In the current cosmological framework, the diffuse gas (IGM), initially in a highly ionization state, is expected to recombine, i.e. neutral atoms are formed, ~450 thousand years after the Big Bang (the estimated age of the Universe is ~13 billion years) and remain neutral until the first sources of ionizing radiation form and reionize it. Observations of distant quasars (e.g. Fan et al. 2004) provide information on the final stages of the reionization process, while experiments on the cosmic microwave background (CMB) radiation (e.g. Kogut et al. 2003; Spergel et al. 2003) give an estimate of the abundance of electrons produced by it. But observations that map the temporal evolution of reionization are still not available.
It has long been known (e.g. Field 1959) that neutral hydrogen in the IGM may be directly detectable at frequencies that fall in the radio band (in the range 70-170 MHz) and measurements at different frequencies should allow us to probe accurately the structure and the evolution of the reionizing gas. This experiment is especially attractive as a new generation of radio telescope (e.g. LOFAR, PAST, SKA) is under construction. LOFAR, which is being built in the Netherlands, will use almost 40000 antennas grouped into roughly 100 "stations", distributed over an area of about 400 kilometers across. The possible configuration for station layout is shown in Fig. 1.
Scientists at the Max-Planck-Institute for Astrophysics have used previously run simulations of the reionization process (see May 2003 Research Highlight; Ciardi, Stoehr and White 2003; Ciardi, Ferrara and White 2003) to derive the 21cm line emission expected from neutral IGM at radio frequencies (Ciardi and Madau 2003). In Fig. 2 maps of the emission at different observed frequencies are shown. As expected, at longer frequencies, which correspond to later times when the IGM is more ionized, the emission is lower. Inhomogeneities in the gas density and in its ionization state induce fluctuations in the 21cm line emission, with a maximum expected value of order of 10 mK. The next generation of low-frequency radio telescopes should be sensitive enough to measure such fluctuations and to probe the structure of the reionization process directly.
Nevertheless, observations of 21cm line emission from neutral gas in the IGM remains a very challenging project due to contamination from sources which emit at radio frequencies, such as our own Galaxy (e.g. Shaver et al. 1999), radio galaxies (e.g. Di Matteo et al. 2002) or clusters of galaxies. Researchers at the Max-Planck-Institute for Astrophysics have estimated the contribution to such contamination from all possible sources of extra-galactic origin (Di Matteo, Ciardi and Miniati 2004). They find that the emission from the extra-galactic sources is stronger that the primary 21cm line emission, unless bright sources are removed from the observed maps. In this case, on angular scales larger than 1 arcmin, the primary signal will be observable and free from contamination.
In conclusion, the next generation of low-frequency radio telescopes will be able to map, for the first time, the temporal evolution of the reionization process and to shed light on the nature of the sources that caused it.
By B. Ciardi.
Posted By: Brooke