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.
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