The Tozzi-Iliev Diagram

The T-I Diagram (Tozzi & Iliev, private communication, January 2002) is an illustration of the astrophysical signatures in the radio sky, prior to and during The Epoch of Reionization.

The time sequence follows (1) the condensation and heating of neutral gas into low mass Dark Matter halos near the end of the Dark Age but prior to the ignition of the first stars in protogalaxies, (2) the consequences of Reheating of the IGM, prior to Reionization, and (3) the final reionization of the IGM, leading to optical transparency and the beginning of the Observable Universe -- accessible to optical wavelengths at the far right of the diagram. The vertical axis gives theoretical predictions of the magnitude of the brightness temperature fluctuations in milli-Kelvin. For the mini-halos prior to Reheating, the magnitude of the fluctations will be a strong function of size scale, and the T-I diagram indicates larger rms fluctuations on angular scales of 0.1 arcmin than for 1 to 10 arcminute scales.

There is a possibility of a very strong absorption feature developing on wide angular scales during the reheating phase, as the IGM hydrogen spin temperature is coupled to the gas kinetic temperature by resonant scattering of the Lyman-alpha flux generated by the first protogalactic stars (the Wouthuysen-Field Effect); this could give rise to a global spectral signature of absorption against the Cosmic Microwave Background, if there is widespread synchronization of protogalaxy formation. A related absorption signature may appear as rings on smaller angular scales of a few arcminutes around the first Active Galactic Nuclei. Throughout the Reheating period, the hyperfine level populations of the neutral IGM should be coupled to the heated gas kinetic temperature on progressively larger scales, prior to the wave of ionization that will eventually spread through the diffuse IGM and lead to an abrupt decline as the Universe becomes fully ionized and transparent to ionizing radiation.

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