Journal Article

A physical model for the origin of the diffuse cosmic infrared background and the opacity of the Universe to very high energy γ-rays

Joshua D. Younger and Philip F. Hopkins

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 410, issue 4, pages 2180-2192
Published in print February 2011 | ISSN: 0035-8711
Published online January 2011 | e-ISSN: 1365-2966 | DOI:
A physical model for the origin of the diffuse cosmic infrared background and the opacity of the Universe to very high energy γ-rays

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We present a physical model for origin of the cosmic diffuse infrared background (CDIRB). By utilizing the observed stellar mass function and its evolution as input to a semi-empirical model of galaxy formation, we isolate the physics driving diffuse IR emission. The model includes contributions from three primary sources of IR emission: steady-state star formation owing to isolated disc galaxies, interaction-driven bursts of star formation owing to close encounters and mergers, and obscured active galactic nuclei (AGNs). We find that most of the CDIRB is produced by equal contributions from objects at z∼ 0.5–1 and z≳ 1, as suggested by recent observations. Of those sources, the vast majority of the emission originates in systems with low to moderate IR luminosities (LIR≲ 1012 L); the most luminous objects contribute significant flux only at high redshifts (z≳ 2). All star formation in ongoing mergers accounts for ≲10 per cent of the total at all wavelengths and redshifts, while emission directly attributable to the interaction-driven burst itself accounts for ≲5 per cent. We furthermore find that obscured AGNs contribute ≲1–2 per cent of the CDIRB at all wavelengths and redshifts, with a strong upper limit of less than 4 per cent of the total emission. Finally, since electron–positron pair production interactions with the CDIRB represent the primary source of opacity to very high energy (VHE: Eγ≳ 1 TeV) γ-rays, the model provides predictions for the optical depth of the Universe to the most energetic photons. We find that these predictions agree with observations of high-energy cut-offs at ∼ TeV energies in nearby blazars, and suggest that while the Universe is extremely optically thick at ≳10 TeV, the next generation of VHE γ-ray telescopes can reasonably expect detections from out to ∼50–150 Mpc.

Keywords: galaxies: active; galaxies: formation; galaxies: starburst; diffuse radiation; infrared: galaxies; gamma-rays: general

Journal Article.  9926 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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