Solitons in the dark: non-linear structure formation with fuzzy dark matter

Abstract

We present the results of a full cosmological simulation with the new code SCALAR, where dark matter is in form of fuzzy dark matter, described by a light scalar field with a mass of $m_{\rm B} = 2.5 \times 10^{-22}$ eV and evolving according to the Schr\"{o}dinger-Poisson system of equations. In comoving units, the simulation volume is $2.5 ~ h^{-1} {\rm Mpc}$ on a side, with a resolution of $20~h^{-1}{\rm pc}$ at the finest refinement level. We analyse the formation and the evolution of central solitonic cores, which are found to leave their imprints on dark matter density profiles, resulting in shallower central densities, and on rotation curves, producing an additional circular velocity peak at small radii from the center. We find that the suppression of structures due to the quantum nature of the scalar field results in an shallower halo mass function in the low-mass end compared to the case of a $\Lambda$CDM simulation, in which dark matter is expected to cluster at all mass scales even if evolved with the same initial conditions used for fuzzy dark matter. Furthermore, we verify the scaling relations characterising the solution to the Schr\"{o}dinger-Poisson system, for both isolated and merging halos, and we find that they are preserved by merging processes. We characterise each fuzzy dark matter halo in terms of the dimensionless quantity $\Xi \propto \left | E_{\rm halo} \right |/M_{\rm halo}^3$ and we show that the core mass is tightly linked to the halo mass by the core-halo mass relation $M_{\rm core}/M_{\rm halo} \propto \Xi^{1/3}$. We also show that the core surface density of the simulated fuzzy dark matter halos does not follow the scaling with the core radius as observed for dwarf galaxies, representing a big challenge for the fuzzy dark matter model as the sole explanation of core formation.