Quasars as Probes of Late Reionization and Early Structure Formation
  Observations of QSOs at z ~ 5.7 - 6.4 show the appearance of Gunn-Peterson troughs around z ~ 6, and a change in the slope of the IGM optical depth tau(z) near z ~ 5.5. These results are interpreted as a signature of the end of the reionization era, which probably started at considerably higher redshifts. However, there also appears to be a substantial cosmic variance in the transmission of the IGM, both along some lines of sight, and among different lines of sight, in this intriguing redshift regime. We suggest that this is indicative of a spatially uneven reionization, possibly caused by the bias-driven primordial clustering of the reionization sources. There is also some independent evidence for a strong clustering of QSOs at z ~ 4 - 5 and galaxies around them, supporting the idea of the strong biasing of the first luminous sources at these redshifts. Larger samples of high-z QSOs are needed in order to provide improved, statistically significant constraints for the models of these phenomena. We expect that the Palomar-Quest (PQ) survey will soon provide a new set of QSOs to be used as cosmological probes in this redshift regime. 
S.G. Djorgovski, M. Bogosavljevic, A. Mahabal (Caltech)