Early structure formation and reionization in a warm dark matter cosmology

We study the first structure formations in Lambda-dominated universes using large cosmological N-body/smoothed particle hydrodynamics simulations. We consider a standard Lambda cold dark matter (CDM) model and a Lambda warm dark matter (WDM) model in which the mass of the dark matter particles is taken to be m(X) = 10 keV. The linear power spectrum for the LambdaWDM model has a characteristic cutoff at a wavenumber k = 200 Mpc(-1), suppressing the formation of low- mass (<10(6) M-circle dot) nonlinear objects early on. The absence of low- mass halos in the WDM model makes the formation of primordial gas clouds with molecular hydrogen very inefficient at high redshifts. The first star-forming gas clouds form at z approximate to 21 in the WDM model, considerably later than in the CDM counterpart, and the abundance of these gas clouds differs by an order of magnitude between the two models. We carry out radiative transfer calculations by embedding massive Population III stars in the gas clouds. We show that the volume fraction of ionized gas rises up close to 100% by z = 18 in the CDM case, whereas that of the WDM model remains extremely small at a level of a few percent. Thus, the WDM model with m(X) = 10 keV is strongly inconsistent with the high optical depth observed by the Wilkinson Microwave Anisotropy Probe satellite.