Structure formation within the cosmic web

Abstract

In this era of high-precision cosmology we are able to measure and predict properties of the large-scale structure of our Universe to a fine degree. However we still lack a clear and tested understanding of the effects of the large-scale environments on galaxies and their host halos. This thesis focuses on bettering our understanding of this issue by investigating the dependence of galaxies and halos on their location within the cosmic web. An algorithm based on the tidal tensor prescription is developed and applied to the MDR1 1 (h-1Gpc)3 dark matter simulation to classify the geometric environment of every location in the simulated volume as one of the four components of the cosmic web; voids, sheets, filaments and knots. Conditional halo mass functions are extracted to investigate the influence of tidal forces on the abundances and mass distribution of dark matter halos. A theoretical framework based on Gaussian statistics is presented and used to derive predictions for halo abundances in different geometric environments. The Gaussian theory predicts no coupling of tidal forces and, hence, that the halo mass function is independent of geometric environment for a given local mass density. It is shown that the halo mass functions extracted from the simulation are fully consistent with this picture. It is then shown how this method of classifying geometric environments can be extended to observational datasets. The Galaxy And Mass Assembly (GAMA) spectroscopic redshift survey, with its wide field and high completeness, is excellently suited to this study. The geometric environments of the three equatorial GAMA fields are classified, following a thorough analysis of the additional uncertainties introduced when moving to observational datasets. Additionally, the geometric environments of the GAMA galaxies and groups are classified, allowing the influence of the cosmic web on large-scale structure to be investigated. Both the galaxy luminosity function and the group mass function within the observed cosmic web are studied and no evidence of a direct impact of the web is seen. It is found that all modulations can be fully attributed to the indirect dependence of these properties on the local matter overdensity. Whilst these results indicate that there is no strong dependence of the scalar properties of large-scale structure on geometric environment, the final investigation of this thesis presents an attempt to look in more detail at the environmental dependence of stellar properties by investigating stellar-formation histories within the cosmic web.