Gamma-ray probes lift the fog in intergalactic space

The search for the history of the Universe's light emission: The light emitted from all objects in the Universe during its entire history (stars, galaxies, quasars etc..) forms a diffuse sea of photons that permeates intergalactic space, referred to as "diffuse extragalactic background light" (EBL). Scientists have long tried to measure this fossil record of the luminous activity in the Universe, but its direct determination from the diffuse glow of the night sky is very difficult and uncertain. Very high energy (VHE) gamma-rays offer an alternative way to probe this background light, so the researchers of the international H.E.S.S. collaboration observed several quasars (the most luminous VHE gamma-ray sources known to date, see box) with this goal in mind. The results turned out to be rather striking.

The fog of intergalactic photons: When the very energetic gamma-rays (see cartoon below) collide with light near the visible range, matter may be produced, as predicted by Einstein (in this case, an electron-positron pair). A beam of gamma-rays from a distant galaxy is thus attenuated in its way to the Earth, owing to these collisions with the diffuse-light photons. The effect is stronger for more energetic gamma-rays, so the original gamma-ray spectrum gets "reddened", somewhat like the Sun looking redder at sunset because the blue light is more heavily scattered by the atmosphere than the red light. Since the "reddening" depends on the thickness of the absorber (the density of the background light photons, in this case), a measure of this thickness becomes possible.

Measuring the fog of photons: "The main problem is that the distribution of gamma-ray energies (spectra) from quasars can have many different forms, and up to now we couldn't really say if an observed spectrum was 'red' because of a strong reddening or because it was that way at the origin" says Dr. Luigi Costamante, one of the scientists involved in the discovery. But the gamma-ray spectra from these two quasars (named H 2356-309 and 1ES 1101-232 in catalogues), more distant than previous sources and measured thanks to the unprecedented sensitivity of the H.E.S.S. instrument, have provided a breakthrough: they are too "blue" (i.e., have too many gamma-rays at high-energy end of the measured range) to be compatible with the strong reddening implied by a high level of background light. Unless more problematic or exotic scenarios are brought into play, the most likely conclusion is that the level of the fossil light is significantly lower than previously believed.

Expanding the gamma-ray horizon of the Universe: The limit that can be deduced from the H.E.S.S. data on the maximum level of this diffuse light is in fact very close to the lower limit represented by the sum of the light of the galaxies we see with our optical telescopes, such as Hubble. This provides an answer to one of the questions that have puzzled scientists for several years: is this diffuse light caused mainly by the radiation from the very first stars born in the Universe, when it was only few hundred millions years old? The H.E.S.S. result seems to exclude such a possibility, and leaves little room as well for a large contribution from different sources, other than normal galaxies.

A more transparent intergalactic space opens new perspectives for the study of gamma-ray sources outside our galaxy, and the H.E.S.S. scientists continue to explore the gamma-ray sky now being able to look at even larger distances.