ABSTRACT

After recombining at z = 1000, the intergalactic medium (IGM) remains largely neutral, and therefore opaque at wavelengths corresponding to rest frame Ly alpha, until the formation of the first luminous objects. The UV radiation from these first objects re-ionizes the IGM, leading to the transparent IGM as seen today. Current observations (principally the 'Gunn-Peterson' trough in a z = 6.3 QSO and the strength of the first peak in the CMB spatial spectrum), place the epoch of reionization in the range of z = 6.5 to 20. Simple physical arguments, supported by detailed numerical calculations, indicate that reionization is likely to occur in two phases: (i) a slow phase during which each ionizing source is contained within its own Stromgren sphere, and (ii) a fast phase when these spheres overlap. The fast phase manifests itself as a 'cosmic phase transition', when the mean neutral fraction, f, of the IGM goes from f>=1% to f=1e-5 in a short redshift interval. At f>=1% the IGM is opaque to the Ly alpha line, implying that there will be a substantial redshift range, from the formation epoch of the first luminous objects to the epoch of fast reionization, which will be largely opaque at optical wavelengths. Hence the study of the first luminous objects in the universe, and the neutral IGM, will be the responsibility of long wavelength astronomy.

We will discuss methods for studying the neutral IGM during and prior to the epoch of fast reionization using the HI 21cm line. Using the latest numerical models for cosmic reionization, we will show that future large area radio telescopes will reveal the detailed evolution of the neutral IGM in a number of ways: (i) large scale structure through observations of HI 21cm emission, and (ii) intermediate scale structure (the 'HI 21cm Forest') and small scale structure ('minihalos') through absorption observations. These data will also constrain the nature of the objects responsible for reionization: very massive stars, star forming galaxies, or accreting massive black holes.