MessageToEagle.com - Some months ago, astronomers using data from NASA's Fermi Gamma-ray Space Telescope
measured cosmic 'fog' produced by ancient starlight.
Now, researchers from the Laboratoire Leprince-Ringuet (CNRS/École Polytechnique), France have
carried out the first measurement of the intensity of the diffuse extragalactic background
light in the nearby Universe, a fog of photons that has filled the Universe ever since its
Using some of the brightest gamma-ray sources in the southern hemisphere, the study was carried out using
measurements performed by the HESS1 telescope array, located in Namibia and involving CNRS and CEA.
Click on image to enlarge
A Deeper Universe
The Fermi Gamma-Ray Space Telescope has been scanning the entire Universe every 3 hours ever since launch. The gamma-ray map that Fermi accumulates grows more and more sensitive with time as more and more gamma-ray photons are added to it. Each year, the Fermi team (and other scientists) take a detailed look at this map to see what's changed, what new sources have been detected, and which strong sources have changed over this time interval. Earlier this year, the Fermi team's detailed analysis of 2 years of gamma-ray data obtained with Fermi's Large Area Telescope was published as the Fermi 2-year source catalog. The 2-year all-sky map is shown above, along with an inset showing the distribution of the types of sources seen by Fermi so far. Most of the sources seen by Fermi are blazars, types of active galaxies which are oriented in space so that the jet emanating from their central, supermassive black hole is pointing directly at us. Other identified sources include pulsars and supernova remnants, and things like globular clusters, high mass X-ray binary stars, normal galaxies and other
(non-blazar) active galaxies, and a star or two. Interestingly, many of the sources detected by Fermi are as yet unidentified;
what can they be?
The study is complementary to that recently carried out by the Fermi-LAT2 space observatory.
These findings provide new insight into the size of the Universe observable in gamma rays and shed light on the
formation of stars and the evolution of galaxies. They feature on the cover of the 16 January 2013 issue of the
journal Astronomy & Astrophysics online.
The light emitted by all the objects in the Universe (stars, galaxies, etc) ever since its birth fills intergalactic
space with an 'ocean' of photons known as the 'diffuse extragalactic background light'.
The ambient luminosity of our own Galaxy makes it impossible to directly measure this fossil record of the light emitted
in the Universe.
To get around this problem, astrophysicists make use of gamma rays3 (whose energy is
more than 500 billion times greater than that of visible light), which provide an alternative, indirect method
of measuring this light.
A beam of gamma rays emitted by a distant galaxy located several hundred million light years away is
attenuated on its way to Earth due to interactions with diffuse light. More specifically, when a gamma-ray
photon enters into contact with a diffuse photon it may 'disappear', giving rise to an electron and its
antiparticle, a positron, which reduces the intensity of the beam.
The thicker the fog of diffuse photons, the
greater the attenuation, and the smaller the size of the Universe observable in gamma rays.
Finally, absorption by the Earth's atmosphere of the remaining radiation gives rise to a shower of subatomic
particles, which generates a flash of light that can be detected from the ground by HESS, a mainly French-
German telescope array.
Click on image to enlarge
Cosmic history and opacity to gamma-rays at different periods, as measured by HESS and Fermi-LAT. The vertical axis of the graph shows opacity normalized to a reference model (Franceschini et al., 2008), while the horizontal axis shows the distances in light years of the blazars used for the measurements. The blue point on the left shows the range within which the Fermi measurements are statistically significant, and the red point on the right shows the measurements carried out by HESS in the nearby Universe. Credit: HESS Collaboration
HESS detects very-high-energy gamma rays (in the region of a thousand billion eV), while those with lower
energy are directly detected by the Large Area Telescope (LAT) on the Fermi Gamma-Ray Space Telescope.
In this study, the researchers focused on distinctive galaxies called blazars4, which are several billion light
By using HESS to measure the gamma-ray spectra emitted by relatively close blazars, they
evaluated the effect of the interaction of highly energetic gamma rays with the diffuse extragalactic
background light within a sphere of a three billion light year radius.
The Fermi-LAT collaboration did the same for the more distant Universe, from 5 to 10 billion light years away. These measurements made it
possible to estimate, for the first time with a precision of around 20%, the intensity of the starlight contained
within the Universe at wavelengths ranging from the near infrared to the ultraviolet, including visible
A better understanding of this diffuse light, which acts as a record of the Luminous Universe, provides
information about the first stars, shedding light on their formation and on the evolution of galaxies. This new
data could be incorporated into certain cosmological models to better describe the rate and processes of
star formation since the birth of the Universe.
These findings can also be used to define the size of the
Universe observable in gamma rays, and open up the possibility of studying the signatures of more
fundamental mechanisms related to intergalactic magnetic fields, as well as exotic physical phenomena.