Causes and effects of the first quasars.

The light we observe from the most distant known quasars set out when the Universe was about 200 times denser than it is now and less than one-tenth of its present age. The existence of these objects implies that galaxy formation had already, at that early epoch, proceeded to the stage when massive (>10(8)M[symbol, see text]) objects had accumulated in the centers of at least some young galaxies. A specific model is presented to show that the evolution and luminosity function of quasars are compatible with the cold dark matter cosmogony. Most big galaxies probably passed through a quasar phase; the remnant black holes in nearby galaxies may reveal themselves via the flares that occur whenever a star passes too close to them and gets tidally disrupted. The rich absorption spectra of quasars serve as a probe of the intervening medium. The gas responsible for the Lyman alpha absorption lines may be due to primordial gas gravitationally confined in minihalos of dark matter--shallow potential wells whose evolution and relation to dwarf galaxies are briefly discussed. The patchy heat input into the intergalactic medium from early quasars could modulate the environment in which galaxies form, leading to large-scale spatial correlations in the galaxy distribution. This review concludes with general comments on the prospects for a fully quantitative understanding of galaxy formation.