PHD Thesis

Cosmological Radiative Transfer and the Ionisation of the Intergalactic Medium

Gravitational instability during the evolution of the Universe formed a large scale filamentary structure, known as the cosmic web. Baryons embedded in this cosmic web constitute the intergalactic medium (IGM) with hydrogen making up around 76% of its total mass. A large fraction of the baryons are kept in a highly ionised state by an ultraviolet (UV) background field. High resolution spectra of quasars (QSOs) reveal a vast amount of absorption features blue-ward of the QSO’s Lyman-α emission line: the H i Lyman-α forest. The Lyα forest traces the filamentary cosmic web, where a small remaining fraction of neutral hydrogen (H i) in the filaments produces all the absorption. The Lyα forest thus provides an unique method to constrain the history of the cosmic web. However due to the high ionisation state of the IGM, the formation history of the cosmic web can only be inferred with a detailed knowledge of the UV background, requiring the usage of both observations and simulations.

In the first part of this thesis we wish to characterise the formation of the structure giving rise to the Lyα forest. We thus derive the Lyα absorber number density evolution and the differential column density distribution. The number density evolution of high column density absorbers reveal a yet unknown dip in the number density at around z ∼ 2.1. We further show, that this depression in the absorber number density is directly connected to a dip in the differential column density distribution at NH i 1014 cm−2. This is most likely related to the high star-formation rate at z ∼ 2.

A small number of absorbers in the Lyα forest stem from ionic metal lines, indicating that some parts of the gas causing the Lyα forest have been metal enriched. In this thesis we investigate the statistical properties of enriched hydrogen absorbers. Further, we constrain the volume averaged NH i − NC iv relation, which shows a constant relation between the two constituents at high NH i. However, we find that the NH i − NC iv relation drops off steeply at NH i ∼ 1015.2 cm−2. This indicates that the IGM around galaxies is only metal enriched up to a characteristic radius. We argue that these findings are similar to results of simulated density-metallicity relations. These observations help to provide constraints on the coupling between galaxies and their environment.

In the second part of this thesis, we focus on solving the 3D radiative transfer equation numerically. In recent years, solving the 3D radiative transfer has become a new exciting field in numerical cosmology. We have developed a method of adapting the cosmological radiative transfer code CRASH2 for distributed memory clusters. We show that the resulting parallel MPI application pCRASH2 performs and scales well, enabling the simulation of complex and large problems with high resolution.

In the third part of this thesis, we model the QSO line of sight proximity effect using radiative transfer in a cosmological context. Due to the UV radiation emitted by QSOs, the hydrogen ionisation fraction increases in their vicinity. This proximity effect can be used to determine the UV background flux. With simulations, we confirm the assumption of geometrical dilution used in analytical formulations of the effect. Furthermore, we find a relation between the environmental density fluctuations and the proximity effect signal. The density fluctuations of the cosmic web are responsible for a large scatter in measurements of the proximity effect signal derived from Lyα forest spectra. The scatter is found to increase with decreasing redshift and decreases with increased QSO luminosity. Furthermore, we find that the distribution of normalised optical depths, resembles a log- normal distribution at large distances from the QSO. However the distribution becomes increasingly skewed when approaching the QSO. The proximity effect strength is found to be weakly correlated with the host halo mass, and tightly correlated with the mean density in the large scale environment of a QSO host. If a large scale overdensity is present inside a sphere of 10 h−1 Mpc comoving radius, we show that the proximity effect strength decreases, resulting in an overestimation of the UV background flux. We quantify the dependence of this bias on the QSO luminosity, the host halo mass, and the redshift. These results will help to correct this environmental bias.

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