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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.

Galaxy evolution is an interplay of physical processes across a wide range of cosmological size scales, from star formation and black hole growth on sub-pc scales, to interactions with their environments on Mpc scales. The region surrounding individual galaxies within their host halos is known as the Circumgalactic Medium (CGM), and connects galaxies to the wider Intergalactic Medium (IGM). The CGM acts as both a reservoir of inflowing gas and a repository for outflowing gas as it is transported outwards via galactic feedback. Quasar absorption line surveys have revealed the CGM to be a complex, multiphase environment in which ions tracing cool and hot gas are present within the same absorber systems. Cosmological simulations are a powerful tool for building a theoretical understanding of these interconnected processes and resulting observations. They provide a 3D view of galaxies and their host halos, in which we intrinsically know the physical conditions of the constituent dark matter and baryons. As such, simulations are useful both for testing galaxy evolution models against the real universe, and for interpreting complex observations in a physical context. In this thesis I present detailed comparisons of simulations to observations of star formation and quenching in galaxies, and of absorbers in galactic halos in an effort to constrain physical models. I use Simba, a suite of state of the art cosmological simulations with realistic sub-grid physical models based on high resolution zoom simulations, including novel treatments of dust evolution and of black hole growth and feedback. I investigate the nature of gas in galaxies and their halos, and show comparisons with observations. In addition, I isolate the impacts of galactic feedback and ionising radiation on the results by varying elements of the input physical models. In Chapter 3, I present radial profiles of star forming and green valley galaxies in Simba at z=0 and compare with observations of MaNGA spatially resolved spectroscopy. My analysis shows strong central depressions in star formation rate (SFR), specific SFR (sSFR), and gas fraction in Simba's green valley galaxies and massive star-forming systems, qualitatively as observed, owing to Active Galactic Nuclei (AGN) X-ray feedback, which pushes central gas radially outwards. These effects are less pronounced at higher redshift, owing to less powerful AGN feedback. I also present a comparison of galaxy half light radii to observations at various redshifts, finding that predictions for star-forming galaxy sizes are accurate but that quenched galaxies are impacted by numerical heating. In Chapter 4, I characterise the low redshift CGM in Simba and examine the impact of differing feedback prescriptions. Low mass galaxies live in multiphase, diverse halos, whereas high mass galaxies live in halos dominated by hot gas. Halo baryon fractions are generally < 50% of the cosmic fraction due to stellar feedback at low masses, and 'jet-mode' AGN feedback at high masses. I present a comparison of absorption statistics to the COS-Halos and COS-Dwarfs surveys: Simba reproduces HI absorption well around star forming galaxies and broadly reproduces absorption of selected metal lines, including the observed dichotomy in OVI absorption between star forming and quenched galaxies. These predictions of metal line absorption are sensitive to the choice of photo-ionising background. Varying the galactic feedback prescription shows that stellar feedback enriches the CGM, while AGN feedback sets the gas temperature phase. In Chapter 5 I extend this analysis with a theoretical perspective of the CGM in Simba using a sample of CGM absorbers from a representative sample of Simba galaxies, selected across a range of stellar masses and sSFR. Absorbers are more abundant around low mass, star forming galaxies; the CGM of green valley galaxies is more similar to that of quenched galaxies. The absorber overdensity depends on the sSFR of the galaxy, while the absorber temperature is set by galaxy stellar mass. Absorption from low ions arises from cold, dense CGM gas, whilst absorption from higher ions traces hotter, more diffuse gas. I also examine the proportion of absorption arising from collisionally ionised gas, and from gas that is associated with satellite galaxies. Finally, in Chapter 6 I develop a tree-based machine learning mapping between the observable properties and the underlying physical conditions of the absorbers in the CGM absorber sample from Chapter 5. Such a mapping has the potential to be used to interpret CGM observations within a full cosmological context, assuming the Simba galaxy evolution model.

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.

