Abstract
Abstract Motivated by the discovery of the ultra-strong emission-line starburst galaxies (EELGs) known as “green pea galaxies,” in this work we consider their contribution to the intergalactic flux of ionizing UV at high redshifts. Most galaxies that have been observed show a precipitous drop in the flux blueward of their Lyman limit. However, recent observations of EELGs have discovered that many more Lyman continuum photons escape from them into intergalactic space than previously suspected. We calculate their contribution to the extragalactic background light. We also calculate the effect of these photons on the absorption of high-energy γ-rays. For the more distant γ-ray sources, particularly at z ≥ 3, an intergalactic opacity above a few GeV is significantly higher than previous estimates which ignored the Lyman continuum photons. We calculate the results of this increased opacity on observed γ-ray spectra, which produce a high-energy turnover starting at lower energies than previously thought, and a gradual spectral steepening that may also be observable.
Highlights
Within the past decade several empirically based calculations of the Extragalactic Background Light (EBL) have been published (Stecker et al 2012, 2016; Helgason and Kashlinsky 2012; Scully et al 2014)
We make a comparison of the two models that differ only by the inclusion or exclusion of the Lyman continuum (LyC) photons that we add following the prescription of the previous section
The newly discovered green pea class of extreme emission line starburst galaxies (EELGs), and other galaxies with large escape fractions of FUV and EUV photons in the Lyman continuum (LyC) region, are the most typical galaxies found at high redshifts
Summary
Within the past decade several empirically based calculations of the Extragalactic Background Light (EBL) have been published (Stecker et al 2012, 2016; Helgason and Kashlinsky 2012; Scully et al 2014). Γ-rays emitted at a redshift of 3 interacting with 15 eV ionizing photons would be absorbed within the energy range for detectability by the Fermi-LAT space telescope. FUSE only detected LyC in two dwarf starbursts (fesc = 3% - 5%) within its FUV sensitivity range (Leitet et al 2013) At this impasse it seemed that only the much rarer quasars were contributing significantly to the flux of ionizing photons into the EBL. Since the amount of IGM absorption varies from one sight-line to another, interpretations from emitted to observed radiation are difficult to make Another problem is that some claimed detections of LyC turned out to be artifacts of emission at longer wavelengths, from contaminating low-redshift galaxies that had not been recognized close to the line-of-sight (Vanzella et al 2015). Several recent discoveries of substantial fluxes of ionizing photons from certain types of starburst galaxies is changing this situation
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