Redshifted 21 centimeter emission from the pre-reionization era. 1. Mean signal and linear fluctuations

We use cosmological simulations of reionization to predict the possible signal from the redshifted 21 cm line of neutral hydrogen in the pre-reionization era and to investigate the observability of this signal. We show that the separation of the mean (global) signal over the whole sky from the known foreground contamination may be feasible but very challenging. In agreement with previous studies, we demonstrate that measuring angular fluctuations in the H 1 signal is likely to be extremely difficult if not impossible because of the overwhelming contamination from the Galactic and extragalactic foregrounds. However, we show that the sharp H 1 fluctuations in the frequency domain should be easily separable from the relatively smooth spectra of the foregrounds and that these fluctuations should be detectable even at moderate angular resolution (similar to10'-20').