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

TL;DR

This study uses CROC simulations to compare early star formation histories in cold and warm dark matter cosmologies, finding minor age differences that are within observational uncertainties and highlighting the challenges in distinguishing cosmologies based on high-redshift galaxy properties.

Contribution

It provides a detailed comparison of stellar ages in CDM and WDM models, revealing small differences and emphasizing the importance of systematic uncertainties in cosmological simulations.

Findings

Minor age differences (~5 Myr) between CDM and 3 keV WDM galaxies.

No significant difference between CDM and 6 keV WDM in stellar ages.

Simulation uncertainties are comparable to differences between cosmologies.

Abstract

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.