Paris, April 19 2006

Gamma rays pierce the intergalactic fog

By observing gamma rays(1) emitted by two quasars with the HESS telescopes located in Namibia, researchers at CNRS (IN2P3/INSU) and CEA-Dapnia, working within the framework of the HESS collaboration, have been able to show that the quantity of fossil light in the visible Universe, a fog of photons which pervades the entire Universe, is far smaller than thought so far. This finding, published in the 20 April issue of the journal Nature, is important for our understanding of the formation and evolution of galaxies.

The light emitted by all the objects in the Universe (stars, galaxies, quasars, etc) ever since its birth fills intergalactic space with an “ocean” of photons known as the “diffuse extragalactic background light”. Direct measurement of this fossil record of the light emitted in the Universe is difficult, because other sources of light (such as dust heated up in the solar system) outshine it and dazzle the instruments. Very high-energy gamma rays can be used as an alternative way of investigating and measuring this diffuse extragalactic light indirectly.

A beam of gamma rays emitted by a distant galaxy is attenuated during its journey to Earth due to interactions with diffuse-light photons. This absorption effect attenuates the initial spectrum of the gamma rays, the most energetic part of which is preferentially absorbed. Measuring the spectrum thus makes it possible to measure the density of the diffuse photons. The unmatched sensitivity of the HESS telescopes has led to the discovery of two quasars, which are both the most distant and the brightest very high-energy gamma emitters ever observed. By measuring the gamma ray spectra of these quasars with the HESS telescopes, astrophysicists have set about measuring the density of diffuse photons in the Universe.

The gamma spectra of the quasars contain too many high-energy gamma rays compared to the density of intergalactic light expected. So the most likely conclusion is that the level of fossil light in the visible Universe is significantly lower than previously thought. The Universe is therefore more transparent to gamma rays than predicted, which opens up new prospects for the study of gamma sources from outside our Galaxy for the HESS researchers.

These findings make it possible to postulate that diffuse extragalactic light may simply be the sum of the light from normal galaxies (as observed by optical telescopes such as Hubble) and that a major contribution from the very earliest stars in the Universe can be ruled out.

The HESS collaboration

The HESS experiment (High Energy Stereoscopic System), located in Namibia in south-west Africa, uses four telescopes 13 m in diameter, which currently make up the most sensitive high-energy gamma ray detector in the world.

The HESS telescopes are the fruit of several years' work by an international collaboration between over 100 scientists and engineers from Germany, France, the UK, Ireland, the Czech Republic, Armenia, South Africa and the host country, Namibia. The instrument was inaugurated in September 2004, and its first observations have already led to many major discoveries, one of which is the first ever resolved astronomical image in high-energy gamma rays of the remains of a supernova.








For comprehensive information about this discovery:


Four images provided by HESS telescopes

© Hess Collaboration -

HESS telescopes


1) Gamma radiation is made up of photons, just like visible light or X-rays, but it has far higher energy.


“A low level of extragalactic background light as revealed by g-rays from blazers”, HESS Collaboration, Nature, 20 April 2006.


Luigi Constamante
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Felix Aharonian
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Michael Punch
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Berrie Giebels
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Delphine Kaczmarek
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Pascal Newton
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