This thesis describes the application of a Monte Carlo radiative transfer code to accretion disc winds in two types of systems spanning 9 orders of magnitude in mass and size. In both cases, the results provide important new insights. On small scales, the presence of disc winds in accreting white dwarf binary systems has long been inferred from the presence of ultraviolet absorption lines. Here, the thesis shows that the same winds can also produce optical emission lines and a recombination continuum. On large scales, the thesis constructs a simple model of disc winds in quasars that is capable of explaining both the observed absorption and emission signatures – a crucial advance that supports a disc-wind based unification scenario for quasars. Lastly, the thesis also includes a theoretical investigation into the equivalent width distribution of the emission lines in quasars, which reveals a major challenge to all unification scenarios.

All massive nearby galaxies, including our own, host supermassive black holes. Active galactic nuclei (AGN) are seen when such black holes accrete, and when they produce powerful jets of synchrotron-emitting plasma, they are termed radio-loud AGN. The close correlation between black hole mass and galaxy bulge mass in elliptical galaxies indicates that AGN feedback may be the key to the regulation of galaxy formation. It is thus necessary to fully understand the structure of AGN, the way that they are fuelled, and their duty cycle, in order to study the feedback processes and get a clear picture of galaxy formation. In this thesis, independent methods are developed to constrain the accretion disk and radio jet angles to the line of sight. H IX emission from a sub-sample of high-redshift quasars is measured from near-infrared spectroscopy and modelled as sums of different components, including the characteristic double-peaked profile which results from a thin, rotating accretion disk. Comparing the models using Bayesian evidence, almost all quasars were found to have infrared spectra consistent with the presence of a disk. The jet inclination angles of the same set of quasars were constrained by fitting a model, including the effect of Doppler boosting and the receding torus model for dust obscuration, to the radio \ spectral energy distribution. The fitted disk and jet angles correlate strongly, and are consistent with a model in which the radio jets are launched orthogonally to the plane of the accretion disk, as expected if the jet is powered by energy drawn from the spin of the black hole. Both disk and jet angles correlate with the observed linear source size, which is a projection effect; when deprojected using the fitted angles, the distribution of source sizes agrees with a scenario in which the sources expand into the surrounding medium at a constant rate up to ~ 1 Mpc and then shut off, probably as the nuclei become quiescent. The accretion disk angle was found to correlate weakly with the low-frequency radio luminosity, which provides direct, albeit tenuous, evidence for the receding torus model.

Abstract: Black holes, lurking at the centers of distant galaxies, feed on gas to become quasars. The bulk of the powerful emission from radio-quiet quasars originates from two structures close to the central black hole: the accretion disk and the hot corona. The disk-corona relationship and its dependence on the rate of gas accretion onto the central black hole is not well understood, in part because of the lack of knowledge regarding the corona temperature, density and geometry. To gain a better understanding of the accretion physics that affect the growth and evolution of quasars, as well as the evolution of their host galaxies, I perform a large-scale study of quasar optical through X-ray spectral energy distributions (SED). I have cross-correlated optical DR5 Sloan Digital Sky Survey (SDSS) quasars with the XMM-Newton archive of X-ray observations to obtain 792 optically-selected quasars with X-ray observations, 473 of which have X-ray spectra. I investigate relations between accretion rate, optical and X-ray luminosity, and X-ray slope. I compare the observed correlations with population synthesis simulations to determine which correlations are intrinsic to the physics of quasar accretion, and which are simply due to selection effects or the consequence of another correlation. I discuss the results with respect to physical models. At low accretion rates, the disk-corona structure may change significantly. I investigate this possibility in the case of red quasars by using optical and X-ray information to disentangle the effect of absorption and low accretion rates on red quasar SEDs. I find that 7 out of 17 of the reddest SDSS quasars are not well described by absorption. Instead, the red optical colors appear to be intrinsic to the accretion physics, and are perhaps related to the low accretion rates ( L/L Edd -0.05) observed for these objects. By extending the intrinsically red quasar SEDs to the infrared and ultraviolet wavelengths, I constrain standard disk models. By combining studies of large-scale trends with case studies of quasar accretion under extreme conditions, I investigate current models of disk-corona interactions.

Observing the temperature profiles of accretion disks around black holes is a fundamental test of an important astrophysical process. However, angular resolution limitations have prevented such a measurement for distant quasars. We present a new method for determining the size of quasar accretion disks at a range of wavelengths, thus constraining their temperature profiles. The technique uses single-epoch, multi-wavelength optical and nearinfrared imaging of gravitationally lensed quasars in conjunction with X-ray imaging, and takes advantage of the presence of microlensing perturbations to the magnifications of the lensed images. The dependence of these perturbations on the angular size of the source, combined with the temperature structure of quasar accretion disks, causes the flux ratio anomalies due to microlensing to appear chromatic. This allows us to probe regions of the quasar that are too small to be measured by any other technique. We apply this method to observations of 12 lensed quasars, and measure the size of the accretion disk of each in 8 broadband filters between 0.36 and 2.2 microns (in the observed frame). We find that the overall sizes are larger by factors of 3 to 30 than predicted by the standard thin accretion disk model, and that the logarithmic slope of the wavelength-dependent size is ~ 0.2 on average, much shallower than the predicted slope of 4/3. This implies that the temperature is a steeper function of radius than the thin disk model predicts. With this new approach to determining quasar accretion disk sizes, we are thus able to rule out the standard thin disk model as the source of the (rest-frame) ultraviolet and optical continuum in these bright quasars.

Accretion disks, active galaxies, quasars, X-rays.