Early Galaxy Formation in Warm Dark Matter Cosmologies

We present a framework for high-redshift (z≳ 7) galaxy formation that traces their dark matter (DM) and baryonic assembly in four cosmologies: cold dark matter (CDM) and warm dark matter (WDM) with particle masses of {{m}_x} = 1.5, 3, and 5 keV. We use the same astrophysical parameters regulating star formation and feedback, chosen to match current observations of the evolving ultraviolet luminosity function (UV LF). We find that the assembly of observable (with current and upcoming instruments) galaxies in CDM and {{m}_x}≥slant 3 keV WDM results in similar halo mass-to-light ratios (M/L), stellar mass densities (SMDs), and UV LFs. However, the suppression of small-scale structure leads to a notably delayed and subsequently more rapid stellar assembly in the 1.5 keV WDM model. Thus, galaxy assembly in {{m}_x}≲ 2 keV WDM cosmologies is characterized by (1) a dearth of small-mass halos hosting faint galaxies and (2) a younger, more UV-bright stellar population, for a given stellar mass. The higher M/L (effect 2) partially compensates for the dearth of small-mass halos (effect 1), making the resulting UV LFs closer to CDM than expected from simple estimates of halo abundances. We find that the redshift evolution of the SMD is a powerful probe of the nature of DM. Integrating down to a limit of {{M}_UV}=-16.5 for the James Webb Space Telescope (JWST), the SMD evolves as log (SMD) \propto -0.63(1+z) in {{m}_x}=1.5 keV WDM, as compared to log (SMD) \propto -0.44(1+z) in CDM. Thus, high-redshift stellar assembly provides a powerful test bed for WDM models, accessible with the upcoming JWST.