School of Physics - Theses
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ItemSimulations of source recovery and completeness in galaxy surveys at high redshift( 2018)The search for and characterisation of galaxies at high-redshift is a very active topic in Astrophysics. Thanks to advances in observations from space, the redshift frontier is approaching the epoch of formation of first generation objects. Thus, these samples of galaxies can give us insight into the processes that govern galaxy formation and evolution. One of the key observables used to characterise galaxy populations throughout the cosmic history is their luminosity function (number of galaxies per unit luminosity per unit volume), which requires knowledge and characterisation of the completeness and selection functions of a survey, in addition to the catalogue of discovered objects. In this thesis, we present a search for high-redshift galaxies (redshift z > 6) in two in the Hubble Space Telescope surveys, the Brightest of Reionizing Galaxies Survey (BoRG), and the Reionization Lensing Cluster Survey (RELICS) using a photometric selection technique (the Lyman break dropout selection). We aim at using the resulting galaxy candidates to estimate a new measurement of the luminosity function at z ~ 10. To achieve that, we develop GLACiAR, an open Python-based tool available on GitHub, which is designed to estimate the completeness and selection functions in galaxy surveys. The code is tailored for multiband imaging datasets aimed at searching for high-redshift galaxies through the Lyman Break technique, but it can be applied broadly. The code generates artificial galaxies that follow Sérsic profiles with different indexes and with customisable size, redshift and spectral energy distribution properties, adds them to input images, and measures the recovery rate. We finally apply GLACiAR to quantify the completeness and redshift selection functions for J-dropouts sources (redshift z ~ 10 galaxies). Our comparison with a previous completeness analysis on the same dataset shows overall agreement, but also highlights how different modelling assumptions for artificial sources can impact completeness estimates.
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ItemThe fundamental plane and peculiar velocities from the 6dF galaxy survey( 2012)Early-type galaxies (ellipticals and lenticulars) are observed to populate the relation known as the Fundamental Plane that links their effective radius, R_e, stellar velocity dispersion, σ, and mean surface brightness, I_e. We have measured Fundamental Plane parameters in the near-infrared J, H and K passbands for ~10^4 of the brightest early-type galaxies in the 6dF Galaxy Survey (6dFGS). We improve upon previous regression techniques used to derive the Fundamental Plane by developing a robust maximum likelihood algorithm for fitting the galaxy distribution in Fundamental Plane space with a 3D Gaussian model. We exploit this large near-infrared-selected sample of galaxies to investigate trends in the Fundamental Plane with stellar population, morphology and environment. The 6dFGS galaxies exhibit clear stellar population trends in Fundamental Plane space, with age varying most strongly orthogonal to the plane. Remarkably, none of the stellar population parameters vary along the long axis of the plane, which corresponds to luminosity density. The Fundamental Plane slopes show little variation with either morphology or environment, but the Fundamental Plane size zeropoint is systematically larger for galaxies in lower density environments and for early-type spiral bulges. We speculate that age drives all the trends with residuals about the plane through its correlation with environment, morphology and metallicity. Using the Fundamental Plane, we measure distances and peculiar velocities for ~10^4 6dFGS galaxies to form the largest and most homogeneous peculiar velocity sample to date. Using a maximum-likelihood approach, we measure the overall bulk galaxy motions from the 6dFGS velocity field for the local volume of the universe, finding broad agreement with the predicted velocity field constructed from the 2MASS Redshift Survey. The local volume out to 16 120 km/s is found to have a bulk motion of 337 km/s in the direction (l,b) = (313°±9°,14°±10°), in good agreement with the results of other recent studies. A comparison of the observed and predicted fields is used to constrain parameters relating the distribution of galaxies and matter. We obtain a redshift-space distortion parameter β = 0.29±0.06 and a bias parameter for the 6dFGS velocity sample of b = 1.69±0.36. The 6dFGS velocity field provides an independent probe of cosmological parameters defining models of large-scale structure formation. Next steps include: (i) combining the 6dFGS sample in the south with the SDSS sample in the north to obtain an all-sky velocity field; (ii) deriving additional constraints on cosmological parameters from the velocity power spectrum analysis; and (iii) comparing the Fundamental Plane distances and velocities for early-type galaxies with the Tully-Fisher distances and velocities for spiral galaxies that will be obtained with the WALLABY survey on the Australian SKA Pathfinder.
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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.