Absorption effects in the expanding Universe: spectral transmittance functions of the intergalactic medium for distant sources

We construct two self-consistent analytic approximations to the neutral hydrogen fraction, x HI(z), and the helium ionization fractions, x HeI(z), x HeII(z), and x HeIII(z), that are consistent with current constraints inferred from quasar spectra, galaxy surveys, and CMB polarization measurements. These approximations describe observationally motivated early- and late-reionization scenarios. Using these histories, we analyse the formation of broad absorption troughs in the continuum spectra of high-redshift sources over 1 at most z s at most15. We assume that neutral hydrogen and helium in a homogeneous diffuse intergalactic medium reside predominantly in their ground states and absorb radiation through the Lyman-series lines and continua of HI, HeI, and HeII. We compute the wavelength-dependent optical depths for the first 39 Lyman-series lines of HI and HeII, the first 10 lines of HeI, and the corresponding continua, and use them to derive spectral transmittance functions, T(λ;z s). As illustrative applications, we apply them to toy-model continuum spectra of starless haloes and to model spectra of a low-metallicity dwarf galaxy at different redshifts. Spectral features in sources at 5 z s 7 caused by intergalactic absorption are found to be particularly sensitive to the adopted hydrogen and helium ionization histories