HI Signatures of Reionization

The exploration of the end of the Dark Ages will be one of the most exciting field of the next decade. While most of the proposed observations must await the next–generation telescopes, the observational window of the redshifted 21cm line offers the possibility to investigate the physics of reheating and reionization on a short term. Here we describe several possible signatures detectable in the wavelength range 100-200 MHz. Among the physics that can be investigated: the epoch of reheating and reionization; topology and timescales of reheating; the nature of the ionizing sources; the baryon distribution at redshift z ∼ 10. Such a good deal of information is within reach of present–day, or near–future radio facilities. 1. Exploring the Dark Ages The Dark Ages are ended by the appearance of the first stars and/or quasars, that reheat the diffuse cosmic baryons and then reionize the Universe (see Rees 1999). The reionization is defined as the epoch when the volume–filling factor of HII regions is ≃ 1, and the Universe becomes transparent to ionizing radiation (for an extensive review see Loeb & Barkana 2001). The absence of a Gunn– Peterson (GP) effect in the line of sight of distant quasars, put the epoch of reionization (zreion) at redshifts > 5. To date, there are little additional constraints on the physics of the cosmic baryons at such high redshifts. This is also due to the complexities involved by any theoretical model that must include the nature and the birthrate of the first luminous objects, their spectrum and emissivity, their feedback into the surrounding Intergalactic Medium (IGM), etc. A significant step forward has been recently obtained with Keck spectroscopy of the most distant SLOAN quasars (Becker et al. 2001; Djorgovski et al. 2001). The presence of a sudden increase in the Lyα opacity between redshift 5 and 6 may indicate that the Universe is approaching reionization at z ∼ 6. However, Barkana (2001) pointed out that, due to the high Lyα opacity, these observations are also consistent with a post–reionization phase, and that a conclusive proof requires the observation of similar GP absorptions along several additional line of sights. Alternatively, the observation of smoking–gun signatures has been proposed. Among them: dectection of scattered Lyα emission around sources beyond zreion (Loeb & Rybicki 1999); features in galaxy number counts at z > zreion (Barkana & Loeb 2000). These observations would be effective probes of the physics of cosmic baryons and of the nature of the first ionizing sources at the same time, but they must wait for NGST and/or SIRTF,