A tidal disruption event (TDE) occurs when a star crosses the tidal radius of a nearby supermassive black hole (SMBH) and is subsequently torn apart. The fall-back of the stellar debris onto the black hole produces a luminous flare which we can use to understand the black hole and its environment. In particular TDEs allow us to probe SMBHs in otherwise quiescent galaxies and gain insight into the lower mass SMBH population distribution. The fact that TDEs evolve on human timescales make them excellent tools with which to probe accretion physics, allowing us to observe the formation of accretion disks and jets. Over the past decade, with the introduction of high cadence all-sky surveys an increasingly large number of TDEs with well sampled lightcurves and spectra have been discovered. These events have shown a remarkably diverse range of observational features and a strange preference to occur in E+A, or post-starburst galaxies. These two factors have given rise to two key questions: 'What processes produce the observed emission in TDEs?' and 'Why is it that TDEs occur where and when that they do?'. In this thesis I present three separate studies which aim at answering these questions. The first project is a detailed spectroscopic analysis of the TDE AT2018hyz during outburst. The dataset used is a combination of spectra from a number of telescopes resulting in high-cadence spectroscopic follow-up. This allowed me to monitor closely the evolution of spectroscopic features. The Balmer and He II emission line profiles varied dramatically throughout the TDE's evolution. Most notably from ∼ 30 − 100 days after optical lightcurve peak the Balmer lines develop a double-peaked shape. This, combined with an unusually low Hα/Hβ ratio lead me to determine that the observations show the formation of an accretion disk before it either dissipates or gets obscured from view. At the time this was the most clear sign of accretion disk formation in a TDE from optical spectra. The second project is my analysis of another TDE, AT2019qiz. Here I present and analyse very late time optical and X-ray spectra. The optical spectra show high ionisation coronal emission lines which have never before been seen in a TDE and a jump in the flux of narrow lower ionisation lines. The X-ray spectrum shows an order-of-magnitude jump in soft X-ray flux compared to a previous observation taken closer to the peak of the TDE outburst. The coronal line emission implies a source of highly ionising extreme UV photons which is predicted by accretion models but until now not shown in observations. From the emission line widths I show that the coronal lines originate from a region further out from the SMBH than the broad line region, but within the narrow line region distance. In addition, the distances imply that the structure reprocessing the material must have already been in place at the time of outburst, likely originating from previous nuclear activity. The finding raises questions as to why these lines appear in AT2019qiz and not other TDEs and I recommend systematically searching for them in late time spectra of other TDEs. In the final project I analyse spectra of a sample of 11 host galaxies of tidal disruption events. By measuring spectral indices of the galaxies such as D n 4000 and Lick Hδ A , I find that 6 of the objects fit the literature definition of a post- starburst galaxy. This is backed up by comparisons of my measurements to galaxy models from the Bagpipes synthetic population synthesis code. In the second half of the project I conduct detailed fitting to my host galaxy spectra using Bagpipes with the goal of uncovering the star formation histories. I find that the objects previously defined as post-starburst have had a burst of star-formation within the last few Gyrs, and that one object with ongoing star-formation may be undergoing a burst at the time of TDE outburst. In general my results are consistent with other's findings that the TDE host-galaxy population is over- represented by E+A galaxies, and that this may be related to the presence of nuclear star clusters in TDE hosts.

The work presented in this thesis is centred around Symmetric Achromatic Variability, a rare phenomenon observed in the light curves of blazar sources. SAV is hypothesised to arise through gravitational milli-lensing when relativistically moving components traverse the lensing caustics created by some intervening massive object(s). It was first identified due to the presence of symmetric U-shaped features within 15GHz blazar light curves produced using the Owens Valley Radio Observatory 40m telescope. In this thesis I present a model to describe the light curve of a lensed blazar source. I then introduce a fitting procedure which uses nested sampling to fit this model to blazar light curves, with the goal of using this procedure to identify SAV within the the OVRO data set. I also demonstrate how this procedure can then be used to generate artificial blazar light curves based on real OVRO data. I then show the results of running the fitting procedure with both real and simulated blazar light curves. Additionally, I propose a framework for a transdimensional alternative to standard nested sampling algorithms, where the number of model parameters, N, is itself included as a free parameter. Such an approach could have wide-ranging applications, including for the problem of SAV identification. I describe the process by which we explore the parameter space, including the introduction of a novel concept - the 'flattened' particle space. Using both data and an analytical approach, I investigate the performance of this method. Finally, I detail an ongoing campaign to monitor the SAV candidate J1415+1320 using e-MERLIN at both L- and C-band, in order to gather evidence for or against the achromaticity of the SAV events. I then present and analyse light curves produced from the most recently available data.

