School of Physics - Theses
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ItemMassive Black Holes( 2023-08)Black holes are one of the most fundamental astrophysical objects in our universe. In this thesis I look at massive black holes (MBH) with masses $10^{4}-10^{10}$ times that of our sun. In particular, I investigate how their gravitational influence distorts photon trajectories and describe how this can be used to study MBH. This phenomena, known as gravitational lensing, results in changes in shape and brightness of the images of the source as seen by a distant observer. The most striking manifestation of gravitational lensing is multiple images, known as \emph{strong} gravitational lensing. Strong gravitational lensing also results in the magnification of one or more of the images above that which would have been observed in the absence of deflecting matter. The number of cosmological black holes (MBH that do not belong to a galaxy core) is not well constrained. Gravitational lens statistics is one of the few ways to probe their number density. The fraction of sources experiencing strong gravitational lensing (multiple-image formation) is proportional to the number density of gravitational lenses which are able to form such images. GRBs are short bursts of $\gamma$-rays which signify the birth of a stellar mass black hole. Gravitational lensing of time-series data (light-curves) manifests as repetition of the primary signal as a lensed ``echo''. I describe the Bayesian parameter estimation and model selection software \pygrb{} which I wrote for this thesis. I use \pygrb{} to analyse GRB lens candidates from the Burst And Transient Source Experiment (BATSE) GRB catalogue to determine how similar the putative GRB lensed echo images are. I find one convincing candidate -- GRB~950830 -- which passes all our tests for statistical self-similarity. I conclude that GRB~950830 was gravitationally lensed by a $(1+z_l)M_l\approx\unit[5.5\times 10^4]{\msun}$ intermediate mass black hole (IMBH). Furthermore, based on the occurrence rate of this lensing event, I am able to estimate that the density of IMBH in the universe is $n_\textsc{imbh}=\unit[6.7^{+14.0}_{-4.8}\times10^{3}]{Mpc^{-3}}$. I also study the merger of black holes, looking at the recoiling quasar E1821+643 (E1821 hereafter). E1821 has a mass of $\mbh \sim \unit[2.6\times10^9]{\msun}$ and is moving with a line-of-sight velocity $v_\text{los}\approx \unit[2,070\pm50]{\kms}$ relative to its host galaxy. I use Bayesian inference to infer that E1821+643 was likely formed from a binary black hole system with masses of $m_1\sim 1.9^{+0.5}_{-0.4}\times \unit[10^9]{M_\odot}$, $m_2\sim 8.1^{+3.9}_{-3.2} \times \unit[10^8]{M_\odot}$ (90\% credible intervals). Given our model, the black holes in this binary were likely to be spinning rapidly with dimensionless spin magnitudes of ${\chi}_1 = 0.87^{+0.11}_{-0.26}$, ${\chi}_2 = 0.77^{+0.19}_{-0.37}$. I find that E1821+643 is likely to be rapidly rotating with dimensionless spin ${\chi} = 0.92\pm0.04$. Recoiling black holes are one method to populate the universe with massive black holes, however, these are expected to be rare. Massive black holes carry with them a tight cluster of stars and stellar remnants. These stars will pass through the optical caustic(s) of the black hole occasionally, which may lead to observable brightening of the star. Magnifications of greater than one million can easily be achieved, which I term ``Gargantuan Magnification Events'' (GMEs). I estimate the rate at which this lensing occurs, including the distribution of magnifications and event durations. I consider GMEs of pulsars in orbit of MBH as a possible generating mechanism for Fast Radio Bursts (FRBs). I find that pulsar GMEs are able to account for $0.1-1\%$ of the total FRB rate as observed by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) radio observatory. These seemingly unrelated problems all tied together in the end. This thesis is a study of black holes, their interaction with light and matter, and how they evolve through cosmic time. Many lifetimes of work have gone into generating the theory behind the sentence just prior. I hope that my contributions embellish these theories.
