Effects of neutrino masses and asymmetries on dark matter halo assembly

Abstract

Massive cosmological neutrinos suppress the Large-Scale Structure (LSS) in the Universe by smoothing the cosmic over-densities, and hence structure formation is delayed relative to that in the standard Lambda-Cold Dark Matter (ΛCDM) model.We characterize the merger and mass accretion history of dark matter halos with the halo formation time a 1/2, tree entropy s and halo leaf function ℓ(X) and measure them using neutrino-involved N-body simulations. We show that a non-zero sum of neutrino masses Mν delays the a 1/2 for halos with virial mass between 1013 M ⊙ and 3 × 1013 M ⊙, whereas a non-zero neutrino asymmetry parameter η 2 has the opposite effect. While the mean tree entropy s̅ does not depend significantly on either Mν or η 2, the halo leaf function does. Furthermore, the dependencies of ℓ on Mν and η 2 have significant evolution in redshift z, with the relative contributions of Mν and η 2 showing a sigmoid-like transition as a function of z around z ≈ 0.6. Together with the matter power spectrum, these halo parameters allow us to break the parameter degeneracy between Mν and η 2 so that they can both be constrained in principle.