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Luminosity - Luminosity-function evolution We would really...

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Luminosity-function evolution . We would really like to know not only that something about the AGN population changes with cosmic time (redshift), but what . It might be the total number of AGN, the characteristic luminosity of each one, or the duty cycle of intermittent activity - the fraction of time a typical object spends in an ``on" state. One approach to sorting these out is to watch the luminosity function change with redshift. There exist redshift-dependent selection effects in optical surveys, so we need to sidestep these here. Most prominent are those for color and objective-prism selection. Below redshifts of 0.4 or so, QSOs almost uniformly appear much bluer than faint high-latitude stars. At z ~0.4, Mg II λ 2798 emission leaves the U band and makes QSO colors less distinct. Similarly, at very high redshifts Lyman α emission dominates; as soon as it passes the bluer passband of a pair QSOs become quite red. Multicolor selection reduces
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