Prepared by S. Cristiani
The study of quasars at the highest observable redshifts has a wide ranging relevance to AGN astrophysics, galaxy formation, and the growth of large scale structure. The mere existence of quasars at high redshift and the properties of the their host galaxies and local environment is a constraint on theories for the formation of galactic scale objects (e.g. Efstathiou and Rees 1988, Haehnelt and Rees 1993).
Over the past years, much observational effort has gone into the construction of complete samples of quasars in order to determine the luminosity function and its evolution with redshift. The status of the major quasar surveys and selection techniques have been reviewed by Hartwick & Schade (1990) and in two conference proceedings (Osmer et al. 1988; Crampton 1991). At present the quasar population is believed to undergo strong luminosity evolution up to redshifts of , and this evolution slows down between redshifts 2 and 3. The only large complete samples of QSOs with redshifts greater than 3 have been produced from optical surveys (Warren, Hewett & Osmer 1991; Irwin, McMahon & Hazard 1991; Schneider, Schmidt & Gunn 1994). The form of the evolution at these redshifts is less clear than at lower redshifts, and it appears that a combination of luminosity evolution and density evolution is taking place. The space density of the less optically luminous quasars ( ) appears to decline rapidly at z>3 (Schmidt, Schneider & Gunn 1991, Warren, Hewett & Osmer 1994), whereas no evidence for a decline was found for the higher luminosity optically selected quasars (MB<-28) (Hazard, McMahon & Sargent 1986; Miller, Mitchell & Boyle 1990; Irwin et al. 1991). Evidence for a decline in space density at high redshifts suggests that the epoch of galaxy formation may have been found (with the caveat that quasars probably represent the extreme of the matter distribution and are not necessarily tracers of the galaxy population). There is, however, much controversy over these results owing to the complex, redshift-dependent selection effects which affect optical surveys for high-redshift quasars.
The limitation preventing from the detection of quasars and the estimate of their luminosity function is essentially a technical one: at a typical QSO has a magnitude fainter than I=23 and to cover a sufficiently large area to discover rare objects like quasars in practicable time an 8m class wide field imager is required.
The optimum survey strategy depends crucially on 3 factors: the relative surface densities of faint low-mass M-stars, the space density of z>5 quasars and the cosmological evolution of the intervening absorption-line population, All three are highly uncertain and the answers to all three questions are astrophysically relevant. It can be envisaged an V-I vs. I-Gz approach to the selection of QSOs on 10 sq.deg. At the limiting magnitudes , and 10 observing nights with the LBT would be required to settle the above issues.