This thesis presents a study of a sample of objects with a high X-ray-to-optical flux ratio,\ud which are believed to be a good tracer of highly obscured, high redshift active galaxies\ud (AGN). Such objects have particular importance since they may account for a significant\ud fraction of the accretion history of the Universe and because of their links with the\ud co-evolution of super-massive black holes and their host galaxies.\ud In this thesis the results are presented for a new sample of bright, X-ray selected objects\ud with extreme X-ray-to-optical flux ratios (“EXOs”), constructed from a cross-correlation\ud of the 2XMMp X-ray and the SDSS-DR5 optical catalogues. Investigation of the\ud optical/NIR and X-ray colours constrains the fraction of obscured sources to be over half\ud of the sample. Optical and X-ray spectroscopic analysis for a sub-sample of the EXOs\ud confirms these results and reveals the presence of a large number of type-2 QSOs. The\ud discovery of the source with the currently highest X-ray-to-optical flux ratio value is also\ud reported and its properties investigated.\ud Finally, a population study of a complete sample of bright X-ray selected AGN from the\ud Subaru/XMM-Newton Deep Survey (SXDS) is presented. Through detailed X-ray spec-\ud tral analysis, the average properties of the sample are investigated and the fraction of\ud absorbed sources and its dependence on the intrinsic X-ray luminosity is studied. The\ud properties of the high X-ray-to-optical flux ratio sources in this SXDS sample is compared\ud with the EXO sample drawn from the the 2XMMp/SDSS catalogue cross-correlation
To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.