Towards a complete census of active galactic nuclei in the nearby Universe

As it is well established that almost all galaxies host a central supermassive black hole (SMBH), it is natural to ask, how many of these SMBHs are actively accreting? Studies aimed at answering this question can better define the fraction of nearby galaxies that harbour active galactic nuclei (AGNs), provide constraints on the growth of local SMBHs, and further our understanding towards the connection between AGN activity and galaxy evolution. In the first part of this thesis, we use high-quality mid-infrared (IR) spectroscopy to investigate the ubiquity of AGN activity in a complete volume-limited sample of nearby galaxies to D<15 Mpc. We present analyses based on the detection of high-excitation emission lines to unambiguously identify AGNs in even the most heavily dust-obscured and gas-rich galaxies. We find that almost half of the AGN population are not identified in large-scale optical surveys, most likely due to strong circumnuclear star-formation activity and/or extinction through the host galaxy. In the second part of this thesis, we use sensitive hard X-ray (2-10 keV) and mid-IR constraints to calculate bolometric luminosities of these D<15 Mpc AGNs and combine these luminosities with well-constrained SMBH masses to estimate relative mass accretion rates. We use these data to calculate the volume-average SMBH growth rate of galaxies in the local Universe and find that AGNs hosting SMBHs with M_BH ~ 10^6-10^7 M_o are dominated by optically unidentified AGNs. These relatively small SMBHs are acquiring a significant proportion of their mass in the present-day and are amongst the most rapidly growing in the local Universe. In the third part of this thesis, we using [NeV]3427A as an unambiguous indicator of AGN activity to assess the incidence of AGNs in a statistically significant galaxy sample derived from the Sloan Digital Sky Survey (SDSS). On the basis of [NeV], we find strong empirical evidence that the theoretical maximum starburst limit provides a good identification threshold for AGNs. However, we find that only ~27% of Seyfert galaxies have strong detections of [NeV]3427A in their optical spectroscopy. Using spectral stacking analyses we determine that the lack of high-ionisation emission line signatures are due to poor spectral sensitivity and/or high levels of dust extinction. In the final part of this thesis, we use a suite of mid-IR, optical and X-ray luminosity indicators to search for Compton-thick AGNs at z~0.03-0.2, a region of parameter space which is currently poorly constrained by deep narrow-field X-ray surveys. We use the XMM-Newton serendipity survey and the SDSS to select a sample of 14 candidate Compton-thick AGNs. On the basis of the optical [OIII], mid-IR [OIV] and 6um AGN continuum luminosities we conservatively find that the X-ray emission in almost half of our sample appear to be attenuated by a factor >15, i.e., they are likely to be obscured by Compton-thick material with N_H > 1.5 x 10^24 cm^-2. Under the reasonable assumption that our 14 AGNs are representative of the overall X-ray undetected AGN population in the SDSS-XMM parent sample, we estimate that >20% of the optical Type-2 AGN population are likely to be obscured by Compton-thick material. These Compton-thick AGNs identified in our sample harbour some of the most rapidly growing SMBHs in the nearby Universe. Overall, the techniques and results presented in this thesis have arguably provided the most complete and unambiguous census of AGN activity in nearby galaxies to date.