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One of the enduring and most fundamental questions of modern astrophysics is understanding how galaxies form and evolve through cosmic time. Key to this question is understanding the interplay and feedback between star-formation (SF) and accretion onto supermassive black holes (SMBHs). High sensitivity, high angular resolution radio observations are a crucial part of these investigations, uniquely providing spatially resolved tracers of SF and accretion (Active Galactic Nuclei, AGN) in high redshift galaxies. Deep wide-field radio surveys of extragalactic legacy fields trace a large range of spatial scales and brightness temperature sensitivities, with sensitivities to the different radio-emitting components in galaxies at cosmologically-significant distances. At ~GHz frequencies, arcsecond angular resolution scales offer a view of the diffuse radio emission largely associated with star formation regions (e.g., Condon et al., 2012; Vernstrom et al., 2015), with sub-arcsecond angular resolutions sensitive to and able to spatially resolve emissions from both AGN and star formation (e.g., Muxlow et al., 2020), while milliarcsecond angular resolution scales are mostly sensitive to compact emissions mainly associated with AGN (e.g., Herrera Ruiz et al., 2017; Radcliffe et al., 2018, 2021b, Njeri et al., 2022). At sub-milliJansky levels, these high redshift radio source populations are composed of a high fraction of star forming regions and radio quiet AGN with radio emissions at sub-kpc spatial scales (Orienti et al., 2015). For a better understanding of AGN and their host galaxies, a census of AGN at sub-mJy flux regimes is necessary. Therefore, the goal of this PhD, is to disentangle AGN contribution from SF processes over a range of redshifts, using unprecedented large field-of-view, ultra-deep and high-resolution radio imaging (0''.007-1''.6)provided by the European VLBI Network (EVN), the Very Long Baseline Array (VLBA), the enhanced Multi-element Radio Linked Interferometer (e-MERLIN) and the Karl G. Jansky Very Large Array (VLA). Combining these spatial scales, we provide a unique census of AGN identification through; i) radio morphology, ii) source sizes and flux density distribution, iii) brightness temperature, iv) variability, and v) multi-wavelength analysis such as radio-excess via the radio-far infrared correlation. The majority of our radio-detected AGN host galaxies are radio-quiet, with radio emission mostly being powered by a mix of star formation and AGN. By combining different spatial scales (parsecs to sub-kpc), we present a high spatial dynamic view of individual AGN galaxies at z > 1 and offer a direct view of the transition between compact radio emission and SF emission, important for star formation rates (SFR) measurements. Furthermore, we detect a possible rare dual AGN (potentially a binary SMBH) system. Ultimately, as progress is made in radio instrumentation, this work will provide a unique set of scientific and technical benchmarks for future high sensitivity high spatial resolution surveys of the static and dynamic radio sky at high redshifts.

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.

Radio astronomy is heading into a new era where we can use the wide field of view and excellent sensitivity of instruments such as LOFAR, the MWA, ASKAP, and MeerKAT to commensally search for and monitor radio variable and transient sources. In this thesis we present searches for, and investigations of, radio variable and transient sources in images from the 'more' Karoo Array Telescope (MeerKAT). We describe the fast imaging pipeline for the more Transients and Pulsars (MeerTRAP) commensal search for fast radio transients and demonstrate its use on observations of the Vela pulsar. We will use the rapid imaging pipeline to localise the transients to determine their properties and to enable rapid follow-up with other instruments. We present the discovery and investigation of the first radio transient discovered with MeerKAT: MKT J170456.2-482100. This source was found commensally in ThunderKAT observations of the low-mass X-ray binary GX 339-4. We find that MKT J170456.2-482100 is an RS CVn-like stellar binary consisting of a K type sub-giant and companion of unknown type, and that the radio flares that we observe are likely due to magnetic activity of the K star. Further investigation is needed to determine the cause of faint features in the optical spectra and to determine the nature of the companion. We detect radio emission from X-ray flaring star EXO 040830-7134.7, the first radio star detected with MeerKAT. We find that this star is a magnetically active, rotating M-dwarf and that the radio emission we observe is likely caused by the magnetic activity. We investigate and present a solution to systematic flux density variability in the light curves of MeerKAT detected sources over time, though further investigation is required to determine the source of the systematics. Finally, we present the discovery of 25, 2.6 per cent of the radio sources in the field, new long-term variable sources in images made from two years of weekly monitoring observations of GX 339-4 with MeerKAT. This is an unprecedented data set, and the discoveries we present are only scratching the surface of what we can find out using these data. We present the radio light curves and spectral index variability of the new sources, and speculate on their nature. Many of the new variable sources are consistent with refractive scintillation of active galactic nuclei but other sources require further multi-wavelength studies to ascertain their nature. This thesis represents the new and exciting research into the radio variable sky that we can now perform using MeerKAT.

