School of Physics - Theses
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ItemQuasar Broad Emission Line Regions and Gravitational Microlensing( 2023-08)This thesis has focussed on hydrogen and helium emission line generation in the Broad Emission Line Region (BELR) of quasars and also on the gravitationally lensed quasar LBQS1009-0252. Quasars are a class of galaxy characterised by an active nuclear region in which a high rate of accretion onto a central supermassive black hole results in the release of vast amounts of broadband energy over a prolonged period of time. These incredible luminosities—often up to 100,000 times that of a standard galaxy—make quasars some of the most distant, and therefore earliest, sources ever observed in the universe. The relatively tiny size of the innermost region, however, precludes direct observation, and so the physics and geometry of quasars remains enigmatic. Emitted by the accretion disk, light interacts with surrounding gasses in the BELR—so-named because light which is reprocessed and re-emitted from this region tends to be significantly broadened due to wholescale Doppler motions of the gas. As the first gas to ‘see’ light from the accretion disk, understanding and constraining the nature and dynamics of the BELR remains of interest in many astrophysical fields. Following methods pioneered by Ruff (2012), microphysical simulations (such as those produced by the photoionisation code, Cloudy) can be combined with observed spectral data of real sources to model hydrogen line emission from the BELR. This research has confirmed and built further upon the methods presented in Ruff (2012) through the use of updated code and new, high-quality NIR spectral data of 14 quasar sources acquired from the Flamingos-2 (F2) instrument on Gemini South at the Gemini Observatory. The thesis has also gone a step beyond the original method and includes the modelling and analysis of helium emission lines for the first time. Broadly, the results lend support to the conclusions presented in Ruff (2012): hydrogen lines tend to be produced optimally in regions of low incident ionising flux and high gas number density. Helium lines also appear to follow this trend, clustering in a similar parameter space, although there appears to be a tendency towards a flatter distribution across the value for maximum gas number density. This suggests that despite similarities in their physical production and spectral appearance, there are some slight differences in the behaviour of hydrogen and helium emission lines in the BELR, and it is prudent to analyse them separately where possible. This thesis also examined the double-image gravitationally lensed quasar, LBQS1009-0252, via new and relatively high-resolution data from the Gemini Multi-Object Spectrograph (GMOS) at Gemini Observatory. The project investigated both the emission and absorption lines present in the spectra of components A and B, seeking to better understand the spectral differences apparent between the two images. The analysis confirms that the LBQS1009-0252 system likely consists of two images of the same, gravitationally lensed background quasar source, with a third component, LBQS1009-0252C, most likely a foreground and physically unrelated quasar. A comparison and analysis of the overall spectra and emission lines attempts to separate the effects of different light paths through the lensing galaxy, previously identified at z~0.869. A combination of differential extinction due to dust in the macrolens plus a minor component of microlensing is a reasonable explanation for the origin of chromaticity between the spectra of the two component images. The new dataset was of high resolution, allowing for the identification of many more absorption lines than had previously been catalogued. These have been matched to those previously classified as belonging to the lensing galaxy and another known absorber situated at z~1.627. Newly-observed lines were analysed to identify likely absorption species candidates, showing that the presence of at least one more intervening absorption system at z~1.116–1.117 is highly likely.
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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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ItemThe broad emission line region of quasars and gravitational lensing by early-type galaxies( 2012)This thesis has focused on predicting emission line flux ratios from the broad emission line region of quasars under different physical conditions, and measuring the dark matter fraction and total mass density slope within early-type galaxies using gravitational lensing. Quasars are the energetic cores of distant galaxies, and they reside in some of the oldest, most massive objects formed in the universe. Due to their incredible luminosity (as much as $10^5$ times greater than a typical galaxy), quasars can be observed at extremely large distances. Quasars have a unique spectrum, with bright, broad emission lines that are produced by photoionised gas that is close to the central super-massive black hole. Despite the prominence of these broad emission features, the gas physical conditions and the geometry of the emission region are poorly understood. Due to its small scale and large distance, the emission line region cannot be resolved directly — even with the most powerful telescopes — and simulations are required to understand the mechanism that produces the unique quasar spectrum. Using simulations of micro-physical processes, including photoionisation, the broad emission line flux ratios can be calculated for a range of gas densities and distances from the central black hole. Using the photoionisation code, Cloudy, hydrogen and helium line emission was over the range of possible broad emission line region conditions. The hydrogen and helium lines are of particular interest because the line emission has strong dependence on the gas number density and incident ionising flux, whilst having only a negligible dependence on several other free parameters of the model. These simulations were then used to find a set of interesting ratios that can be used to determine the limits on the upper limit on the gas number density, and outer radius of the emission region. This thesis demonstrates a new technique for determining the physical conditions of the broad line emitting gas in quasars, using optical and near-infrared hydrogen and helium emission lines. Near-infrared line ratios are advantageous, as they have a negligible dependence on the amount of internal dust. A locally optimally emitting cloud model of the broad emission line region was applied to four nearby (z $\sim$ 0.2) quasars from the Glikman et al. (2006) sample. By comparing simulated emission line ratios to measured ratios from optical and near-infrared spectroscopy, the physical conditions required to produce the observed emission lines were inferred. The model provides a good fit to three of the objects, and a fair fit to the fourth object. We find that low incident ionising fluxes ($phi <10^{18}$cm^-2 s^-1), and high gas densities (n>10^{12} cm^-3) are required to reproduce the observed line ratios. This analysis demonstrates that the use of composite spectra in photoionisation modelling is inappropriate; models must be fitted to the individual spectra of quasars. This thesis also derives properties of early-type galaxies using a joint gravitational lensing and stellar-dynamics analysis. The sample consists of 11 early-type galaxies from the Strong Lenses in the Legacy Survey (SL2S). The median deflector redshift is 0.5, making it the largest sample of intermediate redshift lenses that have been studied using a joint lensing and dynamics analysis. By combining measured redshifts and stellar velocity dispersions from Keck spectroscopy with lens models from Gavazzi et al. (2012, submitted), the total mass density slope inside the Einstein radius for each of the 11 lenses was derived. The average total density slope was found to be 2.16$\pm$0.9, with an intrinsic scatter of 0.25. The dark matter fraction for each lens within half the effective radius was also determined. The average projected dark matter mass fraction was found to be 0.42$\pm$0.08 with a scatter of 0.25 for a Salpeter initial mass function. By combining the SL2S results with those from previous studies, a mild trend in the cosmic evolution of the total mass density slope was found. This suggests that the total density profile of massive galaxies has become slightly steeper over cosmic time. If this result is confirmed by larger samples, it would indicate that either dissipative processes or off-axis major mergers play an important role in the growth of massive galaxies since a redshift of 1.