The Effect of Warm Dark Matter on Early Star Formation Histories of
  Massive Galaxies: Predictions from the CROC Simulations
  Several 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 suppressed small-scale power (and, hence,
late formation of cosmic structure). We explore the ages of the 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
and 6 keV particles. There are statistically significant differences of $\sim
5\;{\rm Myr}$ between average stellar ages of massive galaxies in CDM and 3 keV
WDM, while CDM and 6 keV WDM are statistically indistinguishable. Even this 5
Myr difference, however, is much less than current observational uncertainties
on the stellar population properties of high-redshift galaxies. The age
distributions of all galaxies in all cosmologies fail to produce a substantial
Balmer break, although uncertainties in dust attenuation are a potentially
significant factor. Finally, we assess the convergence of our simulation
predictions and find that the systematic uncertainties on individual galaxy
properties are comparable to the differences between cosmologies, suggesting
these differences may not be numerically robust.
