THE DETECTION OF HIGH-REDSHIFT QUASARS

The detection of samples of high-redshift (i.e. distant) quasars provides direct information on the physical conditions existing when the Universe was only 10% of its present age. Quasars are the bright nuclei of certain galaxies and are the only astronomical sources that can currently be found in substantial numbers at redshifts z>2. It is probable that quasars are active for only a small proportion of the life of the host galaxy, in which case a substantial fraction of all galaxies must go through a quasar phase at some time in their history. Consequently quasars may be tracers of the galaxy population at high redshift, less active members of which are too faint for detailed study with present-day instrumentation. The main purpose of surveys for high-redshift quasars, then, is to map the evolution of the population, in luminosity and number density, back through the history of the Universe, thereby furnishing clues both to the nature of the quasar phenomenon itself and to the poorly understood process of how galaxies formed. Of special interest is the possibility of detecting the epoch of the birth of the quasar population. The authors summarise the optical and radio techniques employed in the detection of high-redshift (z>3) quasars and attempt to reconcile the contradictory claims in the literature concerning the evolution of their space density at the highest redshifts.