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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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  1. Hauser, M. G. & Dwek, E. The Cosmic Infrared Background: measurements and implications. Annu. Rev. Astron. Astrophys. 39, 249–307 (2001)

    Article  ADS  CAS  Google Scholar 

  2. Nikishov, A. I. Absorption of high-energy photons in the universe. Sov. Phys. JETP 14, 393–394 (1962)

    Google Scholar 

  3. Gould, R. J. & Schreder, G. P. Opacity of the Universe to high-energy photons. Phys. Rev. Lett. 16, 252–254 (1966)

    Article  ADS  CAS  Google Scholar 

  4. Jelley, J. V. High-energy gamma-ray absorption in space by a 3.5°K microwave field. Phys. Rev. Lett. 16, 479–481 (1966)

    Article  ADS  CAS  Google Scholar 

  5. Stecker, F. W., De Jager, O. C. & Salamon, M. H. TeV gamma rays from 3C 279—A possible probe of origin and intergalactic infrared radiation fields. Astrophys. J. Lett. 390, 49–52 (1992)

    Article  ADS  Google Scholar 

  6. Aharonian, F. A. TeV blazars and the cosmic infrared background radiation. Invited, Rapporteur, and Highlight Papers of the Proc. 27th ICRC (Hamburg) (ed. Schlickeiser, R.) 250–262 (Copernicus Gesellschaft, Katlenburg-Lindau, 2001); preprint at (2001)

    Google Scholar 

  7. Costamante, L. & Ghisellini, G. TeV candidate BL Lac objects. Astron. Astrophys. 384, 56–71 (2002)

    Article  ADS  CAS  Google Scholar 

  8. Madau, P. & Pozzetti, L. Deep galaxy counts, extragalactic background light and the stellar baryon budget. Mon. Not. R. Astron. Soc. 312L, 9–15 (2000)

    Article  ADS  Google Scholar 

  9. Santos, M. R., Bromm, V. & Kamionkowski, M. The contribution of the first stars to the cosmic infrared background. Mon. Not. R. Astron. Soc. 336, 1082–1092 (2002)

    Article  ADS  Google Scholar 

  10. Aharonian, F. A. et al. H.E.S.S. observations of PKS 2155-304. Astron. Astrophys. 430, 865–875 (2005)

    Article  ADS  CAS  Google Scholar 

  11. Aharonian, F. A. et al. Observations of H1426 + 428 with HEGRA. Observations in 2002 and reanalysis of 1999 and 2000 data. Astron. Astrophys. 403, 523–528 (2003)

    Article  ADS  Google Scholar 

  12. Primack, J. R., Somerville, R. S., Bullock, J. S. & Devriendt, J. E. G. Probing Galaxy formation with high-energy gamma rays. AIP Conf. Proc. 558, 463–478 (2001)

    Article  ADS  Google Scholar 

  13. Dwek, E. & Krennrich, F. Simultaneous constraints on the spectrum of the extragalactic background light and the intrinsic TeV spectra of Mkn 421, Mkn 501, and H1426 + 428. Astrophys. J. 618, 657–674 (2005)

    Article  ADS  CAS  Google Scholar 

  14. Dwek, E. & Arendt, R. G. A tentative detection of the cosmic infrared background at 3.5 µm from COBE/DIRBE observations. Astrophys. J. Lett. 508, 9–12 (1998)

    Article  ADS  Google Scholar 

  15. Wright, E. L. DIRBE minus 2MASS: confirming the cosmic infrared background at 2.2 microns. Astrophys. J. 553, 538–544 (2001)

    Article  ADS  Google Scholar 

  16. Matsumoto, T. et al. Infrared telescope in space observations of the near-infrared extragalactic background light. Astrophys. J. 626, 31–43 (2005)

    Article  ADS  CAS  Google Scholar 

  17. Cambresy, L., Reach, W. T., Beichman, C. A. & Jarrett, T. H. The cosmic infrared background at 1.25 and 2.2 microns using DIRBE and 2MASS: A contribution not due to galaxies? Astrophys. J. 555, 563–571 (2001)

    Article  ADS  Google Scholar 

  18. Kashlinsky, A. Cosmic infrared background and early galaxy evolution. Phys. Rep. 409, 361–438 (2005)

    Article  ADS  CAS  Google Scholar 

  19. Aharonian, F. et al. The time averaged TeV energy spectrum of MKN 501 of the extraordinary 1997 outburst as measured with the stereoscopic Cherenkov telescope system of HEGRA. Astron. Astrophys. 349, 11–28 (1999)

    ADS  Google Scholar 

  20. Krennrich, F. et al. Discovery of spectral variability of Markarian 421 at TeV energies. Astrophys. J. Lett. 575, 9–13 (2002)

    Article  ADS  Google Scholar 

  21. Djannati-Atai, A. et al. Very high energy gamma-ray spectral properties of MKN 501 from CAT Cherenkov telescope observations in 1997. Astron. Astrophys. 350, 17–24 (1999)

    ADS  Google Scholar 

  22. Malkov, M. A. & Drury, L. O'C. Nonlinear theory of diffusive acceleration of particles by shock waves. Rep. Prog. Phys. 64, 429–481 (2001)

    Article  ADS  CAS  Google Scholar 

  23. Primack, J. R., Bullock, J. S. & Somerville, R. S. Observational gamma-ray cosmology. AIP Conf. Proc. 745, 23–33 (2005)

    Article  ADS  CAS  Google Scholar 

  24. Madau, P. & Silk, J. Population III and the near-infrared background excess. Mon. Not. R. Astron. Soc. 359L, 37–41 (2005)

    Article  ADS  Google Scholar 

  25. Dwek, E., Arendt, R. G. & Krennrich, F. The near infrared background: interplanetary dust or primordial stars? Astrophys. J. 635, 784–794 (2005)

    Article  ADS  CAS  Google Scholar 

  26. Bernstein, R. A., Freedman, W. L. & Madore, B. F. The first detections of the extragalactic background light at 3000, 5500, and 8000 Å. I. Results. Astrophys. J. 571, 56–84 (2002)

    Article  ADS  Google Scholar 

  27. Protheroe, R. J. & Meyer, H. An infrared background-TeV gamma-ray crisis? Phys. Lett. B 493, 1–6 (2000)

    Article  ADS  CAS  Google Scholar 

  28. Fazio, G. G. et al. Number counts at 3 µm < λ < 10 µm from the Spitzer Space Telescope. Astrophys. J. Suppl. 154, 39–43 (2004)

    Article  ADS  Google Scholar 

  29. Totani, T. et al. Diffuse extragalactic background light versus deep galaxy counts in the Subaru deep field: Missing light in the Universe? Astrophys. J. 550L, 137–141 (2001)

    Article  ADS  Google Scholar 

  30. Mattila, K. Has the optical extragalactic background light been detected? Astrophys. J. 591, 119–124 (2003)

    Article  ADS  Google Scholar 

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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 information

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).

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