Letter

Nature 440, 1018-1021 (20 April 2006) | doi:10.1038/nature04680; Received 25 July 2005; Accepted 22 February 2006

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

F. Aharonian1, A. G. Akhperjanian2, A. R. Bazer-Bachi3, M. Beilicke4, W. Benbow1, D. Berge1, K. Bernlöhr1,5, C. Boisson6, O. Bolz1, V. Borrel3, I. Braun1, F. Breitling5, A. M. Brown7, P. M. Chadwick7, L.-M. Chounet8, R. Cornils4, L. Costamante1,9, B. Degrange8, H. J. Dickinson7, A. Djannati-Ataï10, L. O'C. Drury11, G. Dubus8, D. Emmanoulopoulos12, P. Espigat10, F. Feinstein13, G. Fontaine8, Y. Fuchs14, S. Funk1, Y. A. Gallant13, B. Giebels8, S. Gillessen1, J. F. Glicenstein15, P. Goret15, C. Hadjichristidis7, D. Hauser1, M. Hauser12, G. Heinzelmann4, G. Henri14, G. Hermann1, J. A. Hinton1, W. Hofmann1, M. Holleran16, D. Horns1, A. Jacholkowska13, O. C. de Jager16, B. Khélifi1, S. Klages1, Nu. Komin5, A. Konopelko5, I. J. Latham7, R. Le Gallou7, A. Lemière10, M. Lemoine-Goumard8, N. Leroy8, T. Lohse5, J. M. Martin6, O. Martineau-Huynh17, A. Marcowith3, C. Masterson1,9, T. J. L. McComb7, M. de Naurois17, S. J. Nolan7, A. Noutsos7, K. J. Orford7, J. L. Osborne7, M. Ouchrif9,17, M. Panter1, G. Pelletier14, S. Pita10, G. Pühlhofer12, M. Punch10, B. C. Raubenheimer16, M. Raue4, J. Raux17, S. M. Rayner7, A. Reimer18, O. Reimer18, J. Ripken4, L. Rob19, L. Rolland17, G. Rowell1, V. Sahakian2, L. Saugé14, S. Schlenker5, R. Schlickeiser18, C. Schuster18, U. Schwanke5, M. Siewert18, H. Sol6, D. Spangler7, R. Steenkamp20, C. Stegmann5, J.-P. Tavernet17, R. Terrier10, C. G. Théoret10, M. Tluczykont8,9, C. van Eldik1, G. Vasileiadis13, C. Venter16, P. Vincent16, H. J. Völk1 and S. J. Wagner12

  1. Max-Planck-Institut für Kernphysik, PO Box 103980, D 69029 Heidelberg, Germany
  2. Yerevan Physics Institute, 2 Alikhanian Brothers Street, 375036 Yerevan, Armenia
  3. Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, 9 avenue du Colonel Roche, BP 4346, F-31029 Toulouse cedex 4, France
  4. Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Gemany
  5. Institut für Physik, Humboldt-Universität zu Berlin, Newtonstrasse 15, D 12489 Berlin, Germany
  6. LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, F-92195 Meudon cedex, France
  7. University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
  8. Laboratoire Leprince-Ringuet, IN2P3/CNRS, Ecole Polytechnique, F-91128 Palaiseau, France
  9. European Associated Laboratory for Gamma-Ray Astronomy
  10. APC, 11 Place Marcelin Berthelot, F-75231 Paris cedex 05, France
  11. Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
  12. Landessternwarte, Königstuhl, D 69117 Heidelberg, Germany
  13. Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, CC 70, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
  14. Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, F-38041 Grenoble cedex 9, France
  15. DAPNIA/DSM/CEA, CE Saclay, F-91191 Gif-sur-Yvette cedex, France
  16. Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
  17. Laboratoire de Physique Nucléaire et de Hautes Energies, IN2P3/CNRS, Universités Paris VI & VII, 4 Place Jussieu, F-75252 Paris cedex 5, France
  18. Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany
  19. Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic
  20. University of Namibia, Private Bag 13301, Windhoek, Namibia

Correspondence to: L. Costamante1,9 Correspondence and requests for materials should be addressed to L.C. (Email: luigi.costamante@mpi-hd.mpg.de).

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 gamma-ray spectra of distant extragalactic objects by interactions of those photons with the background light photons6. Here we report the discovery of gamma-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 gamma-rays than previously thought.

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