Over the past two decades, detailed studies in the nearby Universe have shown that accreting supermassive black holes (SMBHs; or active galactic nuclei; AGN) can have a significant effect on their host galaxies, suppressing star formation and regulating their growth (known as AGN feedback). One of the most striking examples of AGN feedback in action comes from detailed studies of local AGN which exhibit powerful bi-polar jet outflows that can deposit significant energy into the galaxy halo, heating the surrounding gas and thereby regulating their own growth and suppressing star-formation activity. Therefore, studying the properties of AGN and the galaxies in which they reside is crucial in understanding and further developing our current models of galaxy formation and evolution. AGN can be split into two distinct categories, based on the accretion efficiency of the SMBH: radiative-mode AGN, and jet-mode AGN. Radiative-mode AGN are associated with efficient accretion, typically from cold gas, resulting in the formation of a geometrically thin, optically thick accretion disk that is typically surrounded by a dusty obscuring structure; these AGN are known to drive powerful outflows. Jet-mode AGN are associated with inefficient accretion, typically from hot gas, and display powerful bi-polar synchrotron radio jets that emit the bulk of their energetic output into the surrounding medium in the form of mechanical energy; these AGN are identified as such based on radio observations, showing no signs of AGN activity (e.g. accretion disk or torus) at other wavelengths. Based on the nature of the excitation lines, the jet-mode and radio-loud radiative-mode populations are also known as low-excitation radio galaxies (LERGs) and high-excitation radio galaxies (HERGs), respectively. However, our understanding of these AGN and their feedback effect is built primarily from detailed local Universe observations. Determining the physical mechanisms underpinning triggering and fuelling of AGN and how this affects AGN feedback activity across cosmic time is crucial but lacking. In this thesis, I address this shortcoming using deep observations carried out by the LOw Frequency ARray (LOFAR) telescope: the LOFAR Deep Fields; this forms the deepest radio continuum survey to date at low frequencies. I generated key science-enhanced datasets using this survey and studied the cosmic evolution of AGN feedback from low-luminosity radio-AGN within the past 10 Gyrs and how this feedback affects the growth and evolution of galaxies. In the first science chapter of the thesis, I detail the pipeline I developed to generate new, more robust multi-wavelength catalogues in the LOFAR Deep Fields. The existing catalogues in the literature either did not include the deepest available datasets in each survey field, or were created using different methodologies for detecting sources and measuring their fluxes; all this meant the catalogues were not sufficiently robust for the scientific aims of the thesis. To overcome these issues, I generated new catalogues in two of the Deep Fields by combining information from the ultraviolet to the mid-infrared wavelengths to detect sources and extract their properties in a clean and homogeneous manner. These are some of the best-studied regions of the sky and therefore these catalogues are also expected to provide a legacy value beyond the aims of the LOFAR surveys. Then, in the next chapter, using the multi-wavelength catalogues generated, I identify the host-galaxy counterparts of the radio-detected sources in the LOFAR Deep Fields. Host-galaxy identification and characterisation is crucial, in particular for radio surveys, in determining the photometric redshifts and physical properties (e.g. stellar masses, luminosities, star-formation rates) of the radio-source host galaxies, greatly expanding the scientific scope of the survey. I identified the host-galaxy counterparts of the LOFAR sources using a combination of the statistical Likelihood Ratio method and a visual classification scheme, using a workflow to decide the most appropriate method of identification for each source. This process results in a value-added catalogue of over 80 000 radio sources with multi-wavelength counterparts identified for > 97% of them. In this chapter, I then also investigate the properties of host galaxies of the faint radio population in the LOFAR Deep Fields. In the fourth chapter I focus on studying the evolution of the radio-AGN population and their properties in the LOFAR Deep Fields and how feedback from these AGN evolves across cosmic time. Of particular interest, and the focus of this chapter are the LERGs, which dominate at low radio luminosities and are thought to play a key role in the formation of massive galaxies in the local Universe; however, the evolution of this population beyond z ∼ 1 is not well known. In this chapter, I present the first robust measurement of the LERG luminosity functions out to z ∼ 2.5 and characterise the evolution of their host galaxy properties. This population shows relatively mild evolution across the redshifts examined; this is explained by the different evolution of the LERGs hosted in star-forming galaxies and those hosted in quiescent galaxies. The evolution of the quiescent LERGs show a strong decline in their space densities with increasing redshift, in accordance with the available host galaxies, while there is also an increase in the characteristic luminosity. I also find that unlike in the local Universe, the bulk of the LERGs are hosted by star-forming galaxies at higher redshifts and that the AGN in these galaxies appear to be fuelled by a different mechanism, likely associated with the cold gas, as compared to the LERGs in quiescent galaxies. In the final chapter I present the conclusions and look towards further exploration of this dataset. In particular, I discuss the characterisation of 3% of the radio-sources that were found to be completely invisible at optical and near-infrared wavelengths; an investigation of the far-infrared and radio properties of this subset found that the vast majority of these sources are likely high-redshift star-forming galaxies hosting a radio-AGN. To understand the nature of these extreme sources at early epochs, I present preliminary analysis from recent sub-millimetre follow-up via the sub-millimetre array (SMA) and the James Clerk Maxwell Telescope (JCMT). In addition, I also discuss the spectroscopic follow-up of the radio-AGN population found in the previous chapter with the upcoming multi-object WEAVE spectrograph on the William Herschel Telescope to study the prevalence of AGN activity as a function of different galaxy properties out to high redshifts. Finally, I also describe plans to compare the observational results found in the previous chapters with predictions from the latest cosmological simulations.

Large-scale surveys of the Active Galactic Nuclei (AGN) population performed over the past few decades have highlighted the existence of strong correlations between the central supermassive black hole and their host galaxy. Whilst such samples are incredibly useful, they tend to be incomplete. This skews our understanding of the AGN population as samples consist of largely brighter, more actively accreting objects. In this thesis, I combined SDSS galaxy samples with XMM-Newton serendipitous sources to measure the incidence of AGN as a function of galaxy properties in the nearby (z < 0:35) Universe, carefully accounting for the incompleteness of the X-ray imaging. Regardless of how the galaxy population were split, the probability of hosting an AGN was well described by a power law and highlighted significant amounts of activity at low X-ray luminosities and accretion rates. I identified 61 AGN in dwarf galaxies, of which 40 have not been previously identified. The probability of hosting an AGN in this regime was unaffected by either the stellar mass or redshift of the host galaxy. Then I studied the wider local galaxy population. As with dwarf galaxies, the probability of hosting an AGN is not affected by stellar mass but it does increases with redshift. When comparing my results to higher redshift measurements, I identified a stellar-mass-dependent distribution of black hole growth rates. Finally, I investigated the effect of star-forming activity. I found that the probability of a galaxy hosting an AGN increases with the star formation rate. Throughout, I also compared the effectiveness of AGN selection at X-ray and optical wavelengths. I found that the BPT diagnostic is not as effective as X-ray selection. The optical diagnostic incorrectly classified a significant proportion of the X-ray selected AGN sample as star-forming, and was unable to identify weak AGN emission.

Accretion discs around supermassive black holes act as the power houses of radiation to the most luminous, continuously emitting objects in the Universe; Active Galactic Nuclei (AGN). Accretion discs produce light, which we can observe by turning gravitational potential energy into heat via viscous torques. For discs around black holes, the available gravitational energy is the significant fraction of the rest mass energy of the orbiting matter. Simple, analytical models are capable of explaining the broad features of observed accreting black holes. However, recent observations have challenged existing theories by finding that AGN luminosities vary rapidly and with large amplitudes. In this thesis, we investigate the dynamics of warped discs with numerical simulations to see if such variability can be produced by these discs. Previous works have provided a criterion for warped discs to tear into discrete rings. We examine this possibility and explore how this may connect to observed disc behaviour. We demonstrate an agreement between our numerical results and the predicted criterion for disc tearing in warped discs around supermassive black holes. We also explore how our numerical analysis prove useful to illustrate the observational variability exhibited in AGN discs. In the later chapter, we show that often used initial conditions for accretion discs in simulations that employ Smoothed Particle Hydrodynamics (SPH) are not in dynamical equilibrium, and this leads to the formation of unwanted pressure waves that cannot be effectively damped at higher resolution. We propose a damping scheme, which uses an initial relaxing phase to remove these waves over a timescale comparable with the disc's dynamical timescale.

