The Environments of High Redshift Active Galactic Nuclei

Abstract

In this thesis I study the links between Active Galactic Nuclei (AGN) and their surrounding large scale environments mainly at high redshift. I firstly use Spitzer space telescope data for one of the largest and most uniformly selected samples of radio-loud and radio-quiet AGN at high redshift. It consists of 173 AGN of both type-1 Sloan Digital Sky Survey (SDSS) Quasi- Stellar-Objects (QSOs) and type-2 radio-galaxies at the single cosmic epoch of z ∼ 1. I find significant (8 σ) over-densities of galaxies in the AGNs’ environments when compared to an offset field. Further to this I address the question of whether radio-loud AGN are found, on average, in denser environments than their radio-quiet counterparts. I show that there is a link between the environment and radio luminosity of the most powerful radioloud QSOs and RGs in the sample, and also reconcile the conflicting results in the literature by suggesting that there is only a link to the environment at the highest radio powers. I extend this work to higher redshift with data from the Spitzer extragalactic Representative Volume Survey (SERVS) and type-1 SDSS QSOs in the regions covered by SERVS. This deep data allowed me to study the environments of QSOs in the redshift range 1 < z < 4. Again I find significant (4 σ) over-densities of galaxies around the QSOs in this sample, this time making use of the 3.6-4.5 μm colour to select galaxies more likely to reside at the redshifts of interest. I show that the environments of these QSOs are comparable to those predicted for similarly large black holes in the Durham semi-analytic galaxy formation model (Galform). Finally I use data from the Herschel-Astrophysical Terahertz Large Area Survey with the recently launched Herschel space observatory to study the environments of type-1 QSOs in the far-infrared (FIR). I find a small excess of galaxies around the QSOs for which I find that the star-formation rate increases with increasing redshift. The star-formation rates are estimated by modelling the FIR spectral energy distribution of the galaxies with a modified black-body spectrum. This follows the general increase in starformation rate with redshift observed in the Universe as a whole. I also compare these findings with those made by the Submillimeter Common-User Bolometer Array (SCUBA) of higher redshift QSOs.