Supermassive black holes at the centre of galaxies are strongly linked to relativistic plasma outflows seen in nearly 10% of all active galaxies. These outflows may be responsible for producing the most energetic particles in the universe, and are known to influence the formation and evolution of structure in the universe. While it is clear that jets are transporting matter and energy to great distances, the mechanism for doing so is poorly understood. A longstanding problem in jet physics is the appearance of two flavours of jets. Entrainment appears to play a role in sources with a higher surface brightness closer to the central region. For edge-brightened sources like Cygnus A, what mechanism allows them to sustain a well-collimated relativistic jet out to many kiloparsecs? Previous studies of kiloparsec-scale jets were limited by sensitivity, bandwidth, and resolution. Modern interferometers offer a chance to observe jets across many frequencies with excellent sensitivity and resolution. The work presented in this thesis is part of the e-MERLIN Extragalactic Jets Legacy Project which seeks to probe sub-arcsecond structures at 1.5 and 5 GHz. The project aims to shine new light on key areas such as: (1) What are the transverse structures in the jet like?; (2) Is there any evidence for a spine-sheath along the jet?; and (3) Can we link acceleration to areas of brightening? Presented here are high resolution and sensitivity radio maps of two powerful FRII quasars - 3C334 and 3C263, and one powerful FRII radio galaxy - Cygnus A, with observations from the combined e-MERLIN and JVLA arrays. The 1.5 GHz radio map of 3C 334 is the highest-resolution map to date. The improved maps reveal new knots along the jet and hotspots previously unresolved in the next best resolution maps. Within the counterjet "head" feature, we identify a centrally placed secondary hotspot. While this was identified in previous maps, the feature did not qualify as a hotspot at lower resolutions. Details of the radio structure and an analysis of the apparent jet opening angle is discussed. A spectral index map created at 1.5 and 5 GHz (matched to a common resolution) is provided. Evidence in support of jet precession is outlined based on the alignment of many distinct features within the quasar's lobes. The e-MERLIN+JVLA 1.5 GHz map of 3C263 has revealed a strikingly straight, narrow, and mostly unresolved jet. A previously unresolved knot close to the core is identified. The high-resolution maps reveal a narrow deflecting jet entering the east hotspot. At VLA resolution the feature was considered a part of the jet due to the orientation and placement but was significantly less resolved. The hotspots in the west lobe are resolved into complex smaller features. The e-MERLIN maps of Cygnus A provided a unique resolution to its hotspots and jet at 1.5 and 5 GHz. In both observations the transient observed by the JVLA was detected at a position of alpha =19:59:28.328, delta = +40 4401.913 at 1.5 GHz, and alpha = 19:59:28.3227, delta = +40 44 01.9280 at 5 GHz. The SED indicates the flux at each frequency (S1.5 = 5.2+/-2.2 mJy and S5 = 4.05+/-0.5 mJy) is consistent with the estimated flux (~3-4 mJy). At 5 GHz a structure extending ~0.10 from the centre of the core and perpendicular to the direction of the jet is imaged. This may be support for the higher frequency JVLA detection of a torus, but sidelobes around the core reduce our confidence in the detection.

