Radiative transfer effects on the Lya forest

Strong observational evidence for a fluctuating ultraviolet background (UVB) has been accumulating through a number of studies of the HI and HeII Lyα forest as well as accurate intergalactic medium (IGM) metallicity measurements. UVB fluctuations could arise both from the inhomogeneous distribution of the ionizing sources and/or from radiative transfer (RT) through the filamentary IGM. In this study we investigate, via numerical simulations, the role of RT effects, such as shadowing, self-shielding and filtering of the ionizing radiation, in giving rise to a fluctuating UVB. We focus on possible detectable signatures of these effects on quantities derived from Lyα forest spectra, as photoionization rate fluctuations, η(≡NHeII/NHI) parameter distributions and the IGM temperature at z~ 3. We find that RT induces fluctuations up to 60 per cent in the UVB, which are tightly correlated to the density field. The UVB mean intensity is progressively suppressed toward higher densities and photon energies above 4 Ryd, due to the high HeII opacity. Shielding of overdense regions (Δ>~ 5) from cosmic HeII ionizing radiation produces a decreasing trend of η with overdensity. Furthermore, we find that the mean η value inferred from HI-HeII Lyα forest observations can be explained only by properly accounting for the actual IGM opacity. We outline and discuss several implications of our findings.