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We use $N$-body simulations to show that high-redshift galaxy counts providean interesting constraint on the nature of dark matter, specifically Warm DarkMatter (WDM), owing to the lack of early structure formation these models. Oursimulations include three WDM models with thermal-production masses of 0.8 keV,1.3 keV, and 2.6 keV, as well as CDM. Assuming a relationship between dark halomass and galaxy luminosity that is set by the observed luminosity function atbright magnitudes, we find that 0.8 keV WDM is disfavored by direct galaxycounts in the Hubble Ultra Deep Field at $>\!\!10\sigma$. Similarly, 1.3 keVWDM is statistically inconsistent at $2.2\sigma$. Future observations with JWST(and possibly HST via the Frontier Fields) could rule out $1.3$ keV WDM at highsignificance, and may be sensitive to WDM masses greater than 2.6 keV. We alsoexamine the ability of galaxies in these WDM models to reionize the universe,and find that 0.8 keV and 1.3 keV WDM produce optical depths to the CosmicMicrowave Background (CMB) that are inconsistent at 68% C.L. with currentPlanck results, even with extremely high ionizing radiation escape fractions,and 2.6 keV WDM requires an optimistic escape fraction to yield an opticaldepth consistent with Planck data. Although CMB optical depth calculations aremodel dependent, we find a strong challenge for stellar processes alone toreionize the universe in a 0.8 keV and 1.3 keV WDM cosmology.

The High-$z$ Universe Confronts Warm Dark Matter: Galaxy Counts, Reionization and the Nature of Dark Matter