The Environments of Active Galaxies over Cosmic Time

The overall aim of this thesis is to investigate the environments of AGN, in particular, the density of galaxies in the environments of radio-loud and radio-quiet AGN. This determines whether AGN trace dense environments at high redshifts and whether the environments are important in addressing the problem of radio-loud dichotomy. I extend my research by investigating whether star-formation evolves differently in high-redshift AGN environments compared to the field. I begin by investigating the environments of 169 AGN using Spitzer data at z ∼ 1. I investigate the source density of star-forming galaxies in the environments of radio galaxies, radio-loud quasars and radio- quiet quasars. I do not find any significant overdensity of star-forming galaxies in these environments, although I find tentative evidence for a diff erence in the colours of galaxies in the radio galaxy environments compared to the quasar and field environments. I next use VIDEO data to investigate the environments of the quasars out to z ∼ 3. Firstly, I use a training sample of QSOs and galaxies, which trains a neural network to detect QSOs in the VIDEO data. I detect 274 possible QSOs in the VIDEO data using this method. I am able to determine that the efficiency of the neural network clas- sification is 95 per cent using the training sample. I compare these results to a colour selection method, which detects 88 QSOs in the VIDEO data, and find that the neural network is able to detect ∼ 80 per cent of the colour selected QSOs at Ks = 21. I then investigate the source overdensity using a radial analysis on the environments of the VIDEO QSOs. I find a significant overdensity of galaxies in the environments of the whole QSO sample and in the environments of the radio-loud quasars compared to the radio-quiet quasars. I extend the density analysis by using a second density measure, called the spatial clustering amplitude technique, to compare the environments of the quasars with their radio luminosities, absolute magnitudes and redshifts. I do not fi any significant correlations between environmental density and radio luminosity, absolute magnitude or redshift for the QSOs. I extend this research to investigate the type of galaxies found in the AGN environments. However, I do not find any significant differences between the type of galaxies found in the QSO environments and the background field.