The Nature of Extreme X-ray to Optical Ratio Sources (EXOs)

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