School of Physics - Theses
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ItemNo Preview AvailableHydrogen in the first billion years: a study of the 21-cm signal from the high-redshift Universe( 2024-01)The history of our Universe is reflected in the state of its hydrogen (HI) atoms. After recombination (redshift z ~ 1000), the intergalactic medium (IGM) is composed primarily of neutral hydrogen (HI). The formation of the first stars and the first galaxies in the early Universe during Cosmic Dawn (z ~ 30) triggered the last major phase change of the HI. During the Epoch of Reionisation (EoR), the intense ultraviolet (UV) and X-ray radiation emitted by the first luminous sources carve out ionised hydrogen (HII) bubbles in the IGM. These HII bubbles expand and fill the whole Universe (z ~ 5). By altering the thermal and ionisation state of the IGM, the EoR directly impacts the subsequent formation and evolution of galaxies in the Universe. The 21-cm hyperfine spin-flip of HI is the primary probe of this period, and dedicated observational campaigns are ongoing/under construction to observe this redshifted 21-cm emission. Theoretical models must be on hand to interpret current upper limits as well as future observations. Semi-analytical models (SAMs) are well-suited for this purpose because of their computationally efficient and physically motivated prescriptions of relevant physics. Galaxy formation SAMs typically work by post-processing the dark-matter halo merger trees from dark-matter-only N-body simulations. This thesis updated the Meraxes SAM of coupled galaxy formation and reionisation in this thesis. Specifically, the explicit calculation of the spin temperature of the HI gas was implemented. This involves tracking the thermal state of the IGM, which is influenced primarily by the X-rays. This updated version of Meraxes was deployed on an N-body simulation of side 210 h^(-1) Mpc. Such large cosmological volumes are necessitated by the long mean free paths, ~ O(100 Mpc), of X-rays in the early Universe. At the same time, for a given number of particles, the mass resolution of an N-body simulation is inversely proportional to its volume. Hence, the simulations will not capture the full source population. To overcome this, the dark matter merger trees are augmented by introducing low-mass haloes into the simulations. The augmented simulation is one of the large-volume simulations in the literature that is simultaneously capable of resolving all atomically cooled haloes down from z = 20 and is sufficiently large enough to track the impact of X-rays on the thermal state of the IGM. Taking advantage of the computational efficiency of the Meraxes SAM, the impact of the galaxy X-ray luminosity on the 21-cm statistics, i.e. the 21-cm global signal and 21-cm power spectra (21-cm PS), are explored. Exploiting the large dynamic range of the model, the thesis also shows that the magnitude of the non-Gaussian term in the sample variance of the 21-cm PS is more than twice the magnitude of the Gaussian term at scales relevant to the upcoming Square Kilometre Array (SKA). The thesis then explores the astrophysical constraints that will be achievable with a future detection of the 21-cm PS. Using the Fisher matrix formalism, the fractional uncertainties in the model parameters enabled by a 21-cm detection spanning z in [5, 24] from a 1000 h mock observation with the SKA are forecasted. This work focused on the X-ray luminosity, ionising UV photon escape fraction, star formation and supernova feedback of the first galaxies. It is shown that it is possible to recover 5 of the 8 parameters describing these properties with better than 50 per cent precision using just the 21-cm PS. By combining UV luminosity functions with the 21-cm PS, we can improve our forecast, with 5 of the 8 parameters constrained to better than 10 per cent (and all below 50 per cent).
