The accretion and obscured growth of supermassive black holes

Abstract

It is widely accepted that black holes at the centres of galaxies grow to become ‘super’massive through the process of accretion, though the exact mechanism by which black holes evolve across cosmic time is still poorly understood. Sensitive X-ray observations have enabled studies of the closest regions to growing supermassive black holes (aka Active Galactic Nuclei - AGN), and have revealed the majority are obscured by gas. Unfortunately though, the most obscured AGN are typically too enshrouded in thick gas to be detected in wide-field X-ray surveys. This has led to estimates of the fraction of heavily obscured AGN relative to the total population varying drastically between ∼ 20 – 70%, yet a determination of this fraction is essential to understand the evolution and demographics of all supermassive black holes. The first study presented in this thesis creates a basis for robustly probing extreme obscuration of accreting supermassive black holes. This was done with sensitive highenergy X-ray observations from the NASA NuSTAR observatory, in combination with broadband optical and infrared multi-wavelength diagnostics. The next step was to consider what coeval properties AGN obscuration has with the central supermassive black hole, and whether or not extreme obscuration can in some way be regulated by the central engine. This project was able to provide tentative evidence of an anti-correlation between intrinsic AGN luminosity and obscuration reprocessing, (the Compton-thick Iwasawa-Taniguchi effect) which potentially holds implications for future large surveys of AGN if the effect is not taken into account. There exists a known bias against the most obscured AGN, even in our very most local volume. However, an understanding of the distribution of AGN obscuration is essential for constraining the cosmic origin of the Cosmic X-ray Background. For this reason, the final chapter presents the NuSTAR Local AGN NH Distribution Survey (NuLANDS) – one of the largest legacy surveys of AGN currently underway with the X-ray space telescopes NuSTAR, XMM -Newton and Neil Gehrels Swift Observatory (> 4 million seconds of total exposure), which is aimed at finding the elusive Compton-thick AGN population. Via multiwavelength techniques, NuLANDS is targeting AGN representatively of the underlying population, and has thus already led to the discovery and classification of multiple heavily obscured AGN previously hidden at X-ray wavelengths. The initial results from the survey indicate that ≥ 30% of all AGN in the local Universe are heavily obscured, and that high-energy X-ray selection alone preferentially selects less-obscured AGN. As such, NuLANDS marks a major step in completing the local census of supermassive black hole growth, and will pave the way in quantifying the true distribution of obscured accretion in the local Universe

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Identifiers

ORCID for Peter Boorman: orcid.org/0000-0001-9379-4716

ORCID for Poshak Gandhi: orcid.org/0000-0003-3105-2615

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