21 Centimeter Tomography of the Intergalactic Medium at High Redshift

We investigate the 21 cm signature that may arise from the intergalactic medium (IGM) prior to the epoch of full reionization (z > 5). In scenarios in which the IGM is reionized by discrete sources of photoionizing radiation, the neutral gas that has not yet been engulfed by an H II region may easily be preheated to temperatures well above that of the cosmic background radiation (CBR), rendering the IGM invisible in absorption against the CBR. We identify three possible preheating mechanisms: (1) photoelectric heating by soft X-rays from QSOs, (2) photoelectric heating by soft X-rays from early galactic halos, and (3) resonant scattering of the continuum UV radiation from an early generation of stars. We find that bright quasars with only a small fraction of the observed comoving density at z ~ 4 will suffice to preheat the entire universe at z >~ 6. We also show that, in a cold dark matter dominated cosmology, the thermal bremsstrahlung radiation associated with collapsing galactic mass halos (10¹⁰-10¹¹ M_⊙) may warm the IGM to ~100 K by z ~ 7. Alternatively, the equivalent of ~10% of the star formation rate density in the local universe, whether in isolated pregalactic stars, dwarf, or normal galaxies, would be capable of heating the entire IGM to a temperature above that of the CBR by Lyα scattering in a small fraction of the Hubble time at z ~ 6.

In the presence of a sufficiently strong ambient flux of Lyα photons, the hyperfine transition in the warmed H I will be excited. A beam differencing experiment would detect a patchwork of emission, both in frequency and in angle across the sky. This patchwork could serve as a valuable tool for understanding the epoch, nature, and sources of the reionization of the universe, and their implications for cosmology. We demonstrate that isolated QSOs will produce detectable signals at meter wavelengths within their ``spheres of influence'' over which they warm the IGM. As a result of the redshifted 21 cm radiation emitted by warm H I bubbles, the spectrum of the radio extragalactic background will display frequency structure with velocity widths up to 10,000 km s<sup>-1</sup>. Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with δT/T <~ 10<sup>-3</sup> on scales θ ~ 1°. This scale is set either by the ``thermalization distance'' from a QSO within which Lyα pumping determines the spin temperature of the IGM or by the quasar lifetime.

Radio measurements near 235 and 150 MHz, as will be possible in the near future using the Giant Metrewave Radio Telescope, may provide the first detection of a neutral IGM at 5 <~ z <~ 10. A next generation facility like the Square Kilometer Array Interferometer could effectively open much of the universe to a direct study of the reheating epoch and possibly probe the transition from a neutral universe to one that is fully ionized.