The Copernicus Complexio: Statistical properties of warm dark matter haloes

The recent detection of a 3.5 keV X-ray line from the centres of galaxies and clusters by Bulbul et al. and Boyarsky et al. has been interpreted as emission from the decay of 7 keV sterile neutrinos which could make up the (warm) dark matter (WDM). As part of the Copernicus Complexio (coco) programme, we investigate the properties of dark matter haloes formed in a high-resolution cosmological N-body simulation from initial conditions similar to those expected in a universe in which the dark matter consists of 7 keV sterile neutrinos. This simulation and its cold dark matter (CDM) counterpart have ∼13.4 bn particles, each of mass ∼10⁵ h⁻¹ M_⊙, providing detailed information about halo structure and evolution down to dwarf galaxy mass scales. Non-linear structure formation on small scales (M₂₀₀ ≲ 2 × 10⁹ h⁻¹ M_⊙) begins slightly later in coco-warm than in coco-cold. The halo mass function at the present day in the WDM model begins to drop below its CDM counterpart at a mass ∼2 × 10⁹ h⁻¹ M_⊙ and declines very rapidly towards lower masses so that there are five times fewer haloes of mass M₂₀₀ = 10⁸ h⁻¹ M_⊙ in coco-warm than in coco-cold. Halo concentrations on dwarf galaxy scales are correspondingly smaller in coco-warm, and we provide a simple functional form that describes its evolution with redshift. The shapes of haloes are similar in the two cases, but the smallest haloes in coco-warm rotate slightly more slowly than their CDM counterparts.