Theses - Institute of Astronomy

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Open Access
Spectral Energy Distribution Modelling of X-ray Selected AGNs and Their Host Galaxies
Marshall, Adam
The nature of the relation between active galactic nuclei (AGN), and their host galaxies have been observed in detail throughout the Universe. Such work has found an intrinsic link between central supermassive black hole (SMBH) masses, and host galaxy properties such as the velocity dispersion of stars, and bulge mass. However, the difference in scale between SMBH and their host galaxies has led to debate on how this relation might form, and develop over time. In order to aid in understanding the relation between AGN and their host galaxies, the work throughout this thesis has therefore focused on the development and implementation of a new SED fitting code, using an up-to-date AGN SED to accurately infer both AGN and host galaxy properties. To this end, we explore the intricacies involved in producing useful property inferences using a Bayesian MCMC fitting method, whilst working to avoid common issues such as bimodality and lack of convergence. We then perform SED fitting using our methods to 711 luminous X-ray AGN at 0.7 < z < 4.5 using 10-bands of optical and infra-red photometric data for objects within XMMSERVS. Using these fits, we study the relation between AGN X-ray luminosity and host galaxy stellar mass, along with our ability to predict emission line strength and morphology from photometry alone. In order to further understand the intricacies of SED fitting, we also provide a case study into the effect of AGN SED choice on host galaxy and AGN property inferences, by comparing our AGN SED to another commonly used template. In this work, we show that it is important to consider host galaxy contamination when trying to produce a pure AGN template, and the effect that this contamination can have on AGN and host galaxy property inferences. We also find that the use of lower resolution SEDs can lead to repercussions on property inferences such as host galaxy stellar mass, which may provide incorrect assumptions on the relation between AGN and their host galaxies.
Open Access
Investigating the Characteristics of Exoplanetary Atmospheres and Interiors
Nixon, Matthew; Nixon, Matthew [0000-0001-8236-5553]
The characterisation of exoplanets has made rapid progress in recent years, with observations of bulk properties such as mass and radius combining with detailed atmospheric spectroscopy to provide unprecedented insight into the nature of these remote worlds. However, these high-quality observations also require sophisticated modelling and analysis tools in order to maximise the scientific output from the data. In this thesis I present a number of advances in atmospheric modelling and retrieval, as well as internal structure models, which have been used to investigate the properties of a wide range of planets, from hot Jupiters to temperate mini-Neptunes. I conduct an assessment of the feasibility of supervised machine learning as a tool to carry out atmospheric retrievals of exoplanets. Retrieval methods commonly conduct Bayesian parameter estimation and statistical inference using sampling algorithms such as Markov Chain Monte Carlo or Nested Sampling. Recently several attempts have been made to use machine learning algorithms either to complement or replace fully Bayesian methods in order to improve computational efficiency. However, results from these algorithms sometimes disagree with contemporary Bayesian retrievals. To investigate this, I use the Random Forest supervised machine learning algorithm which has been applied previously for atmospheric retrieval. I extend the machine learning approach to develop a new algorithm, and demonstrate excellent agreement with a Bayesian retrieval of the transmission spectrum of the hot Jupiter HD~209458b. Despite this success, and achieving high computational efficiency, I still find that this machine learning approach is computationally prohibitive for high-dimensional parameter spaces that are routinely explored with Bayesian retrievals with modest computational resources. I discuss the trade offs and potential avenues for the future. I present \textsc{Aura-3D}, a three-dimensional atmospheric retrieval framework for exoplanet transmission spectra. \textsc{Aura-3D} includes a forward model that enables rapid computation of transmission spectra in 3D geometry for a given atmospheric structure and can, therefore, be used for atmospheric retrievals as well as for computing spectra from General Circulation Models (GCMs). In order to efficiently explore the space of possible 3D temperature structures in retrievals, I develop a parametric 3D pressure-temperature profile which can accurately represent azimuthally-averaged temperature structures of a range of hot Jupiter GCMs. I apply this retrieval framework to simulated JWST observations of hot Jupiter transmission spectra, obtaining accurate estimates of the day-night temperature variation across the terminator as well as the abundances of chemical species. I demonstrate an example of a model hot Jupiter transmission spectrum for which a traditional 1D retrieval of JWST-quality data returns biased abundance estimates, whereas a retrieval including a day-night temperature gradient can accurately retrieve the true abundances. The forward model also has the capability to include inhomogeneous chemistry as well as variable clouds/hazes. This new retrieval framework opens the field to detailed multidimensional atmospheric characterisation using transmission spectra of exoplanets in the JWST era. I also present a new internal structure model for super-Earths and mini-Neptunes that enables detailed characterisation of a planet's water component. I use my model to determine how the bulk properties and surface conditions of a water world affect its ocean depth, finding that oceans can be up to hundreds of times deeper than on Earth. I explore the region of mass--radius space in which planets with H-rich envelopes could host liquid H$_2$O. Such envelopes could contribute significantly to the planet radius while retaining liquid water at the surface, highlighting the exciting potential for habitable conditions to be present on planets much larger than Earth. I contribute to internal structure models of a number of sub-Neptunes whose atmospheres are set to be observed using JWST. Before such observations take place, it is vitally important to understand the interior structures of these planets, which strongly affects their possible atmospheric compositions. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the habitable-zone mini-Neptune K2-18b, for which I contribute the H$_2$O EOS. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be $\lesssim$6\%; spanning $\lesssim$10$^{-5}$ for a predominantly water world to $\sim$6\% for a pure iron interior. The thermodynamic conditions at the surface of the H$_2$O layer range from the supercritical to liquid phases, with a range of solutions allowing for habitable conditions. We also investigate the possible compositions of the pair of planets orbiting the star TOI-776. The bulk densities of TOI-776b and c allow for a wide range of possible interior and atmospheric compositions. However, the models indicate that both planets must have retained a significant atmosphere. Upcoming observations will revolutionise our understanding of these planets, helping to uncover the mysteries of the sub-Neptune population. Finally, I discuss the latest developments in exoplanet observations, and consider how these advances may further our understanding of worlds beyond our own.
