Characterising the host galaxies of type II quasars

AGN are thought to play an essential role in the evolution of galaxies. Despite this, the means by which the nuclear activity is triggered remains a matter of much debate. In this thesis, the triggering issue is addressed by investigating the properties of a sample of 20 (0.3 < z < 0.41) type II quasar host galaxies, using high-quality optical imaging and spectroscopic data obtained using Gemini GMOS-S Evidence for galaxy interactions in the form of tidal features (tails, fans, shells), double nuclei and close pairs is presented in Chapter 3. It is found the 75 % of the sample show clear evidence of recent mergers, consistent with the rate found for a matched control sample of early-type galaxies. However, the surface brightnesses of the tidal features in the type II quasars are up to two magnitudes brighter than those of the control sample. Major mergers also lead to intense bursts of star formation. Therefore, an analysis of the stellar populations of the type II quasar host galaxies is presented in Chapters 4 and 5. It is shown that, in 79 % of the type II quasars, spectral synthesis models require the inclusion of a young stellar component with an age < 100 Myr. The stellar masses derived from the spectral synthesis modelling are presented in Chapter 6. It is shown that type II quasars are exclusively hosted by galaxies with stellar mass > 10^10 solar masses, and, in a minimum of 26 % of cases, major gas-rich mergers are required to form the young stellar populations. The results presented in the thesis clearly demonstrate that the host galaxies of type II quasars are dynamic, evolving systems, with compelling evidence that the luminous AGN activity has been triggered by recent mergers in the majority of cases. They also suggest that quasar activity is triggered at around the same time as the merger-induced starbursts