Effects of Neutrino Masses and Asymmetries on Dark Matter Halo Assembly

TL;DR

This study investigates how neutrino masses and asymmetries influence dark matter halo formation and assembly history, revealing their distinct effects and potential for constraining neutrino properties.

Contribution

It introduces new measurements of halo formation and structure parameters using neutrino-involved simulations, highlighting their redshift evolution and degeneracy-breaking potential.

Findings

Non-zero neutrino mass delays halo formation time.

Neutrino asymmetry accelerates halo formation.

Halo leaf function varies significantly with neutrino parameters.

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 ($\Lambda$CDM) 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 $\ell(X)$ and measure them using neutrino-involved N-body simulations. We show that a non-zero sum of neutrino masses $M_\nu$ delays the $a_{1/2}$ for halos with virial mass between $10^{13} M_\odot$ and $3\times 10^{13} M_\odot$, whereas a non-zero neutrino asymmetry parameter $\eta^2$ has the opposite effect. While the mean tree entropy $\bar s$ does not depend significantly on either $M_\nu$ or $\eta^2$, the halo leaf function does. Furthermore, the dependencies of $\ell$ on $M_\nu$ and $\eta^2$ have significant evolution in redshift $z$, with the relative contributions of $M_\nu$ and $\eta^2$ showing a sigmoid-like transition as a function of $z$ around $z \approx 0.6$. Together with the matter power spectrum, these halo parameters allow us to break the parameter degeneracy between $M_\nu$ and $\eta^2$ so that they can both be constrained in principle.