Abstract
The ionizing background of cosmic hydrogen is an important probe of the
sources and absorbers of ionizing radiation, their evolution and relationship,
in the post-reionization universe. Previous studies show that the ionization
rate should be very sensitive to changes in the source population: as the
emissivity rises, absorbers shrink in size, increasing the ionizing mean free
path and, hence, the ionizing background. By contrast, observations of the
ionizing background find a very flat evolution from z~2-5, before falling
precipitously at z~6. We resolve this discrepancy by pointing out that, at
z~2-5, neutral absorbers are associated with the same collapsed halos that
additionally host ionizing galactic sources. Thus, an increasing abundance of
galaxies is compensated for by a corresponding increase in the absorber
population, which moderates the instability in the ionizing background.
However, by z~5-6, gas outside of halos dominates the absorption, the coupling
between sources and absorbers is lost, and the ionizing background evolves
rapidly. Our halo based model reproduces observations of the ionizing
background, its flatness and sudden decline, as well as the redshift evolution
of the ionizing mean free path. Our work suggests that, through much of their
history, both star formation and photoelectric opacity in the universe track
halo growth.
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