A NEW CALCULATION OF THE IONIZING BACKGROUND SPECTRUM AND THE EFFECTS OF He II REIONIZATION

Journal Article · Thu Oct 01 04:00:00 EDT 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/703/2/1416· OSTI ID:21371906

Faucher-Giguere, Claude-Andre; Lidz, Adam; Zaldarriaga, Matias; Hernquist, Lars ^[1]

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1. Department of Astronomy, Harvard University, Cambridge, MA 02138 (United States)

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 ({approx}>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.

• OSTI ID: 21371906
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 703; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
BOSONS
COSMIC RADIO SOURCES
COSMOLOGY
CROSS SECTIONS
ELEMENTARY PARTICLES
ENERGY TRANSFER
EVOLUTION
FREQUENCY DEPENDENCE
GALACTIC EVOLUTION
GALAXIES
HEAT TRANSFER
IONIZATION
LUMINOSITY
MASSLESS PARTICLES
MATHEMATICAL MODELS
OPTICAL PROPERTIES
PHOTOIONIZATION
PHOTONS
PHYSICAL PROPERTIES
QUASARS
RADIANT HEAT TRANSFER
RECOMBINATION
RED SHIFT
SORPTION
SPECTRA
STAR MODELS
THERMODYNAMICS
UNIVERSE