Global radiation signature from early structure formation

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(chi)c(2) = 3 keV. We focus on DM haloes with virial masses M similar to 1010M(circle dot). We find that the first star formation activity is delayed by similar to 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 less than or similar to 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 theWDM (CDM) model. The direct detection of the H-2 emission from early structure formation (z greater than or similar to 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 greater than or similar to 10(12) M-circle dot may be observable for z greater than or similar to 10, even without magnification, provided that our extrapolation from the scale of our simulated haloes is valid.