Infrared identification of z > 5.5 quasar candidates

One of the most challenging questions in astrophysics today is to establish when and how the universe became reionized. Different lines of evidence suggest that the reionization of the IGM took place in several stages rather than in a fast phase transition. However, the redshift range at which reionization occurred is still uncertain and the nature of the sources responsible for the process also remains an open issue.; The redshift(s) of reionization can be constrained using the spectra of the most distant quasars and galaxies and measurements of the cosmic background radiation. The results based on the discovery of Gunn-Peterson (GP) troughs in the spectra of several quasars suggest a universe still highly neutral at redshifts z∼6 along some lines of sight, with the IGM ionization state evolving rapidly. These features suggest that at this redshift regime we could be observing the end of the reionization process, a picture also supported by the results obtained from other probes. On the other hand, WMAP results and data on Lyalpha-emitting galaxies seem to indicate a universe mostly ionized at higher redshifts.; The most obvious sources of reionization are stars and AGN. Although there is a consensus that quasars and AGN could not account for all the IGM reionization, their exact contribution to reionization---as well as that of galaxies---is still a matter of debate.; To investigate the questions outlined, we need to find more high redshift sources and thus, the goal of this work is to identify, from a set of optically selected candidates, quasars at z ≥ 5.5 using z'-J colour to separate them from L and T brown dwarfs, which occupy the same locus in an R- z' vs z' colour-magnitude diagram. The results will better constrain the z ≥ 5.5 quasar luminosity function, improving our understanding of the quasar contribution to reionization.