Fluctuations in 21-cm emission after reionization

The fluctuations in the emission of redshifted 21-cm photons from neutral intergalactic hydrogen will provide an unprecedented probe of the reionization era. Conventional wisdom assumes that this 21-cm signal disappears as soon as reionization is complete, when little atomic hydrogen is left through most of the volume of the intergalactic medium (IGM). However, observations of damped Ly alpha absorbers indicate that the fraction of hydrogen in its neutral form is significant by mass at all redshifts. Here we use a physically motivated model to show that residual neutral gas, confined to dense regions in the IGM with a high recombination rate, will generate a significant post-reionization 21-cm signal. We show that the power spectrum of fluctuations in this signal will be detectable by the first generation of low-frequency observatories at a signal-to-noise ratio that is comparable to that achievable in observations of the reionization era. The statistics of 21-cm fluctuations will therefore probe not only the pre-reionization IGM, but rather the entire process of H II region overlap, as well as the appearance of the diffuse ionized IGM.