Radiative Transfer Effects on the Lyα Forest

Strong observational evidence for a fluctuating ultraviolet background (UVB) has been accumulating through a number of studies of the H I and He II Lyα forest as well as accurate 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 raise 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% 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 He II opacity. Shielding of overdense regions (∆ ∼ > 5) from cosmic He II ionizing radiation, produces a decreasing trend of η with overdensity. Furthermore we find that the mean η value inferred from H I -He II Lyα forest observations can be explained only by properly accounting for the actual IGM opacity. We outline and discuss several implications of our findings.