Open Access
Probing Late Stages of Stellar Evolution with Gaia-selected Planetary Nebulae
Chornay, Nicholas; Chornay, Nicholas [0000-0002-8767-3907]
This thesis concerns the use of data from the Gaia satellite to study Galactic planetary nebulae. Gaia is a recent ESA mission measuring positions, parallaxes, proper motions, and photometry of nearly two billion stars in the Milky Way. Several thousand of these stars are passing through the planetary nebula phase, an important phase that low and intermediate mass stars experience towards the ends of their lives before becoming white dwarfs. The thesis has three main contributions. The first is the development and application of an automated technique for identifying the central stars of planetary nebulae in the Gaia catalogue. The second is a statistical method for identifying photometric variability in these central stars. Periodic photometric variability is often an indicator of binarity, which is now known to be key to the formation and shaping of a significant fraction of planetary nebulae. The method is validated through ground-based follow up observations. Finally, more precise distances from Gaia parallaxes are combined with newly obtained ground-based narrowband imagery to determine and investigate the planetary nebula luminosity function of the local Milky Way. Besides being an important extragalactic indicator, this distribution of nebula magnitudes is related to the evolution of planetary nebula central stars across different masses. Distances and photometry from Gaia allow the brightness of the nebula and the evolution of the central star to be related directly in unprecedented detail.
Embargo
Characterising Exoplanet Atmospheres with High-Resolution Transmission Spectroscopy
Langeveld, Adam; Langeveld, Adam [0000-0002-4451-1705]
Over 5000 exoplanets have been discovered in the last three decades, revealing a truly diverse population of worlds beyond our solar system. The field has accelerated rapidly towards characterising the atmospheres of exoplanets around the nearest and brightest stars with large observational surveys from ground-based and space-based instruments. This thesis contains an overview of state-of-the-art methods for detecting chemical species in the atmospheres of transiting exoplanets. In the first chapter, I briefly review the history, key insights, and modern techniques for detecting exoplanets and studying their atmospheres. The subsequent chapters are each based on specific aspects of the analysis of high-resolution transmission spectra in optical wavelengths, aimed towards answering key questions regarding detrending techniques and the atmospheric properties of highly irradiated giant exoplanets. I begin by assessing the robustness of telluric correction methods for removing contaminating telluric water and oxygen lines from high-resolution spectra. This is a challenging and critical task when observing from the ground since absorption from Earth’s atmosphere often overwhelms the exoplanetary signals by several orders of magnitude. I compare the performance of corrections made using a telluric spectrum derived empirically from airmass and using a model of Earth’s transmission spectrum, focusing specifically on sodium detections in the transmission spectrum of a hot Jupiter. Next, I explore the diversity of ten highly irradiated giant exoplanets with equilibrium temperatures ranging from around 1000 to 4000 K. I conduct a homogeneous survey of sodium absorption using high-resolution transmission spectra extracted from observations made with the HARPS and HARPS-N spectrographs. Using sodium as a tracer, I report uniformly measured atmospheric properties across the sample, including atmospheric heights, net day-night wind speeds, and sodium doublet line ratios. I report a new detection of sodium in one of the planets and confirm previously reported detections in the others. I also discuss how different assumptions and detrending processes can cause discrepancies in the measured absorption features, highlighting the importance of a homogeneous analysis when comparing results across multiple planets. I then use these results to perform a search for new global trends on planetary atmospheric properties, with the goal of understanding how the characteristics of the atmospheres change over a diverse sample of planets. I investigate how the homogeneously measured atmospheric heights (as probed by detections of sodium) vary with macroscopic planetary properties, revealing an empirical relationship describing the relative atmospheric heights as a function of planetary equilibrium temperature and surface gravity. I also use the sodium detections to measure net atmospheric wind velocities for all ten planets and discuss how these results can provide information about common underlying processes and dynamics. Finally, I investigate signatures of other atomic species in optical high-resolution transmission spectra of two ultra-hot Jupiters using the cross-correlation technique. In particular, I search for prominent metallic species that are expected to be present due to the extreme temperatures of these exotic atmospheres. I compare the atmospheric heights derived from high-significance detections of neutral iron between the two targets. I also discuss tentative inferences of asymmetric iron absorption features, where the signals from the atmosphere become progressively more blueshifted in the first half of the transit up to a constant value. This asymmetry is indicative of different atmospheric structure or chemistry between the morning and evening terminators. I conclude with final remarks on promising avenues for future research over the course of the next decade, where high-resolution spectroscopy will play a pivotal role in furthering our understanding about the atmospheric properties and processes of the diverse population of exoplanets.
