2015A&A...575A..21N

We study the possible detection of and properties of very high-energy (VHE) γ-ray emission (in the energy band above 100 GeV) from high redshift sources. We report on the detection of VHE γ-ray flux from blazars with redshifts z>0.5. We use the data from the Fermi telescope in the energy band above 100 GeV and identify significant sources via cross-correlation of arrival directions of individual VHE γ rays with the positions of known Fermi sources. There are thirteen high-redshift sources detected in the VHE band by the Fermi/LAT telescope. The present statistics of the Fermi signal from these sources is too low for a sensible study of the effects of suppression of the VHE flux by pair production through interactions with extragalactic background light photons. We find that the detection of these sources with ground-based γ-ray telescopes would be challenging. However, several sources, including BL Lacs PKS 0426-380 at z=1.11, KUV 00311-1938 at z=0.61, B3 1307+433 at z=0.69, as well as a flat-spectrum radio quasar 4C +55.17 at z=0.89, should be detectable by HESS-II, MAGIC-II, and CTA. A high-statistics study of a much larger number of VHE γ-ray sources at cosmological distances would be possible with the proposed high-altitude Cherenkov telescope 5@5.