The Flatness and Sudden Evolution of the Intergalactic Ionizing Background

Abstract

The ionizing background of cosmic hydrogen is an important probe of the sources and absorbers of ionizing radiation 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 puzzling discrepancy by pointing out that, at z~2-5, optically thick absorbers are associated with the same collapsed halos that host ionizing 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.

16 pages, 7 figures, submitted to MNRAS, revised methods leading to same but more convincing conclusions

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Cited by in corpus (10)

• The Evolution of the Intergalactic Medium
• Reionization and high-redshift galaxies: the view from quasar absorption lines
• Large Fluctuations in the High-Redshift Metagalactic Ionizing Background
• Large fluctuations in the hydrogen-ionizing background and mean free path following the epoch of reionization
• Cosmic Reionization After Planck and Before JWST: An Analytic Approach
• On the decreasing fraction of Strong Ly$\alpha$ Emitters around $z$ $\sim$ $6$-$7$
• The spin-temperature dependence of the 21cm -- LAE cross-correlation
• On the accuracy of common moment-based radiative transfer methods for simulating reionization
• 21-cm signatures of residual HI inside cosmic HII regions during reionization
• Measuring photo-ionization rate and mean free path of HeII ionizing photons at $2.5\le z\le 3.6$: Evidence for late and rapid HeII reionization Part-II