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We discuss the feasibility of the detection of the 21cm forest in the diffuseIGM with the radio telescope LOFAR. The optical depth to the 21cm line has beenderived using simulations of reionization which include detailed radiativetransfer of ionizing photons. We find that the spectra from reionization modelswith similar total comoving hydrogen ionizing emissivity but differentfrequency distribution look remarkably similar. Thus, unless the reionizationhistories 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 ofthe 21cm forest. Because the presence of a strong x-ray background would makethe detection of 21cm line absorption impossible, the lack of absorption couldbe used as a probe of the presence/intensity of the x-ray background and thethermal history of the universe. Along a random line of sight LOFAR coulddetect a global suppression of the spectrum from z>12, when the IGM is stillmostly neutral and cold, in contrast with the more well-defined, albeit broad,absorption features visible at lower redshift. Sharp, strong absorptionfeatures associated with rare, high density pockets of gas could be detectedalso at z~7 along preferential lines of sight.

Prospects for detecting the 21cm forest from the diffuse intergalactic medium with LOFAR