Open Access
On the evolution of gaseous haloes in cosmological simulations of galaxy formation
Bennett, Jake
The study of gaseous haloes holds the key to understanding gas flows in and out of galaxies, which in turn determines how galaxies form and evolve. However, recent observations have revealed them to be very complex, with the presence of significant amounts of multiphase gas, driven by a range of physical process within galaxies as well as by the larger-scale environment. Simulations that realistically model this gas are therefore crucial to interpreting this new data, and ultimately helping us understand how galaxies assemble. However, many cosmological simulations sacrifice resolution in gaseous haloes to reduce their computational cost, meaning their ability to accurately resolve some of the key physical processes, such as turbulence and instabilities, can be limited. Using cosmological simulations, in this thesis I have investigated how gaseous haloes evolve over time. I have implemented a new shock refinement scheme to boost numerical resolution in gaseous haloes. Applied to a massive cluster progenitor, I found significant increases in cold gas mass in filaments and clumps, leading to more widespread star formation. In the hot halo there is a notable boost in turbulent velocities, leading to a doubling of non-thermal pressure support. With the FABLE suite, I studied how the hydrostatic mass bias and turbulent pressure support level in galaxy clusters change as they undergo major mergers and are host to powerful feedback episodes. I found that clusters rarely tend to be in hydrostatic equilibrium for long, with variations being driven by both merger activity and interestingly, at higher redshift, AGN feedback. Such insights could be important for future cluster mass estimates from X-ray and SZ observations, for use as cosmological probes. Finally, I simulated the growth of an ultramassive black hole of mass ∼ 10^10 M⊙ by z = 6. I have studied its impact on the gaseous halo around it, including how powerful AGN feedback can disrupt inflowing cold filaments and increase the thermal pressure of the surrounding gas. I found that strong AGN-driven outflows can deposit significant quantities of metals at very large distances from the central galaxy, which could be a unique signature of the presence of such ultramassive black holes in the very early Universe. Many of the effects predicted by the simulations described in this thesis have the potential to be observed (or ruled out) in the coming decade or two, with an incoming wealth of observational data from observatories like JWST, Athena, CMB-S4, SKA, and LISA. These data, combined with further developments in theory and simulation, will therefore revolutionise our understanding of gaseous haloes.
Open Access
Observational Insights into White Dwarf Planetary Systems
Rogers, Laura
In recent decades the number of known planets has escalated from the eight solar system planets to over 5000 exoplanets; the focus has now shifted to their characterisation. Spectroscopic studies of white dwarfs ‘polluted’ by planetary material are a unique laboratory allowing measurements of their bulk composition. This thesis focuses on these polluted white dwarfs to observationally investigate how the material ultimately accretes onto the white dwarfs, and the inferences made about the composition of exoplanetary bodies. These white dwarfs become polluted due to the scattering of exoplanetary bodies on star grazing orbits where they tidally disrupt, producing dusty debris detectable as excess infrared emission, and then subsequently accrete onto the white dwarf. The scattering and accretion are expected to be stochastic processes with variability predicted on human time-scales. Chapter 2 reports near-infrared (JHK) monitoring campaigns of the dust emission with the UKIRT/WFCAM and NTT/SOFI. Over timescales of hours, days, months, and years no statistically significant variation is found. Chapter 3 reports spectroscopic monitoring campaigns of the metal features in the white dwarfs using SALT/HRS and Magellan/MIKE. Across more than 10 years and thousands of sinking timescales, no unambiguous statistically significant variability in the amount of material in the photospheres of the white dwarfs is found. Both results agree that the processes driving the accretion do so at a constant rate. Polluted white dwarfs with infrared emission tend to be the most heavily polluted systems. Chapter 4 presents the first composition studies from a novel programme which identifies new heavily polluted white dwarfs discovered from their infrared excess. The planetary material that polluted these seven white dwarfs are broadly consistent with rocky material, but show some compositional and geological diversity. Some of the white dwarfs appear to have accreted a fragment of a larger core-mantle differentiated body. Using oxygen budgeting, two white dwarfs are discovered to have accreted oxygen-rich material which may imply water-rich bodies. Also, evidence points towards one of the white dwarfs accreting material from two distinct planetary bodies, this challenges the commonly used assumption that there is one body in the white dwarf's atmosphere at once.
Open Access
Hierarchical Bayesian Spectro-temporal Models of Type Ia Supernovae in the Optical and Near-Infrared: Understanding the Properties of Dust in Supernova Host Galaxies
Thorp, Stephen
With the Vera C. Rubin Observatory Legacy Survey of Space and Time, and the Nancy Grace Roman Space Telescope both on the horizon, Type Ia supernova (SN Ia) cosmology is about to undergo a paradigm shift. Whilst this new era has the potential to bring unprecedented constraints on the nature of dark energy, this will only be possible if we are able to overcome the considerable challenge of controlling the astrophysical sources of systematic uncertainty that will dominate the error budget of these future experiments. In this thesis, I develop and deploy a robust new hierarchical Bayesian framework for modelling the spectral energy distributions (SEDs) of SNe Ia in the optical and near-infrared (NIR). I apply this framework to detailed studies of the dust in SN Ia host galaxies – particularly the distribution of the dust law R_V. This model is able to leverage the optical and NIR in a principled manner, whilst coherently marginalising over all sources of uncertainty. I use this model to analyse the Pan-STARRS grid light curves of 157 low-redshift SNe Ia from the Foundation Supernova Survey. I use these data to robustly model the dust laws in SN Ia host galaxies, and to place constraints on the extent to which R_V differs between low- and high-mass hosts. I do not find statistically significant evidence for strongly different R_V between low- and high-mass hosts, and am able to place upper limits on the R_V population variance. My results indicate that observed correlations between SNe Ia and host-galaxy mass cannot be fully explained by dust. I follow this with an analysis of optical and NIR (B–H band; ≈3500–18000 Å) data of 86 SNe Ia from the Carnegie Supernova Project (CSP). I use this independent sample to uphold my previous results, place tighter constraints on the R_V population variance, and to investigate the dust laws for more-highly reddened SNe Ia (beyond the apparent B–V≤0.3 colour cut typically used in cosmological analyses). I also argue for the critical importance of combining optical and NIR data to break degeneracies when investigating the effect of dust on SNe Ia, and present a simulation-based demonstration of the challenges inherent in estimating R_V and its population distribution. I show that hierarchical Bayes is suited to overcoming these challenges. Finally, I analyse high-redshift data from the RAISIN (SN IA in the IR) Survey. These unique data include Hubble Space Telescope F125W and F160W observations for 37 SNe Ia with 0.22≤z≤0.61, providing the first sample of rest-frame NIR (YJ-band) observations at such high redshifts. I use these data to place constraints on R_V in the host galaxies of high-z SNe Ia. By combining the high-z RAISIN data with a low-z CSP companion sample, I am able to place limits on the possible redshift evolution of SN Ia host galaxy dust laws.
