Spectral and Temporal Studies of Supermassive Black Holes

Abstract

In this thesis, I present analysis and interpretation of the multiwavelength spectra and variability of Active Galactic Nuclei (AGN). The most luminous sustained sources in the Universe, these powerful objects are consistent with being the result of gas accretion on to central galactic supermassive black holes. Due to their compact sizes, the inner regions of AGN cannot be spatially resolved by conventional means, so we must instead use spectroscopy and temporal monitoring to determine their composition and structure.

I undertake a number of studies of the spectral energy distributions (SEDs) of AGN, using data from infrared-X-ray bands and employing a range of numerical models. Results from SED modelling of 11 moderate redshift (1.5<z<2.2) AGN are presented, in which there is a selection bias towards nuclei with cooler accretion discs. I find that the peak of the SED is sampled by our data for 5/11 objects, thereby breaking several of the model degeneracies that affected previous studies. This results in stronger constraints on the physical processes at work in these AGN, and provides a powerful tool with which I examine and discuss the relationships between the various radiating components, including those of the emission line regions, dusty torus and host galaxy. I then explore the nature of four 'hypervariable' AGN, for which the origin of their extreme variability is currently unknown. Through an investigation of their SEDs, I find that either an accretion rate change, or gravitational microlensing by a star in a foreground galaxy, are energetically consistent with the data.

The new insights provided by this work lead me to suggest several worthwhile routes for the future development of research in these areas. With the next generation of telescopes, satellites and surveys on the horizon, it will be possible to build on my results, to further our understanding of AGN.