Supermassive black holes at the centre of galaxies can accrete matter from their surroundings, yielding the class of Active Galactic Nuclei (AGN). These AGN monsters, which expel winds and relativistic jets, are known to play a key role in the formation of the Universe that we see today and so identifying these objects is of paramount importance. While in nearby galaxies this is relatively easy, the task becomes more difficult in distant galaxies, where circumnuclear dust can obscure our view of these monsters. In this thesis, we use radio observations, which are not affected by this obscuration, in order to identify these AGN. In particular we use a technique called Very Long Baseline Interferometry (VLBI) which uses multiple radio telescopes spread across continents to provide the sharpest possible view on the sky. This method effectively isolates compact radio cores which can only be attributed to AGN. This thesis pioneered this technique, provided new calibration methods, and almost tripled the number of radio-detected AGN in a famous deep-field. We compared VLBI to other well-known AGN selection techniques, revealing that VLBI provides an invaluable contribution to our understanding of the AGN content of the Universe. In this thesis, we investigated another measure of AGN activity, namely variability. A radio source that varies in brightness rapidly is most likely due to an AGN. We provided a first insight into the faintest variable radio sources, revealing that only a small percentage of radio sources are variable AGN.

Galaxy clusters are the largest gravitationally-bound objects in the Universe, their unparalleled size providing powerful leverage to probe large-scale structure growth and cosmology. At the same time, clusters and groups of galaxies represent unique astrophysical playgrounds in which galaxies interact with each other and with the intervening gas, both of which are strongly influenced by a range of astrophysical processes such as star formation or feedback from supernovae and active galactic nuclei (AGN). In this thesis I present the 'Feedback Acting on Baryons in Large-scale Environments' (FABLE) project, a new suite of cosmological hydrodynamical simulations of galaxies, groups and clusters designed to further our understanding of the formation and evolution of these fascinating objects. Firstly I perform a detailed comparison of the FABLE simulations to low-redshift observations, demonstrating simultaneous agreement with observational constraints on the total stellar and gas mass contents of groups and clusters and the galaxy stellar mass function. I generate synthetic X-ray spectra for the simulated systems and find good agreement with a range of observed X-ray scaling relations. In addition I show that the radial gas profiles of FABLE groups and clusters are a good match to low-redshift observations. Residual deviations in the thermodynamic properties of the cluster core region suggest that more sophisticated AGN feedback modelling or additional physical processes may be needed to explain the observed properties of cluster cores. Next I extend the analysis of the cluster scaling relations out to high redshift, including a comparison to observational constraints and other simulation predictions. I find that all examined scaling relations deviate from the self-similar prediction in terms of their slope and redshift evolution. These deviations are attributed to a combination of factors, including non-thermal pressure support provided by kinetic motions in the intracluster gas and the effects of non-gravitational physics such as AGN feedback. I also find a significant variation in the scatter about the relations with changing halo mass and redshift, contrary to the assumptions of most observational studies. In addition I investigate the scaling between the Sunyaev-Zel'dovich (SZ) signal and total mass, showing good agreement with cluster data from 'Planck' and the South Pole Telescope over a wide redshift range. I demonstrate the sensitivity of the predicted number of detected clusters in an SZ-selected survey to the assumed SZ scaling relation using several recent observational and simulation constraints. Finally I investigate the halo mass and redshift dependence of the total baryon contents of FABLE clusters and groups and of the stellar mass, size and shape of brightest cluster galaxies (BCGs). In particular I show that the simulations agree with recent constraints on the (lack of) redshift evolution in the total gas and stellar mass of massive clusters. Furthermore, I use the stellar mass profiles of FABLE BCGs to highlight potential biases in observational studies of BCG growth associated with the assumed light profile and the outer radius of the fit.

Active Galactic Nuclei are among the most powerful sources of radiation in the Universe and are known to show variability across the EM spectrum. This thesis will focus on the variability in the optical and UV range, which is dominated by emission from the central accretion disc and the Broad Line Region (BLR). All AGN are powered by the accretion of matter onto a supermassive black hole. The BLR reprocesses part of the continuum generated by the accretion disc, re-emitting the energy in atomic emission lines. Variations in the continuum lead to variations in the broad emission lines, however the manner in which the broad lines respond is complex. Neither the accretion disc nor the BLR are currently fully understood. Over the past years the availability of large data-sets from long term spectroscopic observations, such as the Sloan Digital Sky Survey (SDSS), has greatly improved our ability to study the variability of AGN, expanding the observed timescales to decades. These timescales match the dynamical timescales associated with the BLR, implying that we could be able to track structural changes in this region. The research presented in this thesis will use the response of the broad lines to the continuum to investigate the properties of the BLR, with an emphasis on long timescales. The research is split over two independent, but closely linked studies. The first is a case study of the highly variable active galaxy Markarian 110. The second study focusses on one particular emission line: MgIIλ2798. Both studies rely on optical spectroscopic data. A pipeline was written to fit archival and new spectra included in the study. Mrk 110 is a local Seyfert I, at z=0.035, with available spectroscopic data going back decades. Combining archival data with new observations from the William Herschel and Tillinghast Telescopes, it is possible to track Mrk 110 through dips and peaks in the continuum at a relatively high cadence. Of particular note is the behaviour of HeIIλ4686, which varies significantly more than the other emission lines. This line can be used as a proxy for the ionising (FUV) part of the continuum. Comparing Hα, Hβ, and HeIλ5876 with the FUV flux we note that the response of the line fluxes to continuum changes evolves considerably on the timescale of years. For the highest continuum fluxes it is possible to detect a saturation level of the line responses, correlated with the Reverberation Mapping lags associated with the lines. Analysing the line profiles, we can detect an offset between the narrow and broad components of the Hβ and HeIIλ4686 lines, likely associated with a radial flow in the BLR. The Magnesium line at 2798Å is a prominent feature in most AGN spectra. Its response to continuum changes has been shown to be limited in previous studies. The study presented here will make use of two data-sets. The first is a sample of 43 extremely variable quasars. This sample combines SDSS spectra with new observations from the William Herschel, Magellan, and MTT Telescopes. The second data-set consists of approximately 16,000 SDSS quasars with repeat spectroscopy. The data from both samples indicate that there is in fact a broad range in MgII behaviour. We observe that the line tracks the continuum on average, but also note considerable scatter in this correlation. Combining the results from the Mrk 110 and MgII studies we find that there is evidence in all samples for broad line responsivity that evolves over time. There is also considerable evidence to suggest that the dynamics of both the HeIIλ4686 and the MgII line are not only set by their positions in the gravitational potential.