Embargo
Cosmology from the CMB and Lyman-α forest
de Belsunce, Roger; de Belsunce, Roger [0000-0003-3660-4028]
In this thesis, we study challenges arising in cosmological data analysis using data from the cosmic microwave background (CMB), remnant radiation from the Big Bang, and the Lyman-α forest, absorption features in spectra of distant quasars. A six-parameter standard model of cosmology, the ΛCDM model, can explain to great accuracy how the Universe evolved from a hot, dense state to the web of galaxies that we observe today. However, it leaves fundamental questions about the nature of dark matter and dark energy unanswered, despite these making up 95% of the observable Universe. The advent of large cosmological surveys present a unique opportunity to infer some of the fundamental laws governing our Universe. Extracting the full potential of this data set is an ongoing challenge because of its size and highly non-linear nature. In the first part, we present an end-to-end analysis pipeline for large-angular scale CMB data. We present novel foreground removal techniques, improved modeling of the noise and systematics in the data, and develop and extensively test novel likelihood-approximations. The accurate representation of likelihoods including systematics is challenging: exact likelihoods are either unknown or intractable. We present methods that show how to make reliable inference for the optical depth to reionization (τ) or primordial gravitational waves, parametrised by the tensor-to-scalar ratio (r), from large-angular scale CMB data from the Planck satellite. The methods presented range from exact pixel-based likelihoods, maximum-entropy-based semi-analytic likelihood-approximations to simulation-based, so-called likelihood-free, approaches to constrain cosmological parameters. We exhaust current CMB data sets with the developed methods and discuss their potential for next-generation surveys of the CMB. The upcoming Dark Energy Spectroscopic Instrument (DESI) survey will measure spectra for tens of millions of galaxies and quasars, constructing a three-dimensional map spanning the nearby Universe to 11 billion light years. The Lyman-α forest consists of a series of absorption lines that map the distribution of neutral hydrogen in the intergalactic medium. This allows us to probe the matter distribution of the Universe at intermediate redshifts 2 ≤ z ≤ 5. In the second part of this thesis, we present and develop new continuum fitting methods to extract the un-absorbed flux of a quasar spectrum. We will discuss methods to constrain cosmology using this rich new data set.
Open Access
Characterizing sub-mm observations of protoplanetary disks at super-resolution scales
Jennings, Jeffrey
Current advancements in telescope and instrumentation technology enable us to observe planets as astrophysical objects across epochs that span Gyr of evolution, from their formation in protoplanetary disks and continued growth in debris disks to their dynamical evolution in exoplanetary systems and ultimate accretion onto white dwarfs. Measurements in each of these eras can be used to inform study in the others. Interferometry affords the highest angular resolution of any observing technique in astronomy, and the use of radio interferometry with instruments such as the Atacama Large Millimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) is markedly advancing the protoplanetary disk field. By further improving our methods to reconstruct high fidelity (in terms of both resolution and sensitivity) images from the interferometric observable, we can not only characterize these disks, but detect the dynamical effects of planets within them. Over a large ensemble of sources, this offers the potential to both progress disk science and connect inferences on the embedded planet population to the study of these objects in later epochs. This thesis centers on a new imaging framework for radio interferometric observations and its specific application to detect and characterize substructures in protoplanetary disks. Chapter 1 introduces the basics of protoplanetary disk theory and observations, with a focus on the principles of radio interferometry and its application for disk science. Chapter 2 then presents Frankenstein (frank), the open source code we have developed and applied to fit sub-mm observations of disks in order to search for substructure. Chapter 3 applies frank to the high resolution (30 mas) DSHARP mm sample of 20 disks to identify new substructure in these sources and more accurately constrain known disk features. Major results include discovery of more structured inner disks (at separations within 30 au of the host star). In Chapter 4 we apply frank to the moderate resolution (120 mas) Taurus mm survey, finding that compact disks (those with radii <50 au) routinely exhibit substructure. Chapter 5 concludes by summarizing the frank algorithm and our novel scientific results with this tool, then places this evolving imaging approach in the context of future disk science.
