Exploring the Edge of the Stellar Universe with Gamma-ray Observations

The determination of the densities of intergalactic photons from the FIR to the UV produced by stellar emission and dust reradiation at various redshifts can provide an independent measure of the star formation history of the universe. Using recent Spitzer and GALEX data in conjunction with other observational inputs, Stecker, Malkan and Scully have calculated the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 and for photon energies from 0.003 eV to the Lyman limit cutoff at 13.6 eV in a ΛCDM universe with ΩΛ = 0.7 and Ωm = 0.3. Their results are based on backwards evolution models for galaxies developed previously by Malkan and Stecker. The calculated background SEDs at z = 0 are in good agreement with present observational data and limits. The calculated intergalactic photon densities were used to predict the absorption of high energy γ-rays in intergalactic space from sources such as blazars and quasars, this absorption being produced by interactions of the γ-rays with intergalactic FIR-UV photons having the calculated densities. The results are in excellent agreement with absorption features in found in the very high energy γ-ray spectra of the low-z blazars, Mrk 421 and Mrk 501 at z = 0.03 and PKS 2155-304 at z = 0.12. However, uncertainties in the predicted γ-ray absorption features grow with redshift. Actual measurements of the spectra of γ-ray sources at higher redshifts from detectors such as the (soon to be launched) GLAST space telescope can be used to determine intergalactic photon densities in the distant past, thereby shedding light on the history of star formation and galaxy evolution.