Cosmological simulations of mixed ultralight dark matter

TL;DR

This paper uses cosmological simulations to explore how mixed ultralight and cold dark matter influence structure formation, revealing the stability of density cores and the impact on halo abundance across various mass fractions and particle masses.

Contribution

It introduces a simulation framework combining Schrödinger-Poisson equations with CDM dynamics to study mixed dark matter models in cosmology.

Findings

Stable density cores form when FDM is 50% or less of dark matter.

No stable cores are found when FDM is 10% or less.

The halo abundance matches predictions from the adapted halo model.

Abstract

The era of precision cosmology allows us to test the composition of the dark matter. Mixed ultralight or fuzzy dark matter (FDM) is a cosmological model with dark matter composed of a combination of particles of mass $m\leq 10^{-20}\;\mathrm{eV}$, with an astrophysical de Broglie wavelength, and particles with a negligible wavelength sharing the properties of cold dark matter (CDM). In this work, we simulate cosmological volumes with a dark matter wave function for the ultralight component coupled gravitationally to CDM particles. We investigate the impact of a mixture of CDM and FDM in various proportions $(0\%,\;1\%,\;10\%,\;50\%,\;100\%)$ and for ultralight particle masses ranging over five orders of magnitude $(2.5\times 10^{-25}\;\mathrm{eV}-2.5\times 10^{-21}\;\mathrm{eV})$. To track the evolution of density perturbations in the non-linear regime, we adapt the simulation code AxioNyx to solve the CDM dynamics coupled to a FDM wave function obeying the Schr\"odinger-Poisson equations. We obtain the non-linear power spectrum and study the impact of the wave effects on the growth of structure on different scales. We confirm that the steady-state solution of the Schr\"odinger-Poisson system holds at the center of halos in the presence of a CDM component when it composes $50\%$ or less of the dark matter but find no stable density core when the FDM accounts for $10\%$ or less of the dark matter. We implement a modified friends-of-friends halo finder and find good agreement between the observed halo abundance and the predictions from the adapted halo model axionHMCode.