Modelling the galaxy population
Rimes, Christopher D.
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I describe a combined N-body/phenomenological model for the formation and evolution of galaxies in a hierarchical universe. The aim of this work is twofold: to further our understanding of the galaxy formation process and to investigate how selection effects bias our view of the galaxy population.The cold dark matter (CDM) model for structure formation has been remarkably successful at explaining the large-scale properties of the universe and now forms part of the standard picture of galaxy formation. That the matter content of the universe is dominated by a non-luminous (dark), non-baryonic form of matter is commonly accepted; its existence is neccessary to explain observed mass-to-light ratios of structures on all scales. The evolution of dark matter on large scales has been studied extensively using N-body techniques and the results are in good agreement with the observed large- scale structure of the universe. However, our view of the universe is biased by the fact that we can only directly observe luminous (baryonic) matter, which is concentrated in stars and galaxies. To gain a complete understanding of the universe it is therefore vital to have a model for the relationship between the properties of the observed galaxy population and the underlying dark matter distribution.Semi-analytical and related models are an attempt to bring together all of the ingredients necessary for galaxy formation (dark matter evolution, gas dynamics, star formation and evolution, etc.) in a single model with the power to predict the properties of statistically significant numbers of galaxies. The original models used an analytical approach (hence the name) to modelling dark matter evolution, which contained no information on the distribution of galaxies. A recent development has been the coupling of similar models with N-body simulations, allowing galaxy formation to be understood in its full cosmological context.Another way in which our view of the universe is biased is by the limits placed on observational samples of galaxies. Galaxies are often selected for inclusion in a sample on the basis of their apparent magnitude and such samples are often termed ‘magnitude- limited’ samples. However, all observations have an isophotal limit, imposed by the surface brightness of the night sky, below which galaxies cannot be seen regardless of their total flux. Such low surface-brightness galaxies make an as yet unquantified contribution to the number density of galaxies in the universe, so it is im portant to take these into account when interpreting observations and when comparing the observed properties of the galaxy population to the predictions of galaxy formation models.In this thesis, I present a model for galaxy formation which makes use of a customised N-body simulation with a built-in recipe for the formation of galaxy haloes. The specific aim of this work is to model the formation and evolution of disc galaxies with a view to reproducing the observed distribution of galaxies over luminosity and surface brightness. I then use these results to investigate the role of selection effects by constructing mock surveys, an application that will find much use in interpreting the results of recent and future galaxy surveys.