Accretion and Obscured Growth of Supermassive Black Holes

The Cosmic X-ray Background radiation arises from the summed X-ray emission of all accreting supermassive black holes in the Universe, known as active galactic nuclei (AGN). Population studies of AGN have revealed that on average the majority are obscured behind thick layers of gas and dust, yet the current best estimates for the fraction of heavily obscured systems is very uncertain: from ~10-50 per cent. This thesis aims to bridge the gap between detected, studied AGN and the lesser known heavily obscured AGN that are predicted to play a considerable role in the Cosmilogical make-up of the Universe. NuSTAR is the first X-ray observatory capable of focussing hard X-rays and generating an X-ray spectrum beyond 10 keV. Hence, in combination with many other lower-energy X-ray facilities this project will be able to study the broadband X-ray spectrum of heavily obscured AGN. In combination with multi-wavelength techniques encompassing the optical and infrared, the project will begin to construct a representative understanding of obscuration and accretion rates amongst local AGN, requiring minimal bias corrections.