The Extragalactic Background Light

Light from the First Stars

One of the important questions for cosmology is the level of extragalactic background light (EBL), thought to have been produced by the first stars to form in the Universe. VHE gamma-rays travelling from remote sources interact with the EBL via electron-positron pair production and are absorbed. Studying such effects as a function of photon energy and source distance will provide unique information on the EBL density, and thereby on the history of the formation of stars and galaxies in the Universe. This approach is complementary to direct EBL measurements, which are hampered by strong foreground emission from our planetary system (the zodiacal light) and the Galaxy.


The effect of the extragalactic background radiation on the propagation of gamma rays from active galaxies. © H.E.S.S. Collaboration


However, absorption features in VHE spectra may also be produced by the absorption of gamma rays within the source, rather than on their journey through the intergalactic medium. CTA will be able to provide a sufficiently large sample of VHE gamma-ray sources, and high-quality spectra for individual objects, to allow us to disentangle the effects of intrinsic and extrinsic absorption. For many of the sources, the spectrum will be determined at GeV energies. GeV photons are much less affected by the absorption and thus more suitable for the study of intrinsic properties of the objects. We therefore anticipate that with CTA’s excellent sensitivity at low energies, it will be possible to make robust predictions about the intrinsic spectrum above 100 GeV, for both individual sources and for particular source types.

The End of the Dark Ages

The end of the dark ages of the Universe, the epoch of reionisation, is a topic of great interest to cosmology. Not (yet) accessible via direct observation, most of our knowledge comes from simulations and from observing integrated radiation like the cosmic microwave background. The first (Population III) and second generations of stars are natural candidates for the sources of reionisation. If the first stars were hot and massive, as predicted by simulations, their ultraviolet photons emitted at z > 5 would be redshifted to the near infrared and could leave a unique signature on the EBL spectrum. If the EBL contribution from lower redshift galaxies is sufficiently well known (e.g. as derived from source counts) upper limits on the EBL density can be used to probe the properties of early stars and galaxies. Combining detailed model calculations with redshift-dependent EBL density measurements could allow probing the reionisation/ionisation history of the Universe.

A completely new wavelength region of the EBL will be opened up by observations of sources at very high redshifts (z > 5), which will most likely be gamma-ray bursts. According to high-redshift UV background models, consistent with our current knowledge of cosmic reionisation, spectral cut-offs are expected in the few GeV to few tens of GeV range at z > 5. Thus CTA could have the unique potential of probing cosmic reionisation models through gamma-ray absorption in high-redshift gamma-ray bursts.


Further Reading

Aharonian et al., A Low Level of Extragalactic Background Light as Revealed by Gamma-rays from Blazars, Nature (2006), 440, 7087, p. 1018-1021;

MAGIC Collaboration, Very-High-Energy gamma rays from a Distant Quasar: How Transparent Is the Universe?, Science (2008), 320, 5884, p. 1752;

Mazin, Constraints on Extragalactic Background Light from Cherenkov telescopes: status and perspectives for the next 5 years;