In this thesis the connections between galaxy mergers and the growth of supermassive black holes (SMBHs) are investigated. In chapter 2 we investigate the links between galaxy mergers and active galactic nuclei (AGN) as a function of luminosity, using visual classification of galaxy images and a new measure of morphological disturbance to identify mergers, testing the hypothesis that cataclysmic events such as mergers are required to drive the most rapid periods of accretion. We find no correlation between merger fraction and AGN luminosity, nor any difference in the total merger fractions of AGN and matched control galaxies. We also explore the possibility that the role of galaxy mergers evolves through cosmic time over the redshift range 0.5-2.2, finding a modest evolution, with higher redshift AGN more likely to be found in mergers than controls with 99% confidence. In chapter 3 we compare samples of simulated (Illustris) galaxies to real observations, exploring the efficacy of different techniques of merger-identification as a function of merger parameters (mass ratio, time elapsed since the merger, stellar mass, star formation rate etc.). Detailed analysis of the ability of structural parameters and visual classification to identify mergers allows for reinterpretation of observational studies in which AGN merger fractions are measured. We find the intrinsic merger fractions in relevant studies are likely several times higher than reported, with 50 20% of AGN in the local Universe triggered by galaxy mergers. In chapter 4 AGN are selected in the optical, infrared and radio to investigate the role of mergers in triggering AGN in different physical environments. We use structural parameters to estimate merger fractions. It is found that infrared-selected AGN are more likely to be associated with mergers than their controls, optically selected AGN are equivalent to controls and radio-selected AGN show fewer signs of interactions than their controls. Collectively, this is interpreted as evidence that a substantial fraction of SMBH growth is driven by galaxy mergers, but care needs to be taken to control for various observational biases, which can hide or dilute real underlying causal relations.

The radio jets from active galactic nuclei (AGN) can not only influence the evolution of the entire galaxy, but they can also extend up to many Mpc. Furthermore, they are common throughout galaxy clusters, influencing properties of the intra-cluster medium through the injection of relativistic particles. In this thesis I present deep radio observations using the LOw Frequency ARray (LOFAR), along with ancillary telescopes, to explore radio galaxies and merging clusters. Chapter 1 introduces galaxies, AGN and properties of their corresponding radio jets. Chapter 2 introduces galaxy clusters, their basic thermal and non-thermal properties, and highlights the advantage of radio observations to understand their phenomenology. Chapter 3 introduces radio astronomy, observing with dishes and phased arrays, and gives an overview of a facet based calibration scheme required to correct for direction dependent and ionospheric effects. Chapter 4 presents the discovery of a new giant radio galaxy (GRG), found in an ongoing survey with the LOFAR telescope: the Multifrequency Snapshot Sky Survey (MSSS). Chapter 5 continues the analysis of this new GRG by presenting deep follow up observations using LOFAR and the Giant Meter-wavelength Radio Telescope (GMRT). This new GRG extends for 40â€2on the sky, which at a redshift of 0.0545 gives a total projected physical size of 2.5 Mpc. It is thought to be very old (108 years), having attained its large size by growing in a low density environment. Chapter 6 presents deep LOFAR observations of the massive merging galaxy cluster Abell 1682. These high resolution (5â€2â€2) and low frequency (150 MHz) observations detect new filamentary structures not seen at higher frequencies, which are thought to be the tails from moving radio galaxies. Previous radio studies have speculated that Abell 1682 is the host of an Ultra Steep Spectrum Radio Halo (USSRH). These new observations show areas of steep spectrum emission, but seemingly associated with the tails of an inactive radio galaxy. An initial analysis of LOFAR data at lower frequencies (44 MHz) suggests that this steep spectrum emission extends beyond the associated radio galaxy tails, indicative of a diffuse radio halo between the two sub-clusters centre of masses. In such a scenario, mechanisms favour the re-acceleration of old mildly relativistic electrons to radio-emitting relativistic energies as a result of turbulence injected in the wake of the merger event. Chapter 7 presents some closing remarks and thoughts on future prospects with LOFAR.

A small fraction of cosmic microwave background (CMB) photons scatter off electrons in the ionised gas in collapsed structures. This process, known as the Sunyaev-Zel'dovich effect, is usually broken down into a thermal (tSZ) and a kinematic (kSZ) contribution. While the former is sensitive to the random velocities of the electrons in the hot gas, the latter is sourced by the bulk motion of the entire object. In this thesis I measure the signature of both of these effects by cross-correlating CMB data with different tracers of the large-scale structure. I further study how these effects can be used as probes of cosmology and astrophysics. I first report a statistically significant detection of the kSZ effect. This is achieved by combining a cluster catalogue derived from the first year data of the Dark Energy Survey with CMB temperature maps from the South Pole Telescope. I perform the measurement with a differential statistic that isolates the pairwise kSZ signal, providing the first detection of the large-scale motion of clusters using redshifts derived from photometric data. By fitting the pairwise kSZ signal to a theoretical template, I measure the average central optical depth of the cluster sample. I compare the extracted signal to simulations and find good agreement with respect to the signal-to-noise, the constraint on the optical depth, and the corresponding gas fraction. I next study the potential of the kSZ effect as a probe of cosmology, again focussing on the pairwise method. The main challenge is disentangling the cosmologically interesting mean pairwise velocity from the cluster optical depth and the associated uncertainties on the baryonic physics in clusters. Using the Magneticum cosmological hydrodynamical simulations I calibrate a scaling relation between the amplitude of the tSZ signal and the optical depth. I show that this relation can be used to recover an accurate estimate of the mean pairwise velocity from the kSZ signal, and that this effect can therefore be used as a probe of cosmology. I finally derive constraints on feedback from active galactic nuclei by setting limits on their tSZ signal. By combining all-sky microwave, sub-mm, and far-infrared data from the Planck and AKARI satellites, I break the degeneracy between the tSZ signature and extragalactic dust emission. I test the measurement pipeline with a catalogue of galaxy clusters, finding the expected high-significance tSZ detection together with correlated dust emission. I then measure the tSZ signal of spectroscopically confirmed quasi-stellar objects (QSOs), but obtain only a low-significance hint of a tSZ signature. This analysis leads to a lower mean thermal energy than reported in some previous studies which were contaminated by dust emission. A comparison of these results to hydrodynamical simulations can be used as a probe of QSO host masses.

