Probing High Redshift Radiation Fields with Gamma-Ray Absorption

The next generation of gamma-ray telescopes may be able to observe gamma-ray blazars at high redshift, possibly out to the epoch of reionization. The spectrum of such sources should exhibit an absorption edge due to pair-production against UV photons along the line of sight. One expects a sharp drop in the number density of UV photons at the Lyman edge ǫL. This implies that the universe becomes transparent after gamma-ray photons redshift below E ∼ (mec )/ǫL. Thus, there is only a limited redshift interval over which GeV photons can pair produce. This implies that any observed absorption will probe radiation fields in the very early universe, regardless of the subsequent star formation history of the universe. Furthermore, measurements of differential absorption between blazars at different redshifts can cleanly isolate the opacity due to UV emissivity at high redshift. An observable absorption edge should be present for most reasonable radiation fields with sufficient energy to reionize the universe. Lyα photons may provide an important component of the pair-production opacity. Observations of a number of blazars at different redshifts will thus allow us to probe the rise in comoving UV emissivity with time. Our knowledge of UV radiation fields and energy injection into the IGM at z > 5 is fairly tenuous. There are two main constraints: observations of the integrated background light (Madau & Pozzetti 2000, Bernstein et al 1999), and the fact that no Gunn-Peterson trough is observed in the spectra of the highest-redshift quasar to date (Fan et al 2000), implying that the universe must be reionized by z = 5.8. The upcoming Next Generation Space Telescope (NGST) will be able to image high-redshift star clusters or AGNs in rest frame UV continuum emission (Haiman & Loeb 1997), and their redshifts may be obtained via Hα observations (Oh 1999). Nonetheless, the redshift-binned number counts will be fairly sparse, and one is unlikely to be probing sufficiently far down the luminosity function to get a good measure of the comoving emissivity as a function of redshift. Observations of gamma-ray blazars (“grazars”) probe extragalactic IR and UV radiation fields, by observing the pair production opacity to γ rays at the high energy end (Gould & Schreder 1967, Stecker, De Jager & Salamon 1992, Madau & Phinney 1996, Primack et al 1999). However, all models confined their predictions to low redshift grazars, with the exception of Salamon & Stecker (1998), who computed the γ-ray opacity up to z=3. They concluded that because the stellar emissivity peaks between z=1 and z=2, the γ-ray opacity shows little increase at high redshift, and thus is not dependent on the initial epoch of galaxy formation.