Halo Formation in Warm Dark Matter Models

TL;DR

This paper tests whether warm dark matter (WDM) can alleviate cold dark matter's small-scale problems by running high-resolution N-body simulations that compare ΛCDM and ΛWDM with different $m_X$ values. WDM suppresses small-scale power, producing a characteristic mass scale $M_S$ and leading to a cosmic web where most low-mass halos form later via caustic fragmentation rather than hierarchical merging, while large halos form primarily through standard bottom-up processes. WDM halos exhibit lower core densities and concentrations, with substantially fewer satellites and a preferential placement of dwarfs along dense filaments, offering potential alignment with observations of dwarf galaxies and satellite statistics. The study proposes concrete observational tests, including the ionization history and Ly$\alpha$ forest at $z>6$, the galaxy formation timeline, and detailed halo density and satellite statistics, to distinguish WDM from CDM, and highlights the need for larger, higher-resolution simulations to quantify these signatures.

Abstract

Discrepancies have emerged between the predictions of standard cold dark matter (CDM) theory and observations of clustering on sub-galactic scales. Warm dark matter (WDM) is a simple modification of CDM in which the dark matter particles have initial velocities due either to their having decoupled as thermal relics, or having been formed via non-equilibrium decay. We investigate the nonlinear gravitational clustering of WDM with a high resolution N-body code, and identify a number of distinctive observational signatures. Relative to CDM, halo concentrations and core densities are lowered, core radii are increased, and large halos emerge with far fewer low mass satellites. The number of small halos is suppressed, and those present are formed by `top down' fragmentation of caustics, as part of a `cosmic web' connecting massive halos. Few small halos form outside this web. If we identify small halos with dwarf galaxies, their number, spatial distribution, and formation epoch appear in better agreement with the observations for WDM than they are for CDM.