Supermassive black holes (BHs) and their host-galaxies are thought to evolve in tandem, with the energy output from the rapidly-accreting BH regulating star formation and the growth of the BH itself. The goal of better understanding this process has led to much work focussing on the properties of quasars at high redshifts, $z\gtrsim 2$, when cosmic star formation and BH accretion both peaked. At these redshifts, however, ground-based statistical studies of the quasar population generally have no access to the rest-frame optical spectral region, which is needed to measure H$\beta$-based BH masses and narrow line region outflow properties. The cornerstone of this thesis has been a new near-infrared spectroscopic catalogue providing rest-frame optical data on 434 luminous quasars at redshifts $1.5 \lesssim z \lesssim 4$. At high redshift, $z \gtrsim 2$, quasar BH masses are derived using the velocity-width of the CIV broad emission-line, based on the assumption that the observed velocity-widths arise from virial-induced motions. However, CIV exhibits significant asymmetric structure which suggests that the associated gas is not tracing virial motions. By combining near-infrared spectroscopic data (covering the hydrogen Balmer lines) with optical spectroscopy from SDSS (covering CIV), we have quantified the bias in CIV BH masses as a function of the CIV blueshift. CIV BH masses are shown to be over-estimated by almost an order of magnitude at the most extreme blueshifts. Using the monotonically increasing relationship between the CIV blueshift and the mass ratio BH(CIV)/BH(H$\alpha$) we derive an empirical correction to all CIV BH-masses. The correction depends only on the CIV line properties and therefore enables the derivation of un-biased virial BH mass estimates for the majority of high-luminosity, high-redshift, spectroscopically confirmed quasars. Quasars driving powerful outflows over galactic scales is a central tenet of galaxy evolution models involving 'quasar feedback' and significant resources have been devoted to searching for observational evidence of this phenomenon. We have used [OIII] emission to probe ionised gas extended over kilo-parsec scales in luminous $z\gtrsim2$ quasars. Broad [OIII] velocity-widths and asymmetric structure indicate that strong outflows are prevalent in this population. We estimate the kinetic power of the outflows to be up to a few percent of the quasar bolometric luminosity, which is similar to the efficiencies required in recent quasar-feedback models. [OIII] emission is very weak in quasars with large CIV blueshifts, suggesting that quasar-driven winds are capable of sweeping away gas extended over kilo-parsec scales in the host galaxies. Using data from a number of recent wide-field photometric surveys, we have built a parametric SED model that is able to reproduce the median optical to infrared colours of tens of thousands of AGN at redshifts $1 < z < 3$. In individual objects, we find significant variation in the near-infrared SED, which is dominated by emission from hot dust. We find that the hot dust abundance is strongly correlated with the strength of outflows in the quasar broad line region, suggesting that the hot dust may be in a wind emerging from the outer edges of the accretion disc.

This thesis presents a study of the sub-light year regions of Active Galactic Nuclei (AGN). These environments contain accretion discs that orbit a central super-massive black hole. The luminosity of the AGN inner regions varies over time across all wavelengths with variability at longer wavelengths lagging behind that at shorter wavelengths. Since the AGN themselves are too remote and too compact to resolve directly, I exploit these time lags to infer the physical characteristics of the accretion disc and surrounding gas clouds that emit broad emission lines. These characteristics include the inclination and temperature profile of the accretion disc, and the shape (or light curve) of the luminosity fluctuations that drive the accretion disc variability. This thesis details the work in the first author papers of Starkey et al. (2016, 2017), in which I detail the statistical code, CREAM (Continuum REverberting AGN Markov Chain Monte Carlo), that I developed to analyse AGN accretion disc variability. I apply the code to a set of AGN light curve observations of the Seyfert 1 galaxy NGC 5548 by the AGN STORM collaboration (De Rosa et al., 2015; Edelson et al., 2015; Fausnaugh et al., 2016a; Goad et al., 2016; Starkey et al., 2017). I also present work detailing my variability analysis of the Seyfert galaxies NGC 6814, NGC 2617, MCG 08-11-11 and NGC 4151. This work has contributed to the analysis presented in (Troyer et al. 2016, Fausnaugh et al. submitted). I also investigate the implications of a twin accretion disc structure (Nealon et al., 2015) on the disc time lag measurements across near UV and optical wavelengths. I finish by detailing a modification to CREAM that allows it to merge continuum light curves observed in a common filter, but taken by multiple telescopes with different calibration and instrumental effects to consider.

In this thesis I describe the development of a compact camera for ground-based multi TeV gamma-ray astronomy, using the Imaging Atmospheric Cherenkov Telescope (IACT) technique. The camera is based on multi-anode photomultipliers (MAPM) and is designed for use on the Gamma Cherenkov Telescope (GCT), which is proposed to be part of the Small Size Telescope (SST) array of the Cherenkov Telescope Array (CTA). GCT achieves high performance with a compact and cost efficient design via a Schwarzschild-Couder (SC) dual-mirror optical system. The GCT optical design allows the use of a compact camera of diameter roughly 0.5 m. The curved focal plane is equipped with 32 tiles of 64-pixels MAPM for a total of 2048 pixels of ~0.2° angular size, resulting in a field of view of ~9°. The GCT camera is designed to record the flashes of Cherenkov light from electromagnetic cascades, which last only a few tens of nanoseconds. I give a detailed description of the design, the challenges encountered during testing in the lab, and the performance of the most critical components. I give details on the custom front-end electronics modules that provide the required fast electronics, facilitating sampling and digitization, as well as first level of triggering. The camera-level triggering system is a custom backplane, developed to reject spurious triggers on the night sky background, which typically is of the order of few tens of millions of photons per pixel per second. This is to be compared with the rate of the astrophysical signal, which is of the order of few hundreds of events per second at the relevant energies. Additionally I provide a detailed description of all the software needed for the data acquisition and control of the camera, from the very low level drivers to high level and user friendly processes. I follow the commissioning of the camera, from the individual core components to the integration of the system. I then describe the integration of the camera on the GCT prototype telescope structure, and the achievement of "first light", validating for the first time the full proof-of-concept of an IACT with SC optics. I also report a study I performed on expectations for an extragalactic survey for blazars with CTA. The cumulative source count distribution of blazars is presented, including implications from two different phenomena: axion-like particle (ALP) to gamma-ray oscillations in the intergalactic magnetic field, and secondary gamma rays from hadronic origins. I conclude that a shallow and wide survey will provide the best science return for CTA, that the impact of ALP is modest and that the secondary mechanism of gamma-ray production would allow detection of blazars up to redshift of 1 in the multi-TeV energy band.

In this thesis I use X-ray observations to study the cores and extended structures of radio-loud AGN, to determine their structure, accretion properties and the impact they have on their surroundings. I use new Chandra data and archival XMM-Newton observations ofMarkarian 6 to look for evidence of emission from shocked gas around the external radio bubbles, using spatially resolved regions in Chandra and spectral analysis of the XMM data. The results show that the bubbles in Mrk 6 are indeed driving a shock into the halo of the host galaxy, with a Mach number of 3.9. I also find that the spectrum of the AGN has a variable absorbing column, which changes from 8 × 1021 atoms cm−2 to 3 × 1023 atoms cm−2 on short timescales (2-6 years). This is probably caused by a clump of gas close to the central AGN, passing in front of us at the moment of the observation. Using new and archival Chandra observations of the Circinus galaxy, I match them to pre-existing radio, infrared and optical data to study the kpc-scale emission. As for Mrk 6, I find that the radio bubbles in Circinus are driving a shock into the interstellar medium of the host galaxy, with Mach numbers M 2.7–3.6 and M 2.8–5.3 for the W and E shells respectively. Comparing the results with those we previously obtained for Centaurus A, NGC 3801 and Mrk 6, I show that the total energy in the lobes (thermal+kinetic) scales approximately with the radio power of the parent AGN. The spatial coincidence between the X-ray and edge-brightened radio emission in Circinus resembles the morphology of some SNR shocks, a parallel that has been expected for AGN, but has never been observed before. I investigate what underlying mechanisms both types of systems may have in common, arguing that, in Circinus, the edge-brightening in the shells may be accounted for by a B field enhancement caused by shock compression, but do not preclude some local particle acceleration. I also carry out a systematic study of the X-ray emission from the cores in the 0.02 < z < 0.7 2Jy sample, using Chandra and XMM-Newton observations. I combine the results with the mid-IR, optical emission line and radio luminosities, and compare them with those of the 3CRR sources, to show that the low-excitation objects in our sample show all the signs of radiatively inefficient accretion. I study the effect of the jet-related emission on the various luminosities, confirming that it is the main source of soft X-ray emission for our sources. I also find strong correlations between the accretion-related luminosities, and identify several sources whose optical classification is incompatible with their accretion properties. I derive the bolometric and jet kinetic luminosities for the sample and find a difference in the total Eddington rate between the low and high-excitation populations, with the former peaking at 1 per cent and the latter at 20 per cent Eddington. There is, however, an overlap between the two, indicating that a simple Eddington switch may not be possible. The apparent independence of jet kinetic power and radiative luminosity in the highexcitation population in our plots allows us to test the hypothesis in which jet production and radiatively efficient accretion are in fact independent processes that can coexist in high-excitation objects.