Open Access
The complex interplay between AGN jet feedback and galaxy evolution
Talbot, Rosemary
Supermassive black holes (SMBHs), residing in the nuclei of most galaxies, are capable of imparting extreme amounts of energy to their surroundings. Observations indicate the presence of collimated, relativistic jets as well as powerful, multiphase winds emanating from these active galactic nuclei (AGN). Whilst the clearest observational signatures of AGN jet feedback are found in galaxy clusters, there is growing evidence that these jets are, in fact, largely ubiquitous. Indeed, AGN jets have now been detected in a wide variety of galaxy contexts including dwarfs, Seyferts and ellipticals. Looking towards the future, observational facilities such as JWST, SKA and Athena will greatly expand our understanding of AGN physics by facilitating investigations of SMBH jet feedback at earlier times, lower luminosities and with higher spatial and spectral resolution. Additionally, the LISA mission will expand the field of multimessenger astronomy to the low-frequency range, detecting gravitational waves from the coalescence of SMBHs all the way back to cosmic dawn. Exploring the coupled evolution of AGN and their host galaxies is a highly non-linear, multiscale, problem and, thus, lends itself to exploration via numerical simulation techniques. For numerical simulations to provide firm theoretical predictions for, as well as accurately interpret the wealth of data from these next-generation facilities, however, it is vital that physically motivated models of AGN feedback are applied in realistic galaxy formation scenarios. In this thesis I present a novel numerical scheme to model SMBH spin-driven jet feedback in galaxy formation simulations. I then apply this model, first to simulations of isolated Seyfert galaxies and then to simulations of the major merger of gas-rich galaxies at cosmic noon, exploring how AGN jet feedback affects the properties and evolution of the host galaxies and the SMBHs themselves. In Chapter 1, I provide an overview of the relevant scientific concepts and historical events that set the scene for the work which is presented in the subsequent chapters. In Chapter 2, I introduce my model for black hole accretion through a (warped) α-disc and feedback in the form of a Blandford-Znajek jet and describe its numerical implementation. In the remainder of Chapter 2, I perform benchmarking of the model and explore the effects of AGN jet feedback on the central regions of a typical radio-loud Seyfert galaxy. Chapter 3 follows on directly from Chapter 2 and significantly expands the simulation suite presented therein. Using these simulations, I explore how varying the initial black hole spin magnitude and direction, the density of the medium into which the jet propagates and the rate at which gas is funnelled towards the black hole affect the evolution and properties of the jet-driven outflows. Then, in Chapter 4, I apply my spin-driven AGN jet model to black holes at the centres of gas-rich galaxies undergoing a major merger at z ≈ 2. With these simulations, I explore how the AGN jets affect the stellar component, the gaseous haloes of the galaxies and the dynamics of the merger. Additionally, I examine how the AGN jets self-regulate when subject to the extreme environments present in such mergers. Finally, in Chapter 5, I bring together the main conclusions from these three studies, before discussing some future prospects for work in this field.
Open Access
Automating Searches for Gravitationally Lensed AGN in Wide-Field Surveys
Desira, Christopher
Gravitational lensing is a rare phenomenon which requires the exact alignment between a distant source and a massive foreground galaxy. The light from the source is bent around the galaxy forming spectacular rings, arcs and multiple images which have captured the public imagination since their discovery in 1979. This thesis focuses on searches for lensed systems where the background source is an active galactic nucleus (AGN) - a galaxy powered by their central supermassive black hole (SMBH). Gravitational lensing of the most distant and luminous subclass of AGN, known as quasars, provide a unique way to study the universe. They have been used to constrain the Hubble constant (H0), probe the dark matter content of galaxies and uncover the proposed co-evolution between SMBHs and their hosts. Until recently, these studies have been hindered by low sample sizes. However, increased research into automated approaches such as machine learning, combined with a wealth of data from new wide-field surveys, will enable us to dramatically increase the lensed quasar population. In this thesis we develop a number of searches leveraging classic colour selection as well as supervised machine learning techniques. We target quadruply imaged and high-redshift lensed quasars using photometric data from DES, WISE and Pan-STARRS, together with Gaia astrometry. Candidates were confirmed with follow-up spectroscopy, resulting in the discovery of several new systems. A search for radio-loud AGN was conducted using newly released VLASS data and candidates were prepared for high spatial resolution radio confirmation. We discuss the serendipitous discovery of an exciting lensed radio-loud galaxy with extreme high ionisation UV line emission and damped Lyα absorption. We finally outline a deep learning approach to automate the visual inspection stage of our lens searches in preparation for the release of new surveys such as LSST and Euclid.
Open Access
The Astrometric Contribution of Unresolved Stellar Companions
Penoyre, Zephyr
No astronomical body is alone - some fraction of the mass of a system will always be found in orbiting bodies, ranging from fragmented debris to planets, stars and compact objects. In general we ‘see’ the brighter component: one star often dominates the detectable light, obscuring the companion - and we must turn to indirect methods to infer its presence. Astrometry, the mapping of the motion of stars across the sky, is one such method. A significant mass concentrated in one or more collapsed companions will perturb the path. In the simplest and most ubiquitous case, a binary system, this perturbation is regular and easily described - a metronome beat added to the celestial motion. However, the combination of a star’s proper and parallactic motion with a binary orbit is non-linear, biasing our measurements of the system’s distance and motion and introducing extra astrometric noise. In this thesis we will detail the astrometric impact of a binary companion, how it changes our inferences about systems assumed to be single bodies, and how we can detect companions through the deviations from single body fits. This is especially relevant to the current and ongoing Gaia survey, which is building astrometric tracks at unprecedented (sub milli-arcsecond) precision and for the first time allowing us to detect a large number of unresolved but astrometrically significant binary systems. We present analytic expressions for the binary contribution, as well as an exploration of the behavior of simulated systems and detections of real binary candidates in the local galaxy.
