We find supermassive black holes in the central nucleus of all massive galaxies. They are thought to have grown to their present-day masses through accretion episodes. We observe these accretion episodes as Active Galactic Nuclei (AGN), including luminous quasars. AGN emit strongly across the entire electromagnetic spectrum, from radio waves to Gamma radiation. AGN represent natural
laboratories for high-energy and strong-gravity physics, are believed to influence the evolution of their host galaxies, and provide high-redshift cosmological probes. To make the most of these opportunities requires a detailed understanding of the physics and geometry of their central engines.
In this thesis I present four different studies relating to AGN accretion.
Our current model of AGN accretion involves an optically thick, geometrically thin accretion disk around the central black hole, which (in some cases) responds to illumination from a variable X-ray emitting region. The disk sizes inferred from UV-optical continuum reverberation mapping of local-Universe AGN are a factor ∼ 3 larger than those predicted by the thin-disk model. In Chapter 2 I explore the effects of diffuse continuum emission from the broad-line region on reverberation-based disk size estimates. I find that, to first order, diffuse continuum contamination can explain the observed excess delays relative to the thin-disk model.
Observations of changing-look AGN suggest that the accretion flow can be highly variable, such that a given AGN can lose and/or regain its broad line and continuum emission on timescales of ∼years. In Chapter 3 I present evidence that the changing-look AGN Mrk 590, which lost its broad-line emission between 2003-2006, has re-ignited and now displays an ultraviolet to X-ray
spectral energy distribution consistent with those of typical Type 1 AGN. The X-ray luminosity has returned to roughly half of its historic maximum, while the UV continuum emission has reappeared. The soft X-ray component is not detected in Swift XRT and NuSTAR observations, and has not returned to its previous bright state. I also present a reverberation mapping analysis during a recent X-ray flare, finding that the UV emission is correlated with the X-ray component and lags it by ∼ 2−3 days.
A subset of quasars display strong, heavily blueshifted UV absorption lines with velocity dispersions ΔV ≥ 2000 km s−1. These broad absorption line (BAL) quasars may represent ’ordinary’ quasars observed at a certain angle relative to the central engine. Alternatively, they may represent an early stage in a quasar’s lifetime, during which a ’cocoon’ of gas and dust is expelled from the central region. Both these scenarios have implications for the BAL host galaxies. If we observe BAL absorption depending on the inclination of the central engine, the properties of BAL and non-BAL host galaxies at a given AGN luminosity should be statistically indistinguishable. If BAL sources represent young quasars, we may expect to see signs of recent mergers or on-going star formation in their host galaxies, depending on the triggering mechanism for AGN activity. In Chapter 4 I present a study of the host galaxies of four FeLoBAL quasars. I find no evidence for starburst activity in their hosts, and demonstrate that our data are consistent with quiescent elliptical hosts or dust-obscured starburst galaxies.
In Chapters 5 and 6 I present UV-optical and X-ray SEDs for a sample of z ∼ 2 quasars, of which roughly half are radio-loud, as part of an ongoing effort to i) study the dependence of the observational properties of quasars on the SED shape, and ii) to investigate the origin of radioloudness in quasars. I demonstrate that our sample is representative of the broader quasar population,
in terms of UV-optical SEDs, X-ray emission properties, and inferred black hole masses. The radio-loud and radio-quiet sources in our sample are matched in terms of redshift and V-band apparent magnitude. I demonstrate that their optical-UV SEDs are statistically indistinguishable. I also present an exploratory study of the opportunities to investigate accretion disk physics for our z ∼ 2 sample. I find that observed-frame infrared observations are required in order to adequately constrain the accretion disk properties and to test the thin-disk approximation for these quasars.
In Chapter 7 I offer some closing remarks on future prospects for each of these research projects.