A low level of extragalactic background light as revealed by γ-rays from blazars


The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the ‘first stars’, which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light1. An alternative approach2,3,4,5 is to study the absorption features imprinted on the γ-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons6. Here we report the discovery of γ-ray emission from the blazars7 H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies8. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources—in particular from the first stars formed9. This result also indicates that intergalactic space is more transparent to γ-rays than previously thought.

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Figure 1: SED of the EBL in the wavelength band most affecting these HESS data (0.1–10 µm).
Figure 2: The HESS spectra of 1ES 1101 - 232, corrected for absorption with three different EBL SED values, as labelled in Fig. 1.


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The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of HESS is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the UK Particle Physics and Astronomy Research Council (PPARC), the IPNP of the Charles University, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment. The European Associated Laboratory for Gamma-Ray Astronomy is jointly supported by CNRS and MPG.

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Correspondence to L. Costamante.

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Supplementary Notes

This file contains Supplementary Notes with Supplementary Figures and Legends 1–5 and Supplementary Table 1. The Supplementary Notes describe how EBL absorption modifies the source spectrum, and the impact on the EBL limit of the uncertainties in the gamma-ray measurements and of galaxy evolution effects. We also explain why previous TeV blazar detections did not provide a comparably strong limit. (PDF 188 kb)

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Aharonian, F., Akhperjanian, A., Bazer-Bachi, A. et al. A low level of extragalactic background light as revealed by γ-rays from blazars. Nature 440, 1018–1021 (2006). https://doi.org/10.1038/nature04680

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