Halo abundance and clustering in cosmologies with massive and asymmetric neutrinos

TL;DR

This study explores how massive and asymmetric neutrinos influence large-scale structure formation, revealing that neutrino mass suppresses halo abundance while asymmetry enhances it, with effects varying across redshifts.

Contribution

It is the first comprehensive analysis of how neutrino asymmetry, alongside mass, affects halo formation and clustering using cosmological simulations.

Findings

Neutrino mass suppresses the abundance of massive halos.

Neutrino asymmetry enhances the halo mass function across a broad mass range.

Effects become more pronounced at higher redshifts, with up to 75% enhancement due to asymmetry at z=9.

Abstract

Neutrinos are the most abundant fermions in the Universe and influence the formation of large-scale structure through both their non-zero masses and a possible chemical potential which can be described by a single asymmetry parameter. While most previous studies have focused on the impact of the neutrino mass, the role of neutrino asymmetry remains comparatively unexplored. In this work, we investigate how massive neutrinos ($M_{\nu}=0-0.24\,\mathrm{eV}$) with a non-zero asymmetry parameter ($\eta^{2}=0-0.8$) modify the halo mass function (HMF) and halo bias using cosmological N-body simulations with cosmological parameters consistently refitted to CMB observations. We find that at all redshifts, neutrino mass suppresses the abundance of massive halos, whereas neutrino asymmetry enhances the HMF over a broad mass range. At z=0, the abundance of the most massive halos is reduced by up to ~30% in the largest-mass case ($M_{\nu}=0.24\,\mathrm{eV}$), while neutrino asymmetry ($\eta^{2}=0.8$) produces a maximum ~5% enhancement. These effects become increasingly pronounced at higher redshifts: by z=4 and z=9, the enhancement induced by neutrino asymmetry reaches ~25% and ~75%, respectively, while the corresponding suppression due to neutrino mass deepens to below ~40% and ~70% of the massless case. For halo bias, we find that halos with masses above $10^{13.4}\,\mathrm{M_\odot}$ exhibit an enhanced large-scale bias due to neutrino mass, reaching up to ~5% at z=0, while neutrino asymmetry reduces the bias by a few percent on linear scales. These trends strengthen with redshift, with the enhancement and suppression growing to ~15% and ~10% at z=2, respectively. Linear bias models provide an adequate, though not exact, description of halo bias in massive-neutrino cosmologies. Our results demonstrate that halo abundance and clustering offer sensitive probes of both neutrino mass and asymmetry.