The evolution of galaxies and AGN

Abstract

This thesis presents a study of some current questions regarding the evolution of galaxies and AGN. We first argue that the bright submillimetre number counts may be dominated by obscured AGN rather than ultraluminous starbursts. Through statistical analyses of the ECDFS and WHDF, we show that N_H > 10^22 cm^−2 AGN are typically bright at 870um, with less absorbed AGN being much weaker sub-mm emitters. Overall, X-ray-detected AGN contribute 1.5+/-0.1 Jy deg^−2 to the sub-mm background, ~3% of the total, in agreement with the prediction of an AGN model which also fits the bright sub-mm counts. When Compton-thick, X-ray-undetected sources are included, this non-unified model predicts a total AGN contribution to the sub-mm background of 25-40%. The measured dependences of sub-mm flux on X-ray flux, luminosity and column density all agree well with model predictions. We therefore suggest that sub-mm galaxies may contain the absorbed AGN population which has long been sought to explain the X-ray background.

Moving to shorter wavelengths, we find that AGN may continue to dominate the source counts down to ~200um. Below this, dusty star-forming galaxies are shown to fit the observed number counts and colours reasonably well. Below ~5um starlight takes over from dust as the main source of flux. These conclusions are supported by the good fit to the observations of phenomenological models assuming pure luminosity evolution.

Finally, using a new survey of z~3 Lyman-break galaxies (LBGs), we find evidence for the supernova-driven feedback which is thought to be an essential ingredient of galaxy evolution. By cross-correlating LBG positions with Lya absorption in quasar sightlines, we find (a) that the intergalactic medium shows excess neutral hydrogen within 4 h^−1 Mpc of LBG positions, consistent with their lying in overdense regions, but also (b) that a Lya transmission spike may exist extending out to ~1.5 h^−1 Mpc around LBGs, indicating that galactic winds from LBGs may have ionised the IGM within this radius.