The complete absorption shortward of the Lyman resonance wavelength in the spectra of the most distant known (redshift ~ 6) quasars (Becker et al. 2001) is direct evidence that the end of the epoch of reionization has now been observed. The full transition from isolated HII regions around bright sources, through a period of overlap of the ionized regions, to full transparency should occur over the redshift range 12 - 6 (Gnedin 2000; see also reionization cosmological simulations). Mapping out this fundamental phase change in the Universe is extremely important. The properties of galaxies we see today have been strongly influenced by the physical processes occuring in the dark-matter haloes present during the reionization epoch. These include minhaloes - the smallest bound objects, with virial temperatures below that required for hydrogen-line cooling - and protogalaxies - haloes which have sufficiently high virial temperatures that they may already have collapsed and formed stars.

Once we can find and count quasars within the epoch of reionization, several basic cosmological issues can be adressed, namely: (i) it can be determined how supermassive black holes form from low-mass seeds in minihaloes (Haiman & Loeb 2001); (ii) it can be determined whether accretion-powered objects, ie forming supermassive black holes, or star-formation-powered objects, ie forming galaxies, dominate the reionization of the Universe; (iii) direct observation of the physical properties of forming structures become possible through measurement of line-of-sight absorption as well as Ly-alpha haloes around the quasars (Furlanetto & Loeb 2002); (iv) the distorting effects of reionization on the cosmic microwave background can be established (Lewis & Bridle 2002).

Because optical surveys for quasars have now hit their natural limiting redshift, much hope in finding quasars within the reionization epoch rests on the next generation of near-IR surveys [i.e. those with the WFCAM instrument on the UKIRT, a dataset central to the SISCO project (see the UKIDSS science case)]. However, selection of z~7 quasars from near-IR surveys remains a highly uncertain process because no single example of such an object exists to act as a template.

This project aims over the next few years (ie. before the WFCAM surveys start) to find several quasars beyond the reionization epoch at z~6 and hence to establish, in an unbiased way, their optical properties. This will be achieved using radio selection techniques which already allow us to search almost all of the sky for these rare objects. The most pessimistic extrapolations suggest that there are ~100 radio-loud z>7 objects to be found, so the challenge is to develop efficient techniques to find these from surveys containing millions of objects.

At Oxford we have developed expertise with such sifting techniques (Jarvis & Rawlings 2000; Jarvis et al 2001), and in this project we will develop these methods to find the first samples of z>6 radio-loud quasars. We will measure the decline in space density of these systems at high redshift, establish their optical properties (to refine future near-IR-based surveys), and address the basic cosmological issues concerning reionization identified above.

The discovery of any RADIO-LOUD quasars within the reionziation epoch is a key goal in itself. Because the Universe become opaque at optical wavelengths, it is only through radio observations of the HI absorption line against radio-loud sources that one can hope to make detailed studies of the forming structures in the pre-reioinization Universe (Carilli et al. 2002). The project will involve modelling what is expected from such observations (in prepartion for next generation radio arrays like LOFAR and the SKA), and observational work to detect the strongest HI and molecular absorption lines, which is plausible now with telescopes like the VLA and the GBT.