A number of theoretical mechanisms have been invoked to explain the transformation of star forming galaxies into quenched galaxies, such as the removal of gas by active galactic nuclei- (AGN) and star formation-driven galactic outflows, and the heating of the galaxy halo and subsequent halting of gas accretion caused by virial shocks and preventative AGN feedback. Nonetheless, we still lack a clear understanding of which mechanism is the most important for quenching galaxies. The goal of this thesis is to study galaxy quenching from an observational perspective and to address the following question: why do galaxies stop forming stars? In the first part of this thesis I examine the relationship between molecular gas (observed with ALMA and NOEMA) and star formation on spatially resolved scales in seven green valley MaNGA galaxies. I find that both the star formation efficiency (SFE) and gas fraction of green valley galaxies is suppressed relative to typical star forming galaxies. These two effects are equally important for suppressing star formation in the outer disc, but it is not possible to rank their effect towards the strongly quenched galaxy centre, where SFE is only constrained to an upper limit. I also show that the significant reduction in gas fraction in the central regions is driven by an increase in stellar mass surface density rather than a decrease in molecular gas surface density, which disfavours quenching mechanisms that eject gas from galaxies. In the second part of this thesis I develop a 2D inclined rotating disc model to estimate the kinematic properties of 1862 MaNGA galaxies, and I use a random forest analysis to explore the relationship between galaxy kinematics and quenching. I find that the average velocity dispersion is overwhelmingly the most important kinematic parameter for predicting galaxy quenching. Furthermore, a full and partial correlation analysis shows that many commonly discussed correlations between galaxy properties (such as stellar mass) and quenching are spurious, and indeed at fixed velocity dispersion these correlations are almost entirely removed. I interpret this finding in the context of the BH-σ relation and argue for a scenario in which quenching occurs due to preventative feedback from AGN. In the third part of this thesis I attempt to directly probe preventative feedback from AGN by analysing a deep ALMA band 4 observation of HE0515-4414 to search for the Sunyaev-Zel'dovich (SZ) signal tracing the quasar's hot halo gas. I find marginal evidence (~3.3σ) of large scale (~300 kpc) heating in the visibility plane, as well as ~3.2σ evidence of localised heating to the south west of the central quasar in the image plane. Furthermore, I use SZ maps from the FABLE simulation to demonstrate that ALMA and the ACA are effective at detecting the SZ signal in band 3, and I discuss the optimal strategy for future observations.

The growth of galaxies through star formation and that of the central SMBH are believed to be linked. Both require large amounts of cold gas, which when transferred to the inner kpc region of a galaxy triggers circumnuclear star formation before reaching the sub-pc region where it is accreted onto the SMBH. Thus, a tight connection between central starbursts and AGN activity is expected. Major galaxy mergers have been considered the dominant mechanism for providing the necessary gas supply and driving AGN activity for the most luminous AGN, while secular processes have been associated to less luminous AGN. In this thesis we investigate the star formation histories of the host galaxies of luminous broad-line AGN to determine the connection between star formation and AGN activity. We use 19 novel spectral components in MCMC sampling to decompose the active galaxy spectrum into the pure AGN and the host galaxy spectrum. For the merger case, we select AGN at z ∼ 0.3 with neighbouring galaxies at projected separations up to 1 Mpc to assess the influence of galaxy interactions on the SMBH growth and star formation of the host galaxies. The AGN reside in star forming hosts regardless of separation supporting the star formation - AGN activity connection, but star formation is not caused by ongoing galaxy interactions. The luminosity and spectral shape of the pure AGN spectrum is independent of proximity of neighbour indicating an absence of a merger-AGN connection. We then study the star formation histories of a large sample of AGN hosts as a function of AGN properties (black hole mass, Eddington ratio and bolometric luminosity). We find that all AGN hosts are star forming and show higher specific star formation rates with increasing bolometric luminosity.

