Gamma-Ray Absorption from The Cosmic Lyman Continuum Background

Until recently, it was assumed that the diffuse extragalactic background light (EBL) in the FUV has a sharp and complete cut-off at energies above the Lyman limit. However, newly discovered extreme emission line starburst galaxies (EELGs) and other galaxies with substantial escape fractions of EUV photons in the Lyman continuum (LyC) region are increasingly common at high redshifts. The recent observations indicate that many more LyC photons at high redshifts escape from such galaxies into intergalactic space than was previously suspected. They are probably the major cause of reionization of the intergalactic medium. In this paper we estimate the contribution of these hitherto unconsidered EUV photons from EELGs at all redshifts to the (EBL), and their subsequent effect on the absorption of extragalactic high energy $\gamma$-rays. For the more distant $\gamma$-ray sources, particularly at $z \ge 3$, the intergalactic opacity above a few GeV is significantly higher than previously estimated. We calculate the results of this increased opacity on observed $\gamma$-ray spectra, which produces a high-energy turnover starting at lower energies than previously thought, and a gradual spectral steepening that may also be observable.