Reionization and Galaxy Formation in Warm Dark Matter Cosmologies

We compare model results from a semi-analytic (merger-tree based) framework for high-redshift (z ' 5 − 20) galaxy formation against reionization indicators, including the Planck electron scattering optical depth (τes) and the ionizing photon emissivity ( ˙nion), to shed light on the reionization history and sources in Cold (CDM) and Warm Dark Matter (WDM; particle masses of mx = 1.5, 3 and 5 keV) cosmologies. This model includes all the key processes of star formation, supernova feedback, the merger/accretion/ejection driven evolution of gas and stellar mass and the effect of the ultra-violet background (UVB), created during reionization, in photo-evaporating the gas content of galaxies in halos with Mh <∼ 109M. We find that the delay in the start of reionization in light (1.5 keV) WDM models can be compensated by a steeper redshift evolution of the ionizing photon escape fraction and a faster mass assembly, resulting in reionization ending at comparable redshifts (z ' 5.5) in all the Dark Matter models considered. We find the bulk of the reionization photons come from galaxies with a halo mass Mh <∼ 109M and a UV magnitude −15 <∼ MUV <∼ − 10 in CDM. The progressive suppression of low-mass halos with decreasing mx leads to a shift in the “reionization” population to larger halo masses of Mh >∼ 109M and −17 <∼ MUV <∼ − 13 for 1.5 keV WDM. We find that current observations of τes and the Ultra-violet luminosity function are equally compatible with all the (cold and warm) Dark Matter models considered in this work. Quantifying the impact of the UVB on galaxy observables (luminosity functions, stellar mass densities, stellar to halo mass ratios) for different DM models we propose that global indicators including the redshift evolution of the stellar mass density and the stellar mass-halo mass relation, observable with the James Webb Space Telescope, can be used to distinguish between CDM and WDM (1.5 keV) cosmologies