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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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ItemDark matter halos in the early Universe( 2016)We use high resolution N-Body simulations to study the properties of dark matter halos during the Epoch of Reionization. The halo concentration and spin parameters are measured in the mass range 10^8Msun/ h < M < 10^11M sun/h and redshifts 55 concentration-mass (c(M)) relation that is almost flat and well described by a simple power-law for both NFW and Einasto fits. The equilibrium state of the halo has a significant effect on the resulting concentrations. We also measure the spin distribution and spin mass relation, which has a weak dependence on equilibrium state. The spin virial mass relation has a mild negative correlation at high redshift. The correlation between the local density (the environment) of a halo and its formation history is examined. There is very little correlation between the formation time of a halo with local density, but some correlation between environment and the number of mergers the halo has experienced since formation.
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ItemGravitational lensing and clustering of galaxies in the epoch of reionization( 2015)The epoch of reionization marks the period where neutral hydrogen in the intergalactic medium was ionized by high energy photons emitted by the first stars and galaxies. Observations of galaxies during this period aim to uncover the types of stars and galaxies that were responsible for producing the ionizing flux to complete reionization within one billion years after the Big Bang (by z~6), and study the formation of the first galaxies in the Universe. These galaxies are observed in the near-infrared (NIR) today, and so require space-based observatories with sensitive NIR cameras such as Wide Field Camera 3 on the Hubble Space Telescope (HST). Considerable effort has been dedicated to ultradeep observations with HST in order to identify galaxies in the epoch of reionization. Surveys such as the eXtreme Deep Field (XDF), Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), Early Release Science (ERS) and the Brightest of Reionizing Galaxies (BoRG) have now discovered hundreds of these galaxies. This thesis makes use of these observations to address two important topics: how gravitational lensing affects the observations of such galaxies, and what their spatial distribution can tell us about the underlying dark matter distribution at this early time. Photons emitted by the first galaxies traverse most of the Universe’s history before reaching our telescopes. As such, they are subject to gravitational lensing by foreground galaxies along the line of sight. Gravitational lensing can result in the magnification of high redshift galaxies, skewing their observed luminosities. Because bright galaxies are significantly rarer than their fainter counterparts, the chance of gravitational magnification for observationally bright galaxies is significantly enhanced. This effect is known as magnification bias. We use the largest samples of Lyman-break galaxy (LBG) candidates observed in the first 1:5 billion years after the Big Bang (46.5. The clustering signal at z~7 is detected at >4 sigma, and corresponds to a real-space correlation length of r_0 = 6.7 +0.9/-1.0 cMpc, a galaxy bias of b = 8.6 +/- 0:9, and dark matter haloes of mass M = 10^(11:3+0:2/-0.3) M_sun . We reassess the clustering of LBGs at z=4–6 and find a trend of increasing bias from z=3.8 (b~3.0) to z=7:2 (b~8.6). We use these measurements to infer the fraction of dark matter haloes hosting UV-bright galaxies, and find that values near unity are preferred at z=7.2, which may be explained by the shorter halo assembly time at high redshift.
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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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ItemStudying the epoch of hydrogen reionisation in redshifted 21-cm radiation( 2011)The measurement of the spatial distribution of neutral hydrogen in the high-redshift intergalactic medium, through its 21-cm hyperfine transition, will revolutionise our understanding of the period in the evolution of the early Universe known as the cosmic dark ages. This period began once hot ionised gas, formed in the Big Bang, combined to form neutral gas, and lasted until the first stars, galaxies and quasars reionised most of the neutral hydrogen sometime between 400 000 to 700 million years after the Big Bang. The epoch of reionisation, driven by these first sources of light, is arguably the least understood, but one of the most important, periods in the evolution of the Universe. This thesis is an attempt to explain and explore some of the techniques we may employ in order to advance our understanding of this period and prepare for the first observations to come from a new generation of low-frequency instruments. Using a new efficient semi-numerical ionisation model to create simulations of the ionisation state of the intergalactic medium during the epoch of reionisation, we analyse the impact a percolating intergalactic medium has on redshifted 21-cm observations of high-luminosity quasar-generated regions of ionised hydrogen. We also investigate how a population of quasars modifies the 21-cm power spectrum during this period. The study of the reionisation history of hydrogen and, indirectly, the first galaxies will be limited not by raw sensitivity to the signal, but rather, by bright foreground radiation from Galactic and extragalactic radio sources and the Galactic continuum. We analyse the effect of non-polarised foreground subtraction on redshifted 21-cm observations, and demonstrate a method for removing polarised foregrounds using Faraday tomography.