Clues to the nature of dark matter from first galaxies

Warm dark matter (WDM) remains a viable alternative to the prevailing paradigm of cold dark matter (CDM). Given that the differences between WDM and CDM increase at higher redshift, the high-redshift Universe is an ideal laboratory in which to test these theories. Using many high-resolution simulations of galaxy formation (related to the NIHAO suite of simulations), we study the impact of a 3 keV WDM candidate on not only the properties of the high-redshift galaxies, but also the cumulative effect via reionization. We find that three different effects contribute to differentiate warm and cold dark matter (CDM) predictions: WDM suppresses the number of haloes with mass less than few 10^9 Msol; at a fixed halo mass, WDM produces fewer stars than CDM; and finally at halo masses below 10^9 Msol, WDM has a larger fraction of dark haloes than CDM post-reionization. These three effects combine to produce a lower stellar mass function in WDM for galaxies with stellar masses at and below 10^7 Msol. For z > 7, the global star formation density is lower by a factor of two in the WDM scenario, and for a fixed escape fraction, the fraction of neutral hydrogen is higher by 0.3 at z ~ 6. Overall, we conclude that galaxy formation simulations at high redshift are a key tool to differentiate between dark matter candidates given a model for baryonic physics.