Black hole jets and accretion discs are the most extreme objects in modern astrophysics whilst dark energy is undoubtedly the most mysterious. This thesis focuses on understanding these three topics. The majority of this thesis is dedicated to investigating the structure and properties of black hole jets by modelling their emission. I develop an inhomogeneous jet model with a magnetically dominated parabolic accelerating base, transitioning to a slowly decelerating conical jet, with a geometry set by radio observations of M87. This model is able to reproduce the simultaneous multiwavelength spectra of all 38 Fermi blazars with redshifts in unprecendented detail across all wavelengths. I constrain the synchrotron bright region of the jet to occur outside the BLR and dusty torus for FSRQs using the optically thick to thin synchrotron break. At these large distances their inverse-Compton emission originates from scattering CMB photons. I find an approximately linear relation between the jet power and the transition region radius where the jet first comes into equipartition, transitions from parabolic to conical and stops accelerating. The decreasing magnetic field strength and increasing bulk Lorentz factor with jet power are the physical reasons behind the blazar sequence. I calculate the conditions for instability in a thin accretion disc with an α parameter which depends on the magnetic Prandtl number, as suggested by MHD simulations. The global behaviour of the instability induces cyclic flaring in the inner regions of the disc, for parameters appropriate for X-ray binary systems, thereby offering a potential solution to a long standing problem. Finally, I calculate the effect of an interacting quintessence model of dark energy on cosmological observables. I find that a scalar-tensor type interaction in the dark sector results in an observable increase in the matter power spectrum and integrated Sachs-Wolfe effect at horizon scales.

The two SCUBA HAlf-Degree Extragalactic Survey (SHADES) fields are amongst the richest places in the sky in terms of multi-wavelength coverage. They comprise an eastern section of the Lockman Hole (LH) and the central portion of the Subaru- XMM/Newton Deep Field (SXDF). In this thesis, I have obtained extremely deep, multi-frequency radio imaging of the SHADES fields using the GiantMetre-wave Radio Telescope (GMRT) and the Very Large Array (VLA), at 610MHz and 1.4GHz, respectively. These data are used to analyse the nature of the sub-milliJansky (sub-mJy) radio population, which has been hotly debated in the last few years: are they powered by star-forming or nuclear activity? To tackle the problem, I employ different approaches making use of the large variety of multi-wavelength data in the SHADES fields. I begin by analysing the spectral index, α610MHz 1.4GHz , of radio sources detected in the LH, to explore the dominant emission mechanism. Based on a robust 10 σ detection criterion, I find a constantmedian spectral index of α610MHz 1.4GHz ≈ −0.6 to −0.7 for sources between S1.4GHz ≈ 200 μJy and 10mJy. This result suggests that the galaxy population in the sub-mJy regime is powered by optically-thin synchrotron emission – starforming galaxies or lobe-dominated active galactic nuclei (AGN). Making use of X-ray observations in the LH, I show that the fraction of radio sources detected in the hard X-ray band (between 2 and 10 keV) decreases from 50 to 15 per cent between S1.4GHz ≈ 1mJy and . 100 μJy, which strongly suggests a transition from AGN to star-forming galaxies. Based on the deep, multi-wavelength coverage of the SXDF, I explore the behaviour of the far-infrared (FIR)/radio correlation as a function of redshift. I combine the q24 factor – the logarithmic flux density ratio between Spitzer 24-μm and VLA 1.4- GHz flux densities – with available photometric redshifts and find strong evidence that the correlation holds out to z ≈ 3.5. Based on M82-like k-corrections and using a high-significance (S1.4GHz > 300 μJy) radio sub-sample, I find a mean and scatter of q24 = 0.71 ± 0.47. Monte-Carlo simulations based on these findings show that fewer sources deviate from the correlation at fainter flux densities (i.e. fewer radioloud AGN). I predict that the radio-loud fraction drops from 50 per cent at ∼ 1mJy to zero at . 100 μJy. The validity of the FIR/radio correlation out to very high redshifts adds credibility to identifications of sub-millimetre (submm) galaxies (SMGs) made at radio wavelengths. Based on a sample of 45 radio-identified SMGs in the LH, I find a median radio spectral index of α610MHz 1.4GHz = −0.72 ± 0.07, which suggests that optically-thin synchrotron is the dominant radio emission mechanism. Finally, as anAppendix I include a theoretical treatment that constrains the average geometry of the dusty, torus-like structures believed to obscure a large fraction of the AGN population. I use the distribution of column densities (NH) obtained from deep ∼ 1Msec X-ray observations in the Chandra Deep Field South. I find that to reproduce the wide observed range of NH, the best torus model is given by a classical “donut”- shaped distribution with an exponential angular dependency of the density profile.

All massive nearby galaxies, including our own, host supermassive black holes. Active galactic nuclei (AGN) are seen when such black holes accrete, and when they produce powerful jets of synchrotron-emitting plasma, they are termed radio-loud AGN. The close correlation between black hole mass and galaxy bulge mass in elliptical galaxies indicates that AGN feedback may be the key to the regulation of galaxy formation. It is thus necessary to fully understand the structure of AGN, the way that they are fuelled, and their duty cycle, in order to study the feedback processes and get a clear picture of galaxy formation. In this thesis, independent methods are developed to constrain the accretion disk and radio jet angles to the line of sight. H IX emission from a sub-sample of high-redshift quasars is measured from near-infrared spectroscopy and modelled as sums of different components, including the characteristic double-peaked profile which results from a thin, rotating accretion disk. Comparing the models using Bayesian evidence, almost all quasars were found to have infrared spectra consistent with the presence of a disk. The jet inclination angles of the same set of quasars were constrained by fitting a model, including the effect of Doppler boosting and the receding torus model for dust obscuration, to the radio \ spectral energy distribution. The fitted disk and jet angles correlate strongly, and are consistent with a model in which the radio jets are launched orthogonally to the plane of the accretion disk, as expected if the jet is powered by energy drawn from the spin of the black hole. Both disk and jet angles correlate with the observed linear source size, which is a projection effect; when deprojected using the fitted angles, the distribution of source sizes agrees with a scenario in which the sources expand into the surrounding medium at a constant rate up to ~ 1 Mpc and then shut off, probably as the nuclei become quiescent. The accretion disk angle was found to correlate weakly with the low-frequency radio luminosity, which provides direct, albeit tenuous, evidence for the receding torus model.

