Large-scale structure of the early universe

Abstract

The large-scale structure of the universe is one of the most active areas of research in modern astronomy. However, until recently our knowledge of the distribution of matter on large scales has been primarily limited to two epochs - the present epoch and the epoch of recombination - and consequently it has proved difficult to distinguish between competing models of structure formation and evolution. Quasars are an ideal tracer of the matter distribution on large scales at different periods in the evolution of the universe and can therefore tell us much about the large-scale structure of the early universe. The main subject of this thesis is an investigation into the nature and implications of coherent structures in the spatial distribution of quasars on scales of 100h 1 Mpc known as quasar groups.
The first part of the thesis deals with the overall status of large-scale structure in the universe. After a review of our current knowledge of this subject, I present a new graph-theoretical method for finding large-scale structures in cosmological data sets, based on the minimal spanning tree (MST). The next part of the thesis focuses on observations of the largest quasar group known (Clowes & Campusano 1991a). I describe the observation, reduction and analysis of quasars, selected by the
Automated Quasar Detection (AQD) software of Clowes (1986), as part of quasar surveys of two fields containing the quasar group. I also detail the preparation of a new wide-field survey for quasars, using selection by ultraviolet excess, centred on
the quasar group. The results of the photometry used are presented in an appendix at the end of the thesis. The third part of the thesis considers the nature and theoretical implications of quasar groups. Using the results of the surveys described
in the thesis, I present an analysis of the Clowes & Campusano group. I also calculate the expected density contrast on the scales of the three known quasar groups for a variety of cosmological models and compare these with the observed values. The final part of the thesis deals with the results of other quasar surveys on which I have participated.
The main results of the thesis are:
• the detection of two new candidates for large-scale structure in quasar surveys: a group of ten quasars at z ' 1.9, with dimensions of "- 120 x 90 x 20h3 Mpc 3 , and a grouping of seven Seyfert galaxies at z 0.19, with dimensions of 60 X 30 X lOh' 3 Mpc3;
• the members of quasar groups show no peculiar characteristics compared with a general population of quasars;
• assuming that quasar groups are equivalent to present-day superciusters, they are compatible with CDM and MDM models with o = 1 and H0 75 km s1 Mpc' and also with models involving late-time phase transitions. They are, however, incompatible at> 10o with models involving a non-zero cosmological constant, fo = 0.2 and models involving cosmic strings.
• the discovery of a quasar with ultrastrong ultraviolet Fe II emission The unusual emission features seen in this quasar are explained by Lya fluorescence contributing significantly to the formation of Fe II emission.