Radiative transfer effects on the Lyα forest

Abstract

Strong observational evidence for a fluctuating ultraviolet background (UVB) has been accumulating through a number of studies of the HI and HeII Lya 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 Lya forest spectra, as photoionization rate fluctuations, eta parameter (the HeII to HI column density ratio) distributions and the IGM temperature at redshift about 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 HeII opacity. Shielding of overdense regions (Delta > 5) from cosmic HeII ionizing radiation, produces a decreaseing trend of eta with overdensity. Furthermore we find that the mean eta value inferred from HI-HeII Lya forest observations can be explained only by properly accounting for the actual IGM opacity. We outline and discuss several implications of our findings.