The Impact of Peculiar Velocity and Inhomogeneous Reionization on 21cm Cosmology from the Epoch of Reionization.

21cm tomography has emerged as a promising cosmological probe. The assumption that cosmological information in the 21cm signal can be separated from astrophysical information is based on linear perturbation theory and the anisotropy introduced by peculiar velocity. While it is true that fluctuations in the matter density at such high redshift are likely to be of linear amplitude on the large scales which correspond to the beam- and bandwidths of upcoming experiments, inhomogeneous reionization, coupled to peculiar velocity, and the nonlinearity of smaller scale structure in both density and velocity can leave its imprint on the signal, which might then spoil the linear separation scheme. Using large-scale reionization simulations in a comoving box size 114 Mpc/h, we predict the 21cm brightness temperature fluctuations in redshift space, focusing on the 50% ionized epoch. We find that while the linear separation scheme predicts the angle-averaged power spectrum at the 10% level, the correlation of peculiar velocity with reionization patchiness is significant enough that a cosmic-variance-limited measurement of the 3D power spectrum by a survey limited to the simulation size cannot determine the matter power spectrum to better than a factor of ~ 2. Larger survey volumes will do better but require reionization modelling to interpret systematic departures from the linear separation scheme