Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non‐trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal H ii region expansion; (2) H ii region expansion with evolving temperature; (3) I‐front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized‐neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form.

We study the intergalactic transmission of radiation in the vicinity of the Lyα wavelength. Simulating sightlines through the intergalactic medium (IGM) in detailed cosmological hydrosimulations, the impact of the IGM on the shape of the line profile from Lyα emitting galaxies at redshifts 2.5 to 6.5 is investigated. In particular we show that taking into account the correlation of the density and velocity fields of the IGM with the galaxies, the blue part of the spectrum may be appreciably reduced, even at relatively low redshifts. This may in some cases provide an alternative to the often-invoked outflow scenario, although it is concluded that this model is still a plausible explanation of the many asymmetric Lyα profiles observed.Applying the calculated wavelength dependent transmission to simulated spectra from Lyα emitting galaxies, we derive the fraction of photons that are lost in the IGM, in addition to what is absorbed internally in the galaxies due to dust.Moreover, by comparing the calculated transmission of radiation blueward of the Lyα line with corresponding observations, we are able to constrain the epoch when the Universe was reionized to z 8.5.

We compute the escape of ionizing radiation from galaxies in the redshift interval z = 4 − 10, i.e., during and after the epoch of reionization, using a high-resolution set of galaxies, formed in fully cosmological simulations. The simulations invoke early, energetic feedback, and the galaxies evolve into a realistic population at z = 0. Our galaxies cover nearly four orders of magnitude in masses (10 7.8 − 10 11.5 M ⊙ ) and more than five orders in star formation rates (10 −3.5 − 10 1.7 M ⊙ yr −1 ), and we include an approximate treatment of dust absorption. We show that the source-averaged Lyman-limit escape fraction at z = 10.4 is close to 80% declining monotonically with time as more massive objects build up at lower redshifts. Although the amount of dust absorption is uncertain to 1 − 1.5 dex, it is tightly correlated with metallicity; we find that dust is unlikely to significantly impact the observed UV output. These results support reionization by stellar radiation from low-luminosity dwarf galaxies and are also compatible with Lyman continuum observations and theoretical predictions at z ∼ 3 − 4.

A numerical code for solving various Lyα radiative transfer (RT) problems is presented. The code is suitable for an arbitrary, three-dimensional distribution of Lyα emissivity, gas temperature, density, and velocity field. Capable of handling Lyα RT in an adaptively refined grid-based structure, it enables detailed investigation of the effects of clumpiness of the interstellar (or intergalactic) medium. The code is tested against various geometrically and physically idealized configurations for which analytical solutions exist, and subsequently applied to three different simulated high-resolution "Lyman-break galaxies," extracted from high-resolution cosmological simulations at redshift z = 3.6. Proper treatment of the Lyα scattering reveals a diversity of surface brightness (SB) and line profiles. Specifically, for a given galaxy the maximum observed SB can vary by an order of magnitude, and the total flux by a factor of 3-6, depending on the viewing angle. This may provide an explanation for differences in observed properties of highredshift galaxies, and in particular a possible physical link between Lyman-break galaxies and regular Lyα emitters.

We present new calculations of the inhomogeneous process of cosmological reionization by following the radiative transfer carefully in pre-computed hydrodynamical simulations of galaxy formation. These new computations represent an important step on the way towards fully self-consistent and adaptive calculations which will eventually cover the enormous range of scales from sizes of individual mini-halos to the mean free path of ionizing photons in the post-overlap Universe. The goal of such simulations is to include enough realistic physics to accurately model the formation of early structures and the end of the 'dark ages'. Our new calculations demonstrate that the process by which the ionized regions percolate the Universe is complex, and that the idea of voids being ionized before overdense regions is too simplistic. It seems that observational information pertaining to the reionization epoch may now be in our grasp, through the detection of Gunn-Peterson troughs at z ∼ 6. If so, then the comparison of information from many lines of sight with simulations such as ours may allow us to disentangle details of the ionization history and trace the early formation of structure.