The Effect of Warm Dark Matter on Early Star Formation Histories of Massive Galaxies: Predictions from the CROC Simulations

A few massive ($M_{*} > 10^8 M_{\odot}$), high-redshift ($z = 8-10$) galaxies have recently been discovered to contain stars with ages of several hundred million years, pushing the onset of star formation in these galaxies back to $z\sim15$. The very existence of stars formed so early may serve as a test for cosmological models with little small scale power (and, hence, late formation of cosmic structure). We explore the ages of oldest stars in numerical simulations from the Cosmic Reionization On Computers (CROC) project with Cold Dark Matter (CDM) and two Warm Dark Matter (WDM) cosmologies with 3 keV and 6 keV particles. There are statistically significant differences between average stellar ages of massive galaxies in CDM and 3 keV WDM. However, these differences are much smaller than both the quoted uncertainties in observational data on the ages of galaxies at these redshifts, and the systematic uncertainties in simulation predictions of these ages as assessed by a convergence test. Further theoretical progress will be needed to refine simulation predictions to an accuracy that would enable dark matter particle physics constraints from this probe.