The First Billion Years of a Warm Dark Matter Universe

TL;DR

This study uses cosmological simulations to compare warm dark matter and cold dark matter models, revealing significant differences in early universe structure formation, star formation rates, and luminous object abundance at high redshifts.

Contribution

It provides the first detailed simulation-based comparison of WDM and CDM effects on primordial structure growth and early star formation at high redshifts.

Findings

WDM power spectra and mass functions are up to 2 dex lower than CDM.

Star formation is suppressed in WDM due to damping of small-scale power.

Fewer luminous objects form in WDM at z>10, affecting early galaxy observations.

Abstract

We present results of cosmological N-body hydrodynamic chemistry simulations of primordial structure growth and evolution in a scenario with warm dark matter (WDM) having a mass of 3 keV (thermal relic) and compare with a model consisting of standard cold dark matter (CDM). We focus on the high-redshift universe ($z>6$), where the structure formation process should better reflect the primordial (linear) differences in terms of matter power spectrum. We find that early epochs can be exceptional probes of the dark-matter nature. Non-linear WDM power spectra and mass functions are up to 2 dex lower than in CDM and show spreads of factor of a few persisting in the whole first Gyr. Runaway molecular cooling in WDM haloes results severely inhibited because of the damping of power at large $k$ modes and hence cosmic (population III and II-I) star formation rate (SFR) is usually suppressed with respect to CDM predictions. Luminous objects formed in a WDM background are very rare at $z>10$, due to the sparser and retarded evolution of early WDM mini-haloes during the dark ages and their lack can be fitted with a simple analytical formula depending only on magnitude and redshift. Future high-$z$ observations of faint galaxies have the potential to discriminate between CDM and WDM scenarios by means of cosmic stellar mass density (SMD) and specific SFR, as well. When compared to the effects of alternative cosmologies (e.g. non-Gaussian or dark-energy models) or of high-order corrections at large $z$ (e.g. primordial streaming motions or changes in the pristine IMF) the ones caused by WDM are definitely more dramatic.