6.1. Optical/Infrared Surveys
Building on the success of 2dF and SDSS in delineating the evolution of optically selected quasars at high luminosity, a next logical and important observational step is to map the evolution of lower luminosity objects. They constitute the bulk of the AGN population, and they are also crucial for understanding the nature of the extragalactic ionizing background radiation. At high redshift, there is already evidence that the numbers of quasars are too few to account for the observed level of ionization of the intergalactic medium (McDonald & Miralda-Escudé 2001; Schirber & Bullock 2003).
On the observational front, the slope of the luminosity function of high-redshift quasars is quite uncertain. Although Fan et al. (2001b) have made estimates, and the upper limits on the number of AGNs in the HDF also sets constraints (e.g., Conti et al. 1999), direct observations are needed, because it is the slope that determines the number of faint AGNs.
Among the current surveys for fainter quasars at high redshift are the BTC40 (Monier et al. 2002), COMBO-17 (Wolf et al. 2001), and the NOAO DEEP survey (Januzzi & Dey 1999). All are multi-color imaging surveys. In addition, Steidel et al. (2002) are investigating the AGN population found in a deep spectroscopic survey of Lyman-break galaxies. The main properties of these surveys are:
BTC40 covered 40 deg2 in B, V, I and 36 deg2 in z and was designed to find quasars with 4.8 < z < 6. It reached 3 limiting magnitudes of V = 24.5, I = 22.9, and z = 22.9. To date it has yielded two quasars with redshifts of 4.6 and 4.8 and produced candidates down to I = 22 for future spectroscopy on 8-10 m telescopes.
COMBO-17 uses 17 filters covering the 0.37 - 2.2µ wavelength range to achieve in effect low-resolution spectroscopy for a 1 deg2 area down to R = 26 mag. This survey, which will include 50,000 galaxies as well as quasars and AGNs, is very ambitious and promises to yield very significant results when completed.
The NOAO DEEP survey is covering 18 deg2 of sky in BW, R, I, J, H, K to optical magnitudes of 26 and near-infrared magnitudes of 21. Because of its broad wavelength coverage and faint limiting magnitudes, it will provide very important data on the evolution of AGNs.
The Steidel et al. survey complements the multi-color ones in that it investigates the spectroscopic properties of galaxies at z 3 down to RAB 25.5 mag. It has found that about 3% of the galaxies are AGNs, many of which would not have been detected in deep X-ray surveys, and thus is sampling a part of parameter space not covered to date in other work.
An additional value of all these surveys will be to combine their results with those of the deep radio and X-ray surveys mentioned above. This will help define better the luminosity functions and statistics of quasars, AGNs of different types, and normal galaxies and thereby help improve our knowledge of the evolution of all these objects.