Open Access
Binary evolution in stellar dynamics
(1972) Heggie, Douglas Cameron
Computational studies of the classical gravitational N-body problem have demonstrated the importance in bound systems of energetic binaries formed dynamically. Since analytic and some numerical studies generally neglect three-body interactions, by which these form, it is desirable to establish what role they play in the evolution of a system. Many wide pairs are to be expected, and theoretical results on their properties, such as the distribution of eccentricities, are confirmed by the results of numerical experiments, but their dynamical importance is slight. Without a detailed consideration of encounters, it seems impossible to predict the numbers of close pairs, which must be time-dependent, and ·special rate functions are introduced to describe the evolution of the distribution of their binding energies. While constraints may be laid on these on general grounds, approximate solutions of the three-body problem are generally required in their evaluation. Encounters with a wide pair are impulsive, and a reasonably complete theory is possible. Regularised equations of motion facilitate a theory of wide encounters with close pairs and show that the eccentricity distribution relaxes much more rapidly than that of energy, but otherwise only rough results are available. "Exchange" events may occur, or a bound triple system may form, these being treated by a statistical mechanical argument. Many of the results are confirmed hy specially designed nurnerically experiments, using regularisation. If a cluster contains few close pairs initially, binary evolution is forced by that of the cluster as a whole under collisional relaxation, although the inevitable development of energetic pairs is accompanied by the ejection of fast escapers. Encounters have had little disruptive effect on binaries in the Galaxy within its lifetime. If binaries are formed in similar abundance in clusters, their evolution could force that of the cluster. Systems with a large initial population of binaries deserve further study.
Open Access
High-resolution studies of exoplanetesimal belt formation and evolution
Bennett Lovell, Joshua
Stars are born with gas-rich protoplanetary discs that typically survive for a few Myr before dispersing, after which they are left with the planetary bodies that formed within the protoplanetary system: planets, and belts of planetesimals (km-sized rocky and icy bodies). Planetesimals within belts readily collide and produce detectable levels of dust and gas, known as debris discs, around 20% of main sequence stars. Despite the ubiquity of planets and planetesimal belt detections, these appear quite unlike the planetary system around our Sun. In my thesis, I explore how this planetary system diversity arises, with high-resolution observational studies of planetesimal belts around young stars immediately after the dispersal of the protoplanetary disc (class III stars), and around stars on the main sequence. I introduce the background to my research in chapter 1. I present my published results analysing an ALMA survey of class III stars in chapter 2. In chapter 3, I present a detailed investigation of CO gas observations around the class III star, NO Lup. In chapter 4, I present ALMA and HST images of the debris disc around the main sequence star q1 Eri. Finally, in chapter 5, I summarise my key findings. By observing planetesimal belts at the class III epoch, I interpret these as having formed rapidly within the 2Myr age of Lupus and discuss this in the context of recent theoretical work on planet and planetesimal formation pathways. By analysing the gas of NO Lup, I show this to be outflowing, the first such example at the class III epoch, and discuss this in the context of recent theoretical work on disc dissipation mechanisms, and planetary system formation. By exploring late-stage planet-belt interactions, I set new constraints on the planetary architectures around q1 Eri after 1.4Gyr of evolution. I conclude in my final chapter by summarising these results in the context of recent work by others, and discuss future work to take these analyses further, either with existing instrumentation, or with upcoming/proposed telescopes/observatories.
Open Access
The origin and evolution of warm exozodiacal dust
Rigley, Jessica
Many stars show excess mid-infrared emission which is attributed to warm dust in the habitable zone of the star, known as exozodiacal dust, or exozodis for short. Such dust will be a source of noise and confusion when attempting to detect and characterise Earth-like planets. Therefore, an understanding of exozodiacal dust is crucial to our search for habitable planets and life. In this thesis, I present theoretical models for the origin and evolution of warm exozodiacal dust. Observations find a strong correlation between the presence of warm habitable zone dust and cold belts of planetesimals similar to the Solar System’s Kuiper belt. Given this correlation and the short lifetime of dust grains close to the star, it is probable that exozodiacal dust originates further out in the planetary system and is transported inwards. One possible transport mechanism is Poynting-Robertson (P-R) drag, which causes dust grains to lose angular momentum and spiral in towards the star. Initially, I develop an analytical model for the interplay of P-R drag and catastrophic collisions in a debris disc which predicts the levels of exozodiacal dust dragged into the habitable zone of a star from a cold outer belt. I show that detectable outer belts should produce exozodi levels tens of times higher than our zodiacal cloud via P-R drag, but these levels are insufficient to explain a large fraction of exozodiacal dust detections. In-depth application of the model to the exozodi of β Leo suggests the presence of an additional, warm asteroid belt to explain the radial profile of habitable zone dust. An alternative mechanism is inward scattering of comets, which spontaneously fragment to produce dust. I then develop a numerical model for the zodiacal dust produced by spontaneous fragmentation of Jupiter-family comets in the Solar System. This is able to produce enough dust to sustain the zodiacal cloud, and give the correct radial and size distribution of dust. I show that cometary input to the zodiacal cloud should be highly stochastic, depending on the sizes and dynamical lifetimes of comets scattered in. The comet fragmentation model is then extended to be applicable to other planetary systems, taking into account the different dynamical effects. This model will show how much dust comets produce and its evolution after being released from a comet to give exozodi radial profiles. Finally, I summarise the work in this thesis, and discuss the future outlook and my planned projects for furthering our understanding of exozodiacal dust.
