2019MNRAS.486.3617L

We use cosmological hydrodynamic zoom-in simulations to study early structure formation in two dark matter (DM) cosmologies, the standard cold dark matter (CDM) model, and a thermal warm DM (WDM) model with a particle mass of m_χc² = 3 keV. We focus on DM haloes with virial masses M ∼ 10¹⁰ M_☉. We find that the first star formation activity is delayed by ∼200 Myr in the WDM model, with similar delays for metal enrichment and the formation of the second generation of stars. However, the differences between the two models in globally averaged properties, such as star formation rate density and mean metallicity, decrease towards lower redshifts (z <= 10). Metal enrichment in the WDM cosmology is restricted to dense environments, while low-density gas can also be significantly enriched in the CDM case. The free-free contribution from early structure formation at redshifts z > 6 to the cosmic radio background (CRB) is 3_–1.5⁺¹³ ( 8_–3.5⁺³³) per cent of the total signal inferred from radio experiments such as ARCADE 2, in the WDM (CDM) model. The direct detection of the H₂ emission from early structure formation (z >= 7.2), originating from the low-mass haloes explored here, will be challenging even with the next generation of far-infrared space telescopes, unless the signal is magnified by at least a factor of 10 via gravitational lensing or shocks. However, more massive haloes with M >= 10¹² M_☉ may be observable for z >= 10, even without magnification, provided that our extrapolation from the scale of our simulated haloes is valid.