Shortly after the discovery of quasars, it was understood that these objects are truely remarkable, not only in their physical nature but also in their cosmological evolution. Schmidt (1968, 1970) observed a very sharp decline in the space density of quasars, over roughly a factor of 100 from z ~ 2 to z ~ 0. This decline was so remarkable, that it could be credibly demonstrated from a sample as small as 20 objects. The interpretation was supported by V / Vmax tests, developed for this purpose at the time. The strong evolution clearly applied to both, objects selected optically and those selected from their radio emission. At the time it suggested that quasars were the most dramatically evolving population of objects in the Universe.
It was later understood that quasars are only one part of a larger population of objects commonly called Active Galactic Nuclei or AGN. From an optical perspective, a defining characteristic of AGN is non-stellar continuum radiation. More sensitive observations showed that also light from luminous and young stars is sometimes found in AGN, suggesting that star formation may be occuring in the host galaxy and may have a common trigger with the nuclear activity. AGN have been observed over a wide range of luminosities which presently covers eight orders of magnitude, across MB [- 10, - 30]. This range poses a challenge for observations aiming at understanding the phenomenon: at the lowest luminosities the host galaxy outshines the active nucleus, making it hard to detect. At the highest luminosities, the AGN outshines the host galaxy, making the latter almost impossible to detect, let alone to investigate. Traditionally, unobscured AGN at MB < - 23 have been called QSOs. This definition does not reflect any physical limits, but is motivated by observational practicalities: here the AGN is luminous enough to dominate the combined spectrum over the host galaxy, making its AGN nature easy to recognize.
This article focusses on the more luminous half, MB < - 20, of the range covered by AGN, where > 95% of the total luminosity density is emitted. Besides these bright unobscured type-1 objects, there is an entire class of optically obscured type-2 AGN with a very significant contribution to the cosmic black hole accretion history. However, being optically obscured makes these objects hard to find and study from an optical point of view, although their host galaxies are clearly observable. Hence, this article will give more weight to type-1 objects, here called QSOs irrespective of their luminosity.