In this thesis I study the inner-most regions of Active Galactic Nuclei (AGN) using the reverberation mapping technique, and neutron star low-mass X-ray binaries in quiescence using X-ray observations. Using the 13-year optical monitoring data for the AGN NGC 5548, the luminosity dependence of the Hβ emitting radius was modelled using a delay map, finding that the radius scales with luminosity as predicted by recent theoretical models. Time-delays between the continuum at different wavelengths in AGN can be used to probe the accretion disc. Here, continuum time-delays in a sample of 14 AGN were used to measure the radial temperature profile of the accretion discs, determine the nuclear extinction, and measure distances to the objects. However, the distances measured correspond to a value for Hubble's constant that is a factor of ~2 lower than the accepted value. The implications of this on the thermal disc reprocessing model are discussed. I present two Chandra observations of the neutron star transient in the globular cluster NGC 6440 in quiescence, where the power-law component to the spectrum is seen to be variable between the observations, suggesting that there is ongoing residual accretion. From a Chandra observation of the globular cluster Terzan 1, I have identifed the likely quiescent counterpart to a transient previously observed in outburst, and discuss the other sources within the cluster. Using Chandra and XMM-Newton monitoring observations of two neutron star transients (KS 1731-260 and MXB 1659-29) in quiescence I have found that the neutron star crusts in both sources have now returned to thermal equilibrium with the core. These observations also indicate that the crusts in both sources may have a high thermal conductivity and that enhanced neutrino emission may be occurring in the core. Finally, the discovery of an X-ray transient with XMM-Newton is presented, and the other sources in this observation discussed.

One of the main components of the unified model of AGN (active galactic nuclei) is the circumnuclear dusty torus. The torus often obscures our view of the ''central engine" of the AGN, therefore it is important to understand its size and structure. Reverberation mapping can be used to constrain the size of the torus, and relies on the response of the torus dust emission to the variations in the driving optical/UV emission of the accretion disk. The time lag between the optical emission and the infrared response can be used to constrain the distance between the inner region of the AGN, and the IR emission region of the torus. This project focuses on a sample of 11 AGN, whose mid-IR (3.6 and 4.5 micron) light curves were observed with the Spitzer Space Telescope. The IR light curves were presented in Vazquez (2015). In this work, the optical light curves are constructed with a combination of data from three different ground-based telescopes, in the B, V, and G bands. Cross-correlation analysis has been conducted for four AGN, giving us reverberation lags for these objects. The reliability of the lags have been carefully evaluated by testing their dependence on different analysis procedures. One of these AGN is a changing-look AGN, NGC 6418, whose previous cross-correlation results have been reanalyzed. The optical-IR lag was measured both before and after a large optical flare, and was found to have increased after the flare, likely due to the sublimation of dust in the inner region of the torus. For eight of the AGN, HST images were used along with the galaxy decomposition program GALFIT to model the distribution of galaxy light and hence to remove the starlight contribution from the optical light curves. This is a necessary step in order to use the torus reverberation mapping code, TORMAC, to model the IR light curves, as the dust emission responds to the optical variations. TORMAC was used to model three of the AGN in our sample, which allows us to constrain other torus structural properties. Some of these properties include the radial cloud distribution, the radial extent of the torus, and the torus inclination angle. This dissertation produces reverberation mapping results for four AGN (KAZ 163, MRK 507, NGC 6418, and UGC 10697). micron flux with typical lags between 15-30 days. This project increases the number of AGN with mid-IR dust reverberation mapping measurements, for which there have been few previous studies. One of the AGN (NGC 6418) is found to be a rare changing-look AGN. Not only are we able to measure the size of the torus, but through the use of TORMAC we also have a more complete picture of the structure of the torus in our sample of AGN.

This thesis is concerned with very high energy γ-ray observations of blazars observable from the southern hemisphere. The data presented were obtained using the recently deployed University of Durham Mark 6 high sensitivity Čerenkov telescope. Chapters 1 and 2 are introductory; the subjects of γ-ray astronomy, extensive air showers, Čerenkov light production, the development of the atmospheric Čerenkov technique and the current status of TeV astronomy are discussed. Chapter 3 introduces the telescopes operated by the University of Durham. The Mark 6 telescope, designed to have a low threshold energy and a high resolution imaging system, is discussed in detail. Chapter 4 presents the calibration and analysis techniques routinely applied to data obtained with the Mark 6 telescope. The chapter concludes with a set of moment parameter selections designed to reject a significant fraction of the cosmic ray cascades, while retaining the majority of γ-rays cascades. These selections have resulted in a 5σ detection of PSR 1706-44. Chapter 5 discusses active galactic nuclei and in particular blazars. Topics included are recent high energy observations; the differences between the radio/X-ray selected BL Lacs and flat spectrum radio quasars; the infrared background; high energy flux variability and γ-ray production mechanisms within these objects. The chapter concludes with a list of possible very high energy γ-ray emitting blazars observable with the Mark 6 Čerenkov telescope. Chapter 6 presents the results from four of these very high-energy γ-ray blazar candidates; PKS 0548-322, PKS 1514-24, PKS 2005-489 and PKS 2155-304. There is no evidence for γ-ray emission from these sources, either in the form of a steady flux or variable activity. Three sigma integral flux upper limits above 300 GeV are produced for these objects and the implications of these observations are discussed. The conclusions are presented in Chapter 7 as well as ideas for future work.

In the next few years, large-scale surveys with a time domain element, such as the ten-year Legacy Survey of Space and Time (LSST), will produce an immense data set to study variable and transient astronomical sources such as active galactic nuclei (AGN). AGN are located at the center of most galaxies and have five major components: a supermassive black hole (SMBH), an accretion disk, the broad-line region (BLR), a dusty circumnuclear torus, and the narrow-line region (NLR). The torus is believed to have a two-component structure: a geometrically thin circumnuclear disk and a hollow dusty cone along the polar axis. The torus absorbs the ultraviolet (UV)/optical light from the accretion disk and re-emits it in the infrared (IR). In order to study the properties of the torus, techniques such as dust reverberation mapping are needed since AGN, at large distances, cannot be resolved by direct imaging. Reverberation mapping uses the differences in the UV/optical and IR light travel times (lags) to determine the properties of the torus region. This thesis focuses on the use of the TOrus Reverberation MApping Code, TORMAC, to create simulations of the response of the IR dust emission to an input AGN light curve. Both an observed optical light curve, that of MCG-06-30-15, a narrow-line Seyfert 1 (NLS1) AGN, and a simulated AGN light curve were used as inputs. These simulations are used to determine how observing strategies (time baselines and observing cadences) and practical limitations (such as seasonal gaps, irregular sampling) affect how much information can be recovered about the properties of the torus region. In addition, I examined how various torus parameters affect the lags recovered from reverberation mapping. The work performed in this thesis will help inform the design of future optical-IR monitoring campaigns, as well as, to better understand the uncertainties in previous studies - especially those with sparse sampling and limited time duration.