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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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ItemMeasuring the Epoch of Reionization Signal with Murchison Widefield Array( 2021)With the emergence of the first stars and galaxies at ~100 million years, the Dark Ages ended. The ultraviolet radiation from the first structures started to reionize the surrounding neutral hydrogen, creating ionized bubbles. Gradually the bubbles grew and merged until the intergalactic medium was completely ionized. This transition was a critical phase in the evolution of the Universe, called the Epoch of Reionization (EoR), spanning the redshift range of z ~10-6. Understanding the critical physics happening in this epoch answers many questions about the structure formation and evolution of the Universe. The observation of the neutral hydrogen 21-cm signal is an excellent probe for tracing the ionization process and studying the underlying physics happening therein. Technically, the EoR experiments utilise radio interferometry which provides the required sensitivity and resolution in the frequency range of interest. The sensitivity of current instruments allows us to statistically measure the power spectrum of the EoR signal, while the next generation of instruments is under development with the ultimate goal of direct imaging of EoR. This work measures the EoR signal at a redshift range of z~6-7 with Murchison Widefield Array (MWA), a radio interferometer located in Western Australia. However, detection of the EoR signal is a challenging procedure due to the low amplitude of the signal (~10mK), bright foregrounds (up to 4-5 order of magnitude brighter than signal), ionospheric distortions, Radio Frequency Interference and instrumental effects. An integration time of ~1500 hours MWA EoR data can potentially detect the EoR signal with an S/N of 14 [1]. However, signal detection is currently limited by the aforementioned systematics. They contribute to the power exceeding the thermal noise level. Therefore, detection of the signal requires the development of different strategies to overcome these challenges. This thesis analyzes the EoR data from MWA while developing, improving and providing insight into systematic mitigation approaches to obtain a more precise measurement. The MWA EoR observing program is targeted on three different EoR fields: EoR0, EoR1 and EoR2. In this work, we measure the signal from two fields. First, we calibrate the EoR data from the EoR0 field and develop a data quality metric for refining the data. As a result, the first deep measurements of power spectrum with MWA, using the RTS+CHIPS pipeline, with ~32hr integration is obtained. The lowest upper limit is Delta^2 <= 2.5x10^4 mK^2 at k=0.14 (hMpc)^-1 and z=6.5 which is consistent with previous results from other instruments. Next, we explore some strategies to mitigate the foreground contamination which is a major obstacle in detection of the EoR signal. We modelled the Galactic Diffuse Synchrotron Emission, the dominant foreground at the redshift of interest, over the MWA field. However, since the MWA is a wide field experiment, it requires a full sky model. Therefore, we explored another strategy, i.e. developing a weighting scheme for baselines based on the severity of their contamination. Another accomplishment in this thesis is the analysis of EoR data from the MWA EoR1 field which has a different foreground containing the bright radio galaxy of Fornax-A with a total flux density of ~500Jy at 189 MHz. A precise model of Fornax-A is essential for effective foreground removal. Using the imaging capability of the analysis pipeline and available shapelet fitting tools, the model of Fornax-A in our sky catalogue is improved. While improving our analysis algorithm, we made an effort to mitigate contamination in our measurements by detecting systematic signatures in the data and excluding them. We explored various features in the data, hunting for the source of systematic signatures. We also recognised the visibility noise RMS as a metric to distinguish the more contaminated data within a refined dataset. Eventually, we obtained the upper limits on the EoR signal power spectrum from the MWA EoR1 field at three redshift bands centered at 6.5, 6.8 and 7.1 with the lowest at z=6.5 of Delta^2 <= (73.78 mK)^2 at k=0.13 h Mpc^-1, from ~14 hr data integration. Although it contains a shorter integration time relative to the previous EoR1 analysis[2] (~19 hr), the limits are lower (~1.26 times), thanks to the improvements in the analysis algorithms, foreground modelling and data refinement strategies. We also compared the analysis results from EoR0 and EoR1 fields. It is shown that, due to the improvements achieved in this work, EoR1 can potentially lead to lower limits (at least on large scales) which warrants analyzing longer integrations from this field. In the final chapter, suggestions for further systematic mitigation and obtaining lower limits are provided.
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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 origin of matter and dark matter( 2018)Why is the mass density of dark matter throughout the universe similar to that of ordinary matter? Asymmetric symmetry breaking models can explain this apparent coincidence by exploring how dark matter could naturally have a similar mass to the proton and how the number density of dark matter particles could be almost the same as ordinary nucleons. In this work models of high energy scale mirror symmetry connecting the standard model to an exact copy are spontaneously broken to produce models of asymmetric dark matter, with composite dark matter candidates, that naturally solve the dark matter mystery.
