to 0.4.

No universal core-halo mass relation exists for ULDM halos.

Abstract

UltraLight Dark Matter (ULDM) is an axion-like dark matter candidate with an extremely small particle mass. ULDM halos consist of a spherically symmetric solitonic core and an NFW-like skirt. We simulate halo creation via soliton mergers and use these results to explore the core-halo mass relation. We calculate the eigenstates of the merged halos and use these to isolate the solitonic core and calculate its relative contribution to the halo mass. We compare this approach to using a fitting function to isolate the core and find a difference in masses up to 30%. We analyze three families of simulations: equal-mass mergers, unequal-mass mergers, and halos with a two-step merger history. Setting the halo mass to the initial mass in the simulation does not yield a consistent core-halo relationship. Excluding material "ejected" by the collision yields a core-halo relationship with a slope of 1/3 for simultaneous mergers and roughly 0.4 for two-step mergers. Our findings suggest there is no universal core-halo mass relationship for ULDM and shed light on the differing results for the core-halo relationship previously reported in the literature.