Prospects for detecting the 21cm forest from the diffuse intergalactic
  medium with LOFAR
  We discuss the feasibility of the detection of the 21cm forest in the diffuse
IGM with the radio telescope LOFAR. The optical depth to the 21cm line has been
derived using simulations of reionization which include detailed radiative
transfer of ionizing photons. We find that the spectra from reionization models
with similar total comoving hydrogen ionizing emissivity but different
frequency distribution look remarkably similar. Thus, unless the reionization
histories are very different from each other (e.g. a predominance of UV vs.
x-ray heating) we do not expect to distinguish them by means of observations of
the 21cm forest. Because the presence of a strong x-ray background would make
the detection of 21cm line absorption impossible, the lack of absorption could
be used as a probe of the presence/intensity of the x-ray background and the
thermal history of the universe. Along a random line of sight LOFAR could
detect a global suppression of the spectrum from z>12, when the IGM is still
mostly neutral and cold, in contrast with the more well-defined, albeit broad,
absorption features visible at lower redshift. Sharp, strong absorption
features associated with rare, high density pockets of gas could be detected
also at z~7 along preferential lines of sight.
