Abstract
The cosmological background of ionizing radiation has been dominatedby quasars once the Universe aged by ∼ 2 billion years. Atearlier times (redshifts z ≳ 3), the observed abundance ofbright quasars declines sharply, implying that cosmic hydrogen wasreionized by stars instead. Here, we explain the physical origin ofthe transition between the dominance of stars and quasars as a genericfeature of structure formation in the concordance ΛCDMcosmology. At early times, the fraction of baryons in galaxies growsfaster than the maximum (Eddington-limited) growth rate possible forquasars. As a result, quasars were not able to catch up with the rapidearly growth of stellar mass in their host galaxies.
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