A new calculation of the ionizing background spectrum and the effects of He II reionization

Claude André Faucher-Giguère*, Adam Lidz, Matias Zaldarriaga, Lars Hernquist

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

460 Scopus citations


The ionizing background determines the ionization balance and the thermodynamics of the cosmic gas. It is therefore a fundamental ingredient to theoretical and empirical studies of both the intergalactic medium (IGM) and galaxy formation. We present here a new calculation of its spectrum that satisfies the empirical constraints we recently obtained by combining state-of-the-art luminosity functions and intergalactic opacity measurements. In our preferred model, star-forming galaxies and quasars each contribute substantially to the H I ionizing field at z < 3, with galaxies rapidly overtaking quasars at higher redshifts as quasars become rarer. In addition to our fiducial model, we explore the physical dependences of the calculated background and clarify how recombination emission contributes to the ionization rates. We find that recombinations do not simply boost the ionization rates by the number of re-emitted ionizing photons as many of these rapidly redshift below the ionization edges and have a distribution of energies. A simple analytic model that captures the main effects seen in our numerical radiative transfer calculations is given. Finally, we discuss the effects of He II reionization by quasars on both the spectrum of the ionizing background and on the thermal history of the IGM. In regions that have yet to be reionized, the spectrum is expected to be almost completely suppressed immediately above 54.4 eV, while a background of higher energy (≳0.5 keV) photons permeates the entire universe owing to the frequency dependence of the photoionization cross section. We provide an analytical model of the heat input during He II reionization and its effects on the temperature-density relation.

Original languageEnglish (US)
Pages (from-to)1416-1443
Number of pages28
JournalAstrophysical Journal
Issue number2
StatePublished - 2009


  • Cosmology: Theory
  • Diffuse radiation
  • Galaxies: Evolution
  • Galaxies: Formation
  • Galaxies: High-redshift
  • Quasars: Absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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