A crucial breakthrough in the study of galaxy evolution occurred with the discovery that black holes are ubiquitous in galaxy nuclei, and the masses of supermassive black holes scale with several properties of the host galaxies. The interpretation is that central black holes and their host galaxies co-evolve and regulate one another’s growth. Although the role supermassive black holes play in galaxy evolution is not fully understood, their mass is a key piece of the puzzle. The most commonly adopted methods for determining supermassive black hole mass are reverberation mapping and stellar dynamical modeling. These two techniques rely on independent assumptions, observational data, and analysis procedures, and are almost always applied to different galaxies. Because of the different requirements for each technique, there are only a handful of galaxies that are candidates for both methods, and so each one plays a critical role in probing elements of the techniques themselves and identifying any systematic biases in the results. Currently, only two supermassive black holes have published masses using both methods. In this work, we present mass measurements of the supermassive black holes in two additional galaxies using stellar dynamical modeling. We first present the kinematic and photometric analysis of the lenticular galaxy NGC5273 and demonstrate that the dynamical models reveal a black hole mass of M• = [0.5 − 2.0]x10^7 M⊙. We then discuss the classical spiral galaxy NGC6814 and derive a black hole mass of M• = [0.5−1.2]x10^7 M⊙ from preliminary models. Both galaxies already have black hole masses measured with reverberation mapping, thus, we are effectively doubling the sample of galaxies for this study. The masses for NGC5273 agree, and the masses for NGC6814 are tentatively in agreement, depending on the scale factor used for the reverberation mass.

In this dissertation I present work done from 2018-2023 to investigate the growth of galaxies and supermassive black holes (SMBH) in high redshift overdensities (protoclusters) by studying the star-forming galaxy and active galactic nucleus (AGN) population in the SSA22 protocluster. I examined possible environmental sources of the enhanced star formation and AGN activity in the z = 3.09 SSA22 protocluster using Hubble WFC3 ∼ 1.6 μm observations of the SSA22 field, including new observations centered on eight X-ray selected protocluster AGN. To investigate the role of mergers in the observed AGN and star formation enhancement, quantitative and visual morphological classifications were applied to F160W images of protocluster Lyman break galaxies (LBGs) in the fields of the AGN and z ∼ 3 field LBGs in SSA22 and the GOODS-N field. No significant differences are found between the morphologies and merger fractions of protocluster and field LBGs, though this conclusion is limited by small number statistics in the protocluster. The UV-to-near-IR spectral energy distributions (SED) of F160W-detected protocluster and field LBGs were fit to characterize their stellar masses and star formation histories (SFH). These fits suggest that the mean protocluster LBG is a factor of ∼ 2 times more massive and more attenuated than the mean z ∼ 3 field LBG. These results suggest that ongoing mergers are no more common among protocluster LBGs than field LBGs, though protocluster LBGs appear to be more massive. The larger mass of the protocluster LBGs may contribute to the enhancement of SMBH mass and accretion rate in the protocluster, in turn driving the protocluster AGN enhancement. I also analyzed the physical properties of 8 X-ray selected AGN and one candidate proto- quasar system (ADF22A1) in the SSA22 protocluster by fitting their X-ray-to-IR SEDs. SFHs were recovered for 7 of these systems which are well-fit by stellar population and AGN models. The majority of the protocluster AGN systems, including ADF22A1, are consistent with obscured SMBH growth in star forming galaxies. The SFRs, SMBH accretion rates, and masses estimated from the SED fits for the 9 protocluster AGN systems are consistent with the distributions of SFR, accretion rate, SMBH masses, and stellar masses for a sample of X-ray detected AGN candidates in the Chandra Deep Fields (CDF). The ratio between the sample-averaged SSA22 SMBH mass and CDF SMBH mass is constrained to < 1.41. Furthermore, no statistically significant trends between the AGN or host galaxy properties and their location in the protocluster are found. The similarity of the protocluster and field AGN populations suggests that the observed protocluster AGN fraction enhancement may be driven by the SMBH mass distribution of the entire galaxy population, rather than only the AGN population, consistent with the interpretation of the results of the WFC3 F160W study of LBGs. The results presented here add to the existing body of knowledge on the growth of galaxies and SMBHs in protoclusters, suggesting that galaxies and their black holes grow by the same mechanisms in protoclusters and field environments, and that the observed AGN enhancement is driven instead by more frequent, secular, triggering of AGN episodes in star forming galaxies.

Blazars are a subclass of active galactic nuclei whose observable characteristics are generated by relativistic jets of high-energy plasma with trajectories closely aligned to the line of sight. This orientation results in extreme observed phenomena, such as ultraluminous emission, high amplitudes of variability, and high degrees of optical linear polarization. Furthermore, blazars are the most common extragalactic sources of γ-ray photons and have been proposed as sources of high-energy neutrinos. Long-timescale monitoring of blazars has revealed relatively quiescent states interspersed with active states featuring dramatic brightening events, with timescales ranging from months to years. New, high-time-resolution observations of blazars are revealing dramatic variability on timescales as short as several minutes, the physical drivers of which are not well understood. In this dissertation, I focus on observational signatures that can potentially identify these mechanisms. To characterize this variability, I combine observations of blazars from many telescopes, obtaining a comprehensive view of events occurring in the jets. Using the Very Long Baseline Array, I investigate moving and quasi-stationary features in the parsec-scale jets of 38 blazars. I find that the flow speeds, orientation, brightness temperatures, and opening angles of the jets in flat-spectrum radio quasars (FSRQs), BL Lacertae type objects (BL Lac objects), and radio galaxies (RGs) are statistically different, with FSRQs having the highest speeds and smallest viewing angles. Focusing on a typical source of each subclass, I characterize the optical variability observed at 2-min cadence (with the Transiting Exoplanet Survey Satellite), at gamma-ray energies (Fermi Large Area Telescope), X-ray energies (NICER, NuSTAR, and Swift satellites), and optical (ground-based instruments, especially, the Perkins telescope), including linear polarization measurements. I find that the minimum timescale of RG variability is longer than those of the FSRQ and