Open Access
Telescopes and Techniques for the High-Resolution Characterisation of Exoplanets
Hawker, George
In this thesis I present two strands of research relating to the characterisation of exoplanets in high resolution. The first investigates high-resolution Doppler spectroscopy (HRDS) – a technique for detecting chemical species in exoplanet atmospheres. I present results from analysing high-resolution spectra of three hot Jupiters – HD 209458b, HD 189733b and 51 Pegasi b. Evidence for H2O, CO and of HCN was found in all three datasets – the latter molecule had not previously been found in an exoplanet atmosphere. These studies used HRDS analysis techniques common to the literature but put their associated hyperparameters on an explicit, quantitative footing. This meant different techniques could have their robustness probed and be compared. Doing so revealed that the removal of telluric contamination from infrared spectra should be treated carefully as it has the potential to introduce biases in detections of chemical species. If robust though, such HRDS detections could aid the tracing of exoplanets’ formation and migration histories which are thought to leave significant imprints on their present day atmospheric compositions. The second strand of my research is in developing SUPER-SHARP, an optical alignment technology for use in future large aperture unfolding space telescopes. Such facilities and technologies are needed for conducting the high spatial resolution observations required for characterising Earth-like exoplanets. In this work I have developed an automated alignment routine for telescopes with segmented primary mirrors. Starting from methods documented in the literature, I derived the necessary theory and validated the routine using numerical and ray tracing simulations. I then tested it experimentally by implementing the alignment routine – producing a proof-of-concept self-aligning telescope capable of capturing diffraction-limited thermal infrared imagery. This technology demonstrator has an autonomously aligned segmented primary mirror. It achieves sub-micron alignment through the use of optical metrology and nano-positioning actuators driven by my software in a closed loop control system. Finally, I also prototyped and tested potentially novel primary mirror optics that were invented to improve the compactness/practicality of this alignment technology.
Open Access
Thermal and Radiative Properties of Exoplanet Atmospheres Across the Mass Range
Piette, Anjali
Observations of exoplanet atmospheres have flourished in recent years, revealing a remarkable diversity of thermal, chemical and dynamical conditions. In particular, thermal emission observations of such atmospheres provide unique insights into their temperature profiles, chemistry and energy transport mechanisms. In this thesis, I explore the radiative and thermal conditions in exoplanets across a wide range of masses and irradiation conditions, from isolated brown dwarfs to rocky exoplanets. I begin by investigating important considerations for the atmospheric retrieval of isolated brown dwarfs. These objects provide remarkable laboratories for understanding atmospheric physics in the low-irradiation regime, and can be observed more precisely than exoplanets. As such, they provide a glimpse into the future of high-SNR observations of exoplanets. I introduce novel retrieval methods for isolated brown dwarfs, including a new temperature profile parameterisation and a method for including model uncertainty. I demonstrate this retrieval framework on both simulated and real data, showing that excellent precisions can be achieved on the inferred chemical abundances. I further investigate the temperature profiles and thermal emission spectra of hot Jupiters, including thermal inversions in their dayside atmospheres. In particular, TiO has long been proposed to cause thermal inversions in hot Jupiters and its spectral features in the optical and near-infrared have been detected. I investigate how TiO detections can depend on the molecular line list used to interpret the data, and how this sensitivity varies across different types of atmospheric observations. I also explore the occurrence of thermal inversions due to TiO and assess the performance of photometric metrics used to quantify them over a range in chemical composition, irradiation, gravity and stellar type. Recent optical observations of hot Jupiters in secondary eclipse are providing new constraints on their albedos and cloud/haze scattering. I investigate this for three hot Jupiters spanning a range of temperatures and gravities: KELT-1 b, WASP-18 b and WASP- 43 b. Using self-consistent atmospheric models, I interpret the optical to infrared spectra of these three targets and find that they can be explained by pure thermal emission, without the need for optical scattering by clouds or hazes. Furthermore, I find that inefficient day-night energy redistribution is needed to explain the high infrared fluxes from the daysides of these planets, consistent with previous works. I then explore the atmospheric conditions in mini-Neptunes, whose observations are beginning to provide constraints on their chemical and thermal properties, while also providing clues about their interiors and potential surfaces. With their relatively large scale heights and large planet-star contrasts, mini-Neptunes are currently ideal targets towards the goal of characterising temperate low-mass exoplanets. I explore the effects of irradiation, internal flux, metallicity, clouds and hazes on the atmospheric temperature profiles and thermal emission spectra of temperate mini-Neptunes. Building on recent suggestions of habitability of the mini-Neptune K2-18 b, I find a range of physically-motivated atmospheric conditions that allow for liquid water under the H2-rich atmospheres of such planets. I find that observations of thermal emission with the James Webb Space Telescope (JWST) can place useful constraints on the habitability of temperate mini-Neptunes such as K2-18 b. These results underpin the potential of temperate mini-Neptunes such as K2-18 b as promising candidates in the search for habitable exoplanets. Finally, I explore the characterisation of rocky exoplanet atmospheres across a wide temperature range. JWST will allow unprecedented characterisation of the atmospheres of small, rocky exoplanets. In particular, emission spectroscopy is an ideal technique to observe the secondary atmospheres of rocky exoplanets as such observations are not limited by the small scale height of high-mean-molecular-weight atmospheres. I develop a new retrieval framework tailored for rocky exoplanet atmospheres with unknown atmospheric constituents, and use this to assess the observability of promising, known rocky exoplanet targets. I find that the atmospheres of several known rocky exoplanets across a range of temperatures can be characterised using JWST.
Open Access
The creation and use of a chemical kinetics code to model and understand the atmospheres of hot Jupiters
Hobbs, Richard
Chemical compositions of exoplanets can provide key insights into their physical processes, and formation and evolutionary histories. Atmospheric spectroscopy provides a direct avenue to probe exoplanetary compositions. However, whether obtained in transit or thermal emission, spectroscopic observations probe limited pressure windows of planetary atmospheres and are directly sensitive to only a limited set of spectroscopically active species. It is therefore critical to have chemical models that can relate retrieved atmospheric compositions to an atmosphere's bulk physical and chemical state. To this end we present Levi, a chemical kinetics code for modelling exoplanetary atmospheres. Levi is constructed as a eulerian solver of a series of coupled 1-D continuity equations for the evolution of molecular species. Levi is able to calculate the gas phase hydrogen, oxygen, carbon, and nitrogen chemistry in hot Jupiters to produce abundance profiles of the planets’ atmospheres. We perform extensive testing of Levi to ensure its accuracy, including investigations of how both the boundary and initial conditions of the model could affect the final result, as well as comparing the thermochemical processes to the diffusional and photochemical processes. Levi underwent benchmarking by testing it against already existing codes of the same type, to ensure the code was consistent with previously produced models. We use Levi to run a suite of models across a range of metallicities to produce the abundance of a number of molecules in any hot Jupiter's atmosphere. Our parameter sweep covers metallicities between 0.1x and 10x solar values for the C/H, O/H and N/H ratios, and equilibrium temperatures of hot Jupiters between 1000K and 2000K. We link this parameter sweep to hot Jupiter formation and migration models from previous works to produce predictions of the link between molecular abundance and planet formation pathways, for the spectrally active molecules H2O, CO, CH4, CO2, HCN and NH3. We investigate the detections of numerous molecules in the atmosphere of HD 209458b, and find that within the framework of our model, the abundance of these molecules best matches with a planet that formed by gravitational instability between the CO2 and CO snowlines and underwent disk-free migration to reach its current location. We next extend the underlying chemical network used in Levi. We present and validate a new network of atmospheric thermo-chemical and photo-chemical sulfur reactions. Sulfur was chosen as the element to add due to its importance on planets such as Venus and the existence of previous studies that have shown that sulfur may be significant in hot Jupiter atmospheres. We use Levi to investigate these reactions as part of a broader HCNO chemical network in a series of hot and warm Jupiters. We also investigate how the inclusion of sulfur can manifest in a hot Jupiter's atmosphere indirectly. Sulfur chemistry can result in the depletion of many non-sulfur-bearing species, including CH4, NH3 and HCN, by several orders of magnitude. In summary, we create a 1-D photochemical-kinetic model and show that it can be used to constrain the dynamics of exoplanet atmospheres and their origins. Some of the key directions for future development include expanding the sophistication of the underlying chemical networks, work begun by our introduction of sulfur, and exploring the possibility of 2-D and 3-D dynamical models, to account for zonal redistribution. More developed chemical networks allow us to better constrain the atmospheric chemistry, and thus the overall atmospheric composition, breaking existing degeneracies we highlight, while improving the treatment of the dynamics ensures our modelling gives a more accurate depiction of the disequilibrium chemistry taking place.
Open Access
X-ray Spectroscopy of Cool Core Galaxy Clusters
Liu, Haonan
In this thesis, I present the results of my PhD research on the cooling flow problem in galaxy clusters. The centre of relaxed galaxy clusters has a short radiative cooling time suggesting the presence of a massive cooling flow. However, early studies using high resolution X-ray spectroscopy indicated much lower levels of cooling rate. AGN feedback is thought to be the most likely energy source to balance radiative cooling, though the energy transport and dissipation mechanisms are still under debate. In this work, I study whether AGN feedback can actually balance radiative cooling in a large number of galaxy clusters using high resolution X-ray spectroscopy. The first chapter contains the necessary background of the cooling flow problem and AGN feedback. This is followed by a chapter describing data reduction of XMM-Newton observations. In the third chapter, I present a study of 45 nearby cool core galaxy clusters and groups, where I measure the radiative cooling rate in the softest X-ray band. Then I select a small sub sample of bright clusters to understand the mass temperature profile of the gas in Chapter 4. In Chapter 5, I present a deep study of recent XMM-Newton observations of two luminous clusters at intermediate redshift. Finally, I extend my research on 40 more clusters within a much larger range of distances corresponding to redshift up to 0.6.
Open Access
How to Accurately Map the Milky Way with a Billion Sources: A Journey into the Gaia-verse
Everall, Andrew
Accurately modelling the phase-space distribution of Milky Way sources relies on two vital components: unbiased distance estimators and survey selection functions. Without either, models are susceptible to significant systematic uncertainties. My case study of the tilt of the local velocity ellipsoid demonstrates this. Well-constructed distances for the Gaia DR2 RVS sample return a velocity ellipsoid broadly consistent with spherical alignment. Using the reciprocal parallax distance estimator significantly alters the conclusions. I produce selection functions for catalogues needed to model the phase-space structure of the Galaxy. My spectrograph selection function method is generalisable to many multi- fibre observatories. I supplement this with tools to combine selection functions for unions of samples and transform from observable to intrinsic coordinates. I produce selection functions for Gaia catalogues including astrometry and RVS samples. My model fits the complex behaviour of the Gaia spacecraft impressively well. To enhance our understanding of the published Gaia astrometry, I introduce the Astrometric Spread Function, the expected covariance for a simple point source in Gaia. This reproduces the mean behaviour of published observations to degree level resolution. This is brought together to model the vertical distribution of Milky Way sources. Systematics are minimized by marginalising over parallax uncertainties and regulating the likelihood with Gaia EDR3 selection functions. The veracity of the method is demonstrated on a Gaia-like mock population. Applying to Gaia EDR3, I infer a north-south asymmetry weaker than previously reported and provide updated parameter values for the vertical scale heights of the thin and thick disks, the halo power-law exponent, local stellar mass density and surface density of the Milky Way. My thesis demonstrates the potential of Gaia when distances are well modelled and incompleteness is accounted for. My tools will be invaluable for answering further questions about the Milky Way using future Gaia data.