Early Structure Formation and Reionization in a Warm Dark Matter Cosmology

We study first structure formation in Lambda-dominated universes using largecosmological N-body/SPH simulations. We consider a standard LCDM model and aLWDM model in which the mass of the dark matter particles is taken to be m_X=10keV. The linear power spectrum for the LWDM model has a characteristic cut-offat a wavenumber k=200 /Mpc, suppressing the formation of low mass (< 10^6 Msun)nonlinear objects early on. The absence of low mass halos in the WDM modelmakes the formation of primordial gas clouds with molecular hydrogen veryinefficient at high redshifts. The first star-forming gas clouds form at z~21in the WDM model, considerably later than in the CDM counterpart, and theabundance of these gas clouds differs by an order of magnitude between the twomodels. We carry out radiative transfer calculations by embedding massivePopulation III stars in the gas clouds. We show that the volume fraction ofionized gas rises up close to 100% by z=18 in the CDM case, whereas that of theWDM model remains extremely small at a level of a few percent. Thus the WDMmodel with m_X=10 keV is strongly inconsistent with the observed high opticaldepth by the WMAP satellite.Comment: Minor revision and citation updated. Version accepted by ApJ