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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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ItemThe accretion history of dark matter halos( 2016)The goal of this thesis is to (i) explore the physics that drives universal accretion history of dark matter halos; (ii) determine the relation between the halos accretion history and the halos internal structure; and (iii) disentangle the impact of halos accretion history on galaxy evolution. To address these topics, we first use the extended Press-Schechter (EPS) formalism to derive the halo mass accretion history (MAH) from the growth rate of initial density perturbations. We show that the halo MAH can be well described by an exponential function of redshift in the high-redshift regime. However, in the low-redshift regime the mass history growth slows down because the growth of density perturbations is halted in the dark energy dominated era due to the accelerated expansion of the Universe. As a result, in the low-redshift regime the halo MAH can be described by a power-law function of redshift. We complement this study with the analysis of MAHs of dark matter halos using a suite of cosmological simulations. We explore the relation between the density profile of dark matter halos and their MAHs, and confirm that the formation time, defined as the time when the virial mass of the main progenitor equals the mass enclosed within the scale radius, correlates strongly with concentration. We combine both analysis, analytic and numerical, to show that the halo MAH is the link between halo concentration and the initial density perturbation field. The connection found between the halo MAH and its density profile reached in these studies was vital to derive a semi-analytic, physically motivated model for dark matter halo concentration as a function of halo mass and redshift. Because the semi-analytic model is based on EPS theory, it can be applied to wide ranges in mass, redshift and cosmology. The resulting concentration-mass (c-M) relations are found to agree with the simulations, and because the model applies only to relaxed halos, they do not exhibit the upturn at high masses or high redshifts found by some recent works. We predict a change of slope in the z~0 c-M relation at a mass scale of 10^11 solar masses. We find that this is due to the change in the functional form of the halo MAH, which goes from being dominated by an exponential (for high-mass halos) to a power-law (for low-mass halos). During the latter phase, the core radius remains approximately constant, and the concentration grows due to the drop of the background density. We then connect the evolution of dark matter halos to the evolution of galaxies. We investigate the hot hydrostatic halo formation and its dependence on feedback mechanisms. We find that in the presence of energy sources like stellar feedback, the hot halo mass increases and the mass scale of hot halo formation is reduced. Active galactic nuclei (AGN) do not affect the hot halo as strongly. We develop a semi-analytic approach that makes use of both, the hot halo mass and the fraction of shock-heated gas, to calculate a `critical mass scale' for hot halo formation. We find that this mass scale, where the heating rate produced by accretion shocks equals cooling, is the point in mass above which halos develop a stable hot atmosphere. In the redshift range z=0-4, the critical mass is 10^11.7 solar masses, but it then increases for increasing redshift, in very good agreement with our numerical results. Finally, we investigate the physics that drives the gas accretion rate onto galaxies at the center of dark matter halos. We separately analyze the gas accretion rate onto the interstellar medium (ISM) and onto the galaxy. We find that the accretion rate onto the ISM remains roughly constant in halos larger than 10^11.7 solar masses, whereas the accretion rate onto the galaxy increases with increasing halo mass and flattens in the halo mass range 10^11.7-10^12.7 solar masses, and at redshifts z<2. The flattening is produced by the presence of the hot halo atmosphere that acts as a preventive feedback mechanism. We derive a physically motivated model of gas accretion onto galaxies that accurately reproduces the gas accretion rates from simulations. The model depends on the rate of gas cooling from the hot halo, on the fraction of shock-heated gas, and on the rate of cold gas accretion. We show that the rate of gas cooling from the hot halo is driven by the cooling radius, that it does not continuously decrease with increasing halo mass as generally thought. Instead, it decreases in the halo mass range 10^11.5-10^13 solar masses, and then increases with increasing halo mass, meaning that high-mass halos develop a hot halo cooling flow. We find that the upturn in the cooling radius is due to the change in the gas density profile, which is characterized by an evolving radial slope with halo mass.
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ItemLyα emitters as a probe of galaxy formation and ionisation history( 2016)Current observations suggest that the reionisation of hydrogen in the intergalactic medium had begun by z ∼ 10 and was completed around z ∼ 6. Directly observing this epoch is not possible with existing instrumentation, making it difficult to infer how the increased ionising background during this period affected galaxy formation. This thesis aims to put constraints on the galaxy formation history of the Universe with existing instruments, by modelling and observing the number densities of observed Lyα emitters in the ionised environments around z ∼ 2 − 3 quasars to mimic conditions found during the epoch of reionisation. The main work presented is a model for the ionisation state of the intergalactic medium around star forming galaxies in the vicinity of a luminous quasar, tuned by empirical relationships from conditions at z ∼ 2 − 3. This model suggests that the intense ionising radiation from a quasar offsets the increased density of the intergalactic medium found around it, implying that the direct detection of star forming galaxies by their Lyα emission in the vicinity of z ∼ 2 − 3 quasars is less obstructed by the intergalactic medium than galaxies in the field. The accuracy of this model is compared to existing Lyα galaxy surveys and found to be in good agreement. Discrepancies exist between the expected number of Lyα emitting galaxies this model predicts and the surveyed region around the super-luminous quasar PKS 0424-131, in which no Lyα emission was detected. The modelling done suggests that in order to be consistent with this null detection at the 68% (90%) level, galaxies below 2.5×10^12 M⊙ (4.2×10^12 M⊙) must be omitted. These results suggest that considerable radiative suppression of galaxy formation by PKS 0424-131 is taking place. This hypothesis is tested using observations made on the Baade telescope at the Las Campanas Observatory with the Maryland Magellan Tunable Filter. The unique suitability of tunable filters for the detection of high-redshift galactic Lyα emission is described in detail, along with their idiosyncratic calibration and data reduction processes. The adverse seeing conditions make it impossible to put limits on the impact of ionising radiation of galaxy formation using these observations, and an analysis of the factors that prevented detection is provided. Finally, suggestions are made for ways to improve the chance of success for future observations of this effect using tunable filters, as well as ways to remove spurious ghost reflections in the data analysis that are unique to tunable filter observations.
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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.