The Epoch of Reionization

After recombination, early in the history of the universe, baryonic matter was in the form of neutral hydrogen. Imprinted on the hydrogen distribution were the primordial density fluctuations that were detected and mapped by COBE and the subsequent cosmic microwave background (CMB) experiments. The surface of last scattering delineated by the CMB at a redshift of about 1000 is the most distant structure in the universe that we observe today. The next most distant structures we see are galaxies and quasars at redshifts of 5 to 6. Sometime between a redshift of 1000 and a redshift of 6, the neutral hydrogen collapsed to form structures, the first stars and/or quasars were formed, the galaxies assembled, and the intergalactic medium was reionized. This important time in the history of our universe is commonly referred to as ``the epoch of reionization.''

Optical spectra of the the most distant quasars show Lyman-alpha absorption troughs that suggest that universe was largely neutral above a redshift of 6. However, the recent polarization results from the WMAP mission suggest that reionization occurred at a much higher redshift (about 20). Reconciling these different measurements may involve a fairly complicated reionization history. It should be possible to observe some of the processes at work during the epoch of reionization. Several observable signatures are provided by the neutral hydrogen through observations of the 1.4 GHz line, redshifted to frequencies of tens to hundreds of MHz. The first of these is a possible "reionization step" in the spectrum corresponding to the fairly abrupt transition to full reionization that occured at the end of the epoch of reionization. After the formation of the first stars and/or quasars, the process of reionization probably proceeded relatively slowly as the Stromgren spheres began to grow. However, a sudden transition to complete reionization might occur when the Stromgren spheres overlap and the mean free path of photons increase by around two orders of magnitude. Numerical simulations provide a striking demonstration of this transition.

Another observable signature is the "reheating" of the neutral hydrogen that occurs when the primordial density enhancements collapse under the influence of gravity, the gas becomes hot, and it becomes visible by emitting in the redshifted neutral hydrogen line. The figure below on the left, taken from Tozzi et al. (2000, ApJ, 528, 597), shows a simulation of how these structures would appear to a radio telescope tuned to receive signals at a frequency of 150 MHz, corresponding to a redshift of 8.5. The figure below on the right shows the reheated structures at z=8 but now in the form of a power spectrum. The superimposed light grey area are the measurements errors we expect with the Mileura Widefield Array (see below); the dark grey area are those errors combined with the cosmic variance. Both these figures assume no reionization has taken place at z=8, which is probably unlikely. However, it is useful to consider the un-reionized case as a reference.

As the neutral hydrogen continues to collapse, yet another observable signature occurs when the first stars and/or quasars form. Radiation from these bright objects ionizes the nearby neutral hydrogen, causing regions of ionized gas that appear as dark patches where the 21cm line is no longer radiating. A simulation of this process is shown below (Furlanetto, Sokasian, and Hernquist, 2004, MNRAS, 347, 187) in the nine pictures that show the same slice of the universe at redshifts 12.1, 11.1, 10.4, 9.8, 9.2, 8.7, 8.3, 7.9, and 7.6. At early times (upper left corner), the hydrogen is still neutral, and glows at radio wavelengths. At later times (lower right corner) the hydrogen is almost completely ionized and appears dark. In between there are interesting structures as the ionization fronts propagate through the hydrogen.

The neutral hydrogen features shown above should be detectable with a large radio telescope operating at low frequencies. The Kavli Institute's radio astronomy group is part of a collaboration that is building a low-frequency array at Mileura Station in Western Australia.

Last updated 7 August 2005