The Race Between Stars and Quasars in Reionizing Cosmic Hydrogen

TL;DR

This paper explains the transition from stars to quasars dominating cosmic hydrogen reionization, showing it as a natural outcome of structure formation in the universe, with early galaxy growth outpacing quasar development.

Contribution

It provides a physical explanation for the shift from stellar to quasar dominance in reionization based on structure formation in LCDM cosmology.

Findings

Quasars could not keep pace with early galaxy growth due to Eddington limits.

Stars reionized the universe at z>3, before quasars became dominant.

The transition is a natural feature of structure formation in the universe.

Abstract

The cosmological background of ionizing radiation has been dominated by quasars once the Universe aged by ~2 billion years. At earlier times (redshifts z>3), the observed abundance of bright quasars declined sharply, implying that cosmic hydrogen was reionized by stars instead. Here, we explain the physical origin of the transition between the dominance of stars and quasars as a generic feature of structure formation in the concordance LCDM cosmology. At early times, the fraction of baryons in galaxies grows faster than the maximum (Eddington-limited) growth rate possible for quasars. As a result, quasars were not able to catch up with the rapid early growth of stellar mass in their host galaxies.