School of Physics - Theses
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ItemThe host galaxies of high-redshift quasars( 2020)In the early Universe, we observe supermassive black holes with masses of up to a billion times the mass of the Sun, accreting at or even above the Eddington limit. These high-redshift quasars are some of the most luminous objects in the Universe, and raise many questions about the formation and growth of the first black holes. Investigating their host galaxies provides a useful probe for understanding these high-redshift quasars. In the local Universe, there are clear correlations between the mass of a supermassive black hole and the properties of its host galaxy, indicating a black hole--galaxy co-evolution. Exploring how these black hole--host relations evolve with redshift can give valuable insights into why these relations exist. Studying the host galaxies of high-redshift quasars thus provides vital insights into the early growth of supermassive black holes and the black hole--galaxy connection. In this thesis I use three techniques to study the host galaxies of high-redshift quasars: the Meraxes semi-analytic model, the BlueTides hydrodynamical simulation, and observations with the Hubble Space Telescope. Meraxes is a semi-analytic model designed to study galaxy formation and evolution at high redshift. Using this model, I study the sizes, angular momenta and morphologies of high-redshift galaxies. I also use Meraxes to study the evolution of black holes and their host galaxies from high redshift to the present day. The model predicts no significant evolution in the black hole--host mass relations out to high redshift, with the growth of galaxies and black holes tightly related even in the early Universe. I also examine the growth mechanisms of black holes in Meraxes, finding that the majority of black hole growth is caused by internal disc instabilities, and not by galaxy mergers. I then use the BlueTides cosmological hydrodynamical simulation to investigate the detailed properties of quasar host galaxies at z=7. I find that the hosts of quasars are generally highly star-forming and bulge dominated, and are significantly more compact than the typical high-redshift galaxy. Using BlueTides I make predictions for observations of quasars with the James Webb Space Telescope, finding that detecting quasar hosts at these redshifts may be possible, but will still be challenging with this groundbreaking instrument. Finally, I use observations from the Hubble Space Telescope to obtain deep upper limits on the rest-frame ultraviolet luminosities of six z~6 quasars. I also detect up to 9 potential companion galaxies surrounding these quasars, which may be interacting with their host galaxies. Observations with the upcoming James Webb Space Telescope are needed to detect quasar host galaxies in the rest-frame ultraviolet and optical for the first time.
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ItemNature of quasar disk-wind( 2019)The brightest persistent astrophysical sources in the universe are quasars, a group of active galactic nuclei (AGN) that appear star-like and radiate across all wavelengths. The emitted radiation is believed to be powered by a supermassive black hole at the core of a galaxy. Matter that falls into the black hole is being fed onto the accretion disk, heating up the disk in the process due to friction. A wind emanating from the accretion disk, or a disk-wind, appears ubiquitous in these objects and acts as one effective way to generate the spectral lines observed in the quasar's spectrum. The broad spectral lines, originating from the broad line region (BLR), show diverse properties, specifically in velocity shift, line width, and degree of asymmetry. Yet, the exact structure of the BLR has remained perplexing due to its small size, which means it is unresolved even with the current astronomical instrumentation. Thus, simulations are important. By developing a model of the BLR, an informative analysis of the line profiles allows us to explore some of the key questions about the BLR, emphasising the shape of spectral lines, the disk-wind BLR, and the orientation. We simulate line profile modelling using a simple kinematical disk-wind model of the BLR with radiative transfer in the high velocity limit. The model provides a framework to explore the characteristics of the emission line profile induced by the different geometries and kinematics of the BLR, including the opening angle of the wind and the geometry of the line emitting region. The effect of orientation in these systems is also examined. As a first step, we use the model to simulate a narrow outflowing disk-wind, which has been described in the literature. The primary objective is to determine whether the observed emission line properties are consistent with a narrow wind scenario. We find that the line profiles are more blueshifted for a narrow polar wind model as opposed to intermediate and equatorial models. When viewing at pole-on angles, the simulated emission lines show a narrower line width, which is asymmetric and more blueshifted than that viewed edge-on. The blueward shift of the line profile increases as the line-of-sight and wind intersect. The model is also able to recover a shorter time delay in the red or blue side of the line profiles, consistent with observational evidence in reverberation mapping studies. The second part of the thesis considers the properties of broad absorption line quasars (BALQs). These objects are rare and often display a blueward absorption trough relative to the emission line. One interpretation of the velocity offsets is the unification based on orientation, whereby a BAL is viewed within a constrained narrow wind angle. In order to test whether the BALQs and non-BALQs can be distinguished by their emission features, we conduct statistical tests and machine learning on the two populations. We find that their continuum and emission features are qualitatively similar, which contradicts the narrow disk-wind model in the geometric unification. Therefore, we propose a model of the disk-wind comprising a wide wind opening angle with multiple dense radial streams, where the BAL is detected when the line-of-sight crosses these streams. These findings have lead us to the discovery of a novel orientation indicator of quasars in the ultraviolet-optical regime. We propose a simple yet robust angle-of-viewing probe using the correlation between the velocity shifts and line widths. Our idea is shown to be qualitatively consistent with other orientation proxies. We also perform a wide angle disk-wind simulation and successfully retrieve the predicted correlation with inclination. In addition, we extend our model to estimate the bias in the virial black hole mass due to the scale factor f, which is related to the unknown nature of the BLR. Using a wide disk-wind configuration, we retrieve the f factors for a range of inclination angle. The f factor shows significant dependence with orientation, characterisation of the line width, and location of the emission region in the wind. Therefore, using a constant f value biases the estimation of the mass of the black hole.