Ray-Tracing through the Millennium Simulation

In this thesis, gravitational lensing in the concordance LambdaCDM cosmology is investigated by carrying out ray-tracing along past light cones through the Millennium Simulation, a very large N-body simulation of cosmic structure formation. The method used for tracing light rays substantially extends previous ray-tracing methods that are based on the Multiple-Lens-Plane approximation. Strong lensing is investigated by shooting random light rays through the Millennium Simulation. The probability is evaluated that an image of a small distant light source will be highly magnified, will be highly elongated or will be one of a set of multiple images. It is found that these probabilities increase strongly with increasing source redshift. It is shown that strong-lensing events can almost always be traced to a single dominant lensing object, and the mass and redshift distribution of these primary lenses is studied. The observed lens-mass range extends to lower masses than those found in earlier studies using simulations with lower spatial and mass resolution. Furthermore, effects of additional material along the line-of-sight are investigated. Although strong-lensing lines-of-sight are indeed biased towards higher than average mean densities, this additional matter typically contributes only a few percent of the total surface density. The influence of stellar mass in galaxies on strong lensing is investigated by comparing the results obtained for lensing by dark matter alone to those obtained by also including the luminous matter. The dark-matter component of the lensing matter is constructed directly from the dark-matter particle distribution of the Millennium Simulation, while the luminous component is inferred from semi-analytic galaxy-formation models implemented within the evolving dark-matter distribution of the simulation. It is found that the inclusion of the stellar mass strongly enhances the probability for strong lensing compared to a 'dark-matter only' universe. The identification of the lenses associated with strong-lensing events reveals that the stellar mass of galaxies (i) significantly enhances the strong-lensing cross-sections of group and cluster halos, and (ii) gives rise to strong lensing in smaller halos, which would not produce noticeable effects in the absence of the stars. Even if only image splittings >10 arcsec are considered, the luminous matter can still enhance the strong-lensing optical depths by up to a factor of two. Finally, the potential capabilities of future radio telescopes for imaging the cosmic matter distribution are discussed. The Millennium Simulation is used to simulate large-area maps of the lensing convergence with the noise, resolution and redshift-weighting achievable with a variety of idealised surveys. It is shown that by observing lensing of 21-cm emission during reionization with a sufficiently large radio telescope, an image of the matter distribution could be obtained whose signal-to-noise far exceeds that of any map made using galaxy lensing. These mass images would allow the dark-matter halos of individual galaxies to be viewed directly, giving a wealth of statistical and morphological information about the relative distributions of mass and light. For telescopes like the planned Square Kilometre Array, mass imaging may be possible near the resolution limit of the core array of the telescope.