Feedback mechanisms are invoked in theoretical models and cosmological simulations to regulate or even suppress star formation in galaxies. One feedback route is through galactic outflows, which eject large amounts of gas and thereby remove the fuel for future star formation. Such galactic outflows can be driven by active galactic nuclei (AGN) or processes associated with star formation such as supernovae or radiation pressure from young stellar populations. The goal of this thesis is to study outflows from an observational perspective in different phases to quantify their impact on their host galaxies comprehensively, primarily by using optical and millimetre band data from the Multi Unit Spectroscopic Explorer (MUSE) and the Atacama Large Millimeter/submillimeter Array (ALMA). In the first part of this thesis, I study the physical mechanisms driving outflows as well as whether outflows are capable of suppressing star formation using a study of molecular gas outflows with ALMA and archival data. In a sample of 50 local galaxies with outflow signatures in low-J carbon monoxide (CO) transitions, I find most outflows are consistent with the radiation pressure-driven or the energy-driven (but with a low coupling efficiency) models. Furthermore, most outflows are unable to escape the galaxy's gravitational potential (with 5 % or less of outflowing gas exceeding the escape velocity), and the depletion time-scales of the galaxies' total gas reservoirs are typically of the order of 10^8 years or more. Based on these findings, I conclude that outflows are unlikely to quench their entire host galaxy but might affect the central regions of the galaxy. Second, I investigate the physical conditions of the outflowing gas and the link between different gas phases in the galactic outflows. While there is observational evidence of outflows in many different phases, the connection between them and their relative importance is poorly understood. My results based on MUSE observations of 26 local (ultra)luminous infrared galaxies ((U)LIRGs) suggest that outflows have about three times higher electron density than the galaxy's disc (< n_e,disc > 150 cm^-3 and < n_e,outflow > 500 cm^-3 ), but they are less dust obscured than the galaxy's disc. Furthermore, I establish that the molecular phase is dominant in terms of mass and energy at all AGN luminosities, while the ionized gas phase is negligible in AGN host galaxies and at most a few per cent of the total mass or energy budget in star forming galaxies. The atomic neutral phase is slightly less massive and has lower kinetic power than the molecular phase. Third, I analyse galaxies which exhibit positive feedback, which are instances where outflows trigger or enhance star formation. My results suggest that about 30 % of outflows show signs of star formation inside them, as the gas in these outflows is characterised by SF-like BPT diagnostics. The star formation rate inside these outflows is correlated with their ionized mass outflow rate. These findings suggest this mode of star formation might be more prominent at higher redshifts, and may contribute to the formation of the spheroidal component of galaxies.

Highly supersonic, magnetized and differentially rotating, accretion flows provide an environment for the propagation of waves and the growth of instabilities unlike those normally encountered on Earth. In this dissertation, I investigate the properties of oscillations and instabilities in accretion discs, developing theoretical and numerical models to explore the physical nature of variability encountered in a variety of astrophysical contexts. Through linear theory and semi-analytical calculations, I first consider the physical properties of magnetically altered inertial waves. 'Trapped inertial waves' provide an attractive explanation of the fast variability observed in the emission from low-mass black hole binary systems, but such oscillations can be affected by magnetic tension provided by large-scale poloidal magnetic fields threading the accretion disc. Through local and global analyses, I constrain the modification of trapped inertial waves by poloidal, toroidal and helical magnetic fields. I then investigate the excitation of oscillations in deformed discs with eccentric, non-circular streamlines. Many processes can lead to the growth of eccentricity in accretion discs, and turbulence deriving from the excitation of inertial waves by a local parametric instability provides one mechanism for curbing this growth. However, eccentric discs also provide an environment for the excitation of additional, inherently global oscillations, and I present a framework facilitating a semi-analytical investigation of these modes. I finally employ numerical simulations to explore the dynamics of accretion disc oscillations in the non-linear regime. I first follow the non-linear saturation of inertial waves driven by parametric resonance in non-relativistic discs, and confirm the growth of a second family of large-scale, low-frequency oscillations. Using a pseudo-Newtonian framework to approximate relativistic effects, I then demonstrate the excitation of trapped inertial waves through non-linear coupling with accretion disc deformations in a black hole accretion disc, providing preliminary evidence that trapped inertial waves can be excited even in the presence of MHD turbulence.

In this thesis, I present the results of my research on the X-ray spectra of accreting black holes. The content of this thesis can be broadly separated into two parts: Chapter 2-3 for stellar-mass black holes in X-ray binaries (XRBs) and Chapter 4-6 for supermassive black holes in active galactic nuclei (AGN). This work mainly focuses on the X-ray spectral shape and the spectral variability of these accreting black hole systems. By studying the broad band spectra of XRBs and AGN, we are able to extract some information about the innermost regions of the discs from their X-ray observations. For example, we are able to 1) constrain the size of the coronal region that illuminates the disc; 2) estimate some properties of the discs, such as the density in the disc surface; 3) measure the spin parameters of the accreting black holes. In the meantime, I also study the variability of the innermost regions by comparing different observations or different flux states of individual sources. For example, Chapter 2-3 compare different accretion states of black hole XRBs. Chapter 4 discusses the extreme and fast flux variability on a timescale less than one kilo-second in the narrow-line Seyfert 1 galaxy IRAS 13224−3809. Chapter 5 studies the long-term spectral variability of the Seyfert 1 galaxy 1H0419−577. The main spirit of this thesis is to test for high density discs. A disc reflection model with a variable disc density parameter is used for each source in this work. In the last chapter, I apply the same model to a large sample of Seyfert 1 galaxies in order to compare the disc densities at different black hole mass scales and different accretion rates.

All active galactic nuclei (AGN) are known to vary in the rest-frame UV/optical. Typical variations are on the order of 30% or so and are stochastic in nature. Therefore, the discovery of a number of extreme AGN transients, which are smoothly evolving on year-long timescales and by a factor of four or more, is surprising and necessitates further analysis. Are these objects simply at the extreme end of the variability distribution seen in normal quasars or is there another mechanism which can explain their atypical behaviour? The primary focus for this work is on the possibility that a number of these extreme AGN transients are actually rare, high-amplitude microlensing events, caused by intervening stellar mass object(s). Not only do the microlensing models provide an explanation for the observed variability but they also allow constraints to be placed on the morphology of the emitting regions of the AGN, namely the accretion disc and broad line region (BLR). These transients have been monitored both photometrically and spectroscopically, since their discovery. The majority of spectroscopic observations have been conducted using the William Herschel Telescope. At time of writing (Sept. 2017), there are now 64 confirmed AGN and 235 individual spectra. The spectral reduction pipeline, calibration and initial measurements are described in Chapter 2. This chapter also details the microlensing models and procedures used in interpreting both the light curve information and spectral measurements. This includes: a comprehensive treatment of the simple point-source/point-lens model; quantitative point-lens models which allow for the use of extended sources; and also an initial exploration into more complex lensing morphologies involving multiple lensing objects and/or an external shear. Chapter 3 details the results of the spectroscopic monitoring campaign for the entire transient sample. A general classification scheme is developed which allows for a comparison of the evolutionary trends seen in objects exhibiting similar behaviour. A subset of transient AGN, the most extreme objects in the sample, is also discussed in detail, with a particular focus on the evolution of the continuum, line fluxes and equivalent widths. Chapter 4 details the results of the analysis of four key targets, selected for their suitability in addressing the microlensing hypothesis. For two targets the point-source point-lens model performs very well. Lens parameters for these objects are presented and in one particular case, the data is sufficient to allow constraints to be placed on the size of various components comprising the broad line region. Chapter 5 expands the microlensing analysis to include the entire AGN transient sample. Approximately 10% of objects are well matched by a simple point-source, point-lens microlensing model. In other objects, evidence is seen which requires a more complex lensing scenario to adequately explain. In one class of objects there is also evidence that the accretion disc is being resolved by the lens. Chapter 6 revisits a notable are seen in an AGN which lies behind M31. The analysis reaffirms that this event is well described by a simple microlensing model and provides an independent estimate that the most probable location for the lens is within M31 itself.

The Standard Model of particle physics has enjoyed unprecedented success in predicting experimental results. However, evidence from astrophysical observations points to the existence of a dark sector of particles that interact only very weakly with the Standard Model. In this work, we search for dark sector signatures in X-ray telescope data. Much of this work concerns a class of hypothetical particles, the axion-like particle (ALP). ALPs are a theoretically well-motivated extension of the Standard Model. If ALPs exist, they may lead to intriguing astrophysical signatures: in the presence of a background magnetic field, ALPs and photons can interconvert. We could detect ALPs by searching for photon to ALP conversion. For example, photons produced by point sources in or behind galaxy clusters may convert to ALPs in the cluster's magnetic field. This could lead to observable spectral anomalies. Using this strategy, we place world leading bounds on the ALP-photon coupling. One potential signal of dark matter is an anomalous line in the spectra of galaxies and galaxy clusters. In 2014, an anomalous line was found at an energy of 3.5 keV. The nature and cause of this line is still under discussion. We analyse a scenario in which the 3.5 keV line arises from dark matter decay to ALPs, which interconvert with 3.5 keV photons in astrophysical magnetic fields. We further report an anomalous deficit at 3.5 keV in the spectrum of the Active Galactic Nucleus at the centre of the Perseus galaxy cluster. This motivates the study of a new model in which both features are caused by “fluorescent dark matter” which resonantly interacts with 3.5 keV photons. We analyse observations of Perseus at 3.5 keV to date, and show that they are well explained by this model. Further theoretical and experimental work is needed to discover or exclude fundamental physics effects in X-ray spectra.

I study mid-infrared and star formation properties of AGN samples using infrared observations, and star-forming galaxies using radio observations in order to investigate the link between star formation, AGN activity and radio luminosity. I present the results of these investigations in this thesis. I carried out an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared Survey Explorer (WISE). The combined sample consists of quasars and radio galaxies, and covers a redshift range 0:003 < z < 3:395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs - HERGs) is analysed using large complete samples. The results show that a division in the accretion modes of powerful LERGs and HERGs clearly stands out in the mid-IR radio plane. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN. I examined the link between AGN activity and star formation by constructing matched samples of local (0 < z < 0:6) radio-loud and radio-quiet AGN in the Herschel-ATLAS fields. AGN accretion and jet powers in these active galaxies are traced by [OIII] emission-line and radio luminosity, respectively. Star formation properties were derived using Herschel 250-_m and stellar mass measurements are taken from the SDSS-MPA-JHU catalogue. The stacking analyses show that star formation rates (SFRs) and specific star formation rate (SSFRs) of both radio-loud and radio-quiet AGN increase with increasing AGN power but that radio-loud AGN tend to have lower SFR. Additionally, radio-quiet AGN are found to have approximately an order of magnitude higher SSFRs than radio-loud AGN for a given level of AGN power. The difference between the star formation properties of radio-loud and -quiet AGN is also seen in samples matched in stellar mass. I also investigated the relationship between SFR and low-frequency radio luminosity observed in star-forming galaxies. I used a sample of star-forming galaxies in the 19 local Universe selected from the SDSS-MPA-JHU catalogue. LOFAR observations of the Herschel-ATLAS North Galactic Pole field (NGP) were carried out as part of the LOFAR surveys Key Science Project at an effective frequency of 150 MHz, which provided low-frequency radio luminosity of sample galaxies. SFRs of galaxies in the sample were derived using MAGPHYS spectral energy distribution (SED) fitting. The results of this study show that the slope of L150/SFR is less than unity and not universal for all star-forming galaxies (SFGs) in the local Universe (0 < z < 0:3). The slope of the L150/SFR relation is also found to be steeper than the L1:4/SFR relation, probably due to the contribution from thermal radio emission at 1.4 GHz. If the L150=SFR relation for strongly star-forming objects is explained naively by electron calorimetry, I conclude that low luminosity sources are not ideal calorimeters and differ from the main locus of SFGs at low redshifts. The different gradients we obtain for the far- IR/radio correlation using samples selected at different frequencies reveal the selection effects on relations derived in this thesis.