| Summary: | Many studies have shown that the majority of accretion onto supermassive black holes; i.e., active galactic nuclei (AGNs) are hidden from our view by obscuring "torus" of gas and dust with column densities of N_H ≥ 10²² cm⁻². Arguably, the most efficient method of identifying AGNs is in the X-ray waveband, where even heavily obscured AGNs have been detected. However, a significant fraction of the AGN population have remained hidden in X-rays due to their extreme torus column densities along our line of sight, N_H ≳ 1.5 x 10²⁴ cm⁻²; i.e., Compton-thick (CT). These CTAGNs are predicted to be abundant but their census is far from complete, even in our local universe, due the challenge in identifying them because of their faint fluxes. In this thesis, I present an updated census of the CTAGN population and the N_H distribution of AGN in our local universe, using a volume-limited mid-infrared selected sample of 20 AGNs within D = 15 Mpc. The volume-limited selection within a relatively small volume means that the AGN sample is less limited by flux (unlike most AGN samples), and the mid-infrared selection means that it is unbiased against both torus and host galaxy obscuration (unlike X-ray and optically selected AGN samples, respectively). The N_H values for each AGN are directly measured, by performing broadband X-ray spectroscopy (up to ~2 orders of magnitude in energy range) using data from multiple focusing X-ray observatories, primarily NuSTAR in combination with Chandra and XMM-Newton. For cases in which this is not possible, I use indirect multiwavelength techniques to identify potential CTAGNs. These techniques involve comparing the observed 2-10 keV fluxes from the AGNs to the [OIII]λ5007Å, 12μm, and [NeV]λ14.32μm fluxes, which act as indicators for the AGN intrinsic emission. The CTAGN fraction that I found; i.e., 30⁺²³₋₁₄%, is significantly higher than that observed in the hard X-ray AGN survey by Swift-BAT, but agrees very well with that inferred from the Swift-BAT survey, after taking into account sensitivity effects. I demonstrate that we can identify intrinsically lower luminosity CTAGNs that are missed by the Swift-BAT survey (i.e., down to L₂₋₁₀,_int ~ 10⁴⁰ erg s⁻¹). I provide case studies on two newly identified bona-fide CTAGNs in our local universe (NGC 1448 and NGC 5643), and demonstrate the challenges in characterising the properties of low luminosity AGN, primarily due to significant dilution and contamination by the X-ray emission from the host galaxy. I compare the AGN and host galaxy properties of my sample with that of the Swift-BAT AGN sample, and found the following: (1) the star formation rates between the two samples are consistent with each other; however, (2) my sample has a wider range of AGN Eddington ratio, extending down to a lower Eddington ratio than the Swift-BAT AGN sample; (3) my sample also shows a more diverse optical nuclear spectral type; and (4) dominates at lower black hole mass and galaxy stellar mass.
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