Implementation of metals in a three-dimensional non-LTE radiative transfer code to simulate the intergalactic medium at early cosmological times

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An important issue in modern astrophysical research is to understand precisely the early phases of the evolution of the universe. Recent studies investigating the formation of large-scale structures have shown that the first generation of stars, formed directly from the primordial gas, already played a crucial role during the early phase of the epoch of reionization of the universe. Theoretical studies indicate that the initial mass function (IMF) of this first stellar population differed significantly from the present IMF, being top-heavy and thus allowing for the presence of supermassive stars. The first generation of population III stars was therefore not only very luminous, but due to its lack of metals its output of UV radiation considerably exceeded the UV emission of present stars. Because of their short lifetimes the metals produced in their cores were quickly returned to the environment from which early population II stars with different IMFs and different spectral energy distributions (SEDs) were formed, already much earlier than the time at which the universe became completely reionized (at a redshift of z > 6).In the proposed project we aim to calculate SEDs of very massive stars of different metallicities as input for the 3-dimensional radiative transfer code we are currently developing to simulate the temporal evolution of the ionization of the inhomogeneous interstellar and intergalactic medium, using multiple stellar clusters as sources of ionizing radiation. The objective is not only to simulate the processes which are believed to have lead to the reionized state of the universe, but also to determine possible observational diagnostics to constrain the nature of the ionizing sources. The multi-frequency treatment in our combination of 3-dimensional radiative transfer -- based on ray-tracing -- and time dependent simulation of the ionization structure of all important elements -- including metals -- will allow, in principle, to deduce information about the spectral characteristics of the first generations of stars and their interaction with the surrounding gas on various scales. As our tool will be able to handle distributions of numerous radiative sources characterized by high resolution synthetic SEDs (we will thus not simply use ``black-body'' spectra), and will also yield occupation numbers of the most important energy levels of the elements which will be treated in NLTE and calculated consistently with the 3-dimensional radiative transfer, the ionization state of an inhomogeneous gaseous density structure will be calculated more accurately. On basis of this procedure we will systematically investigate the impact of the increasing metallicity of the radiative sources (simulated cluster structures) in the transition from population III stars to population II/I stars on the ionization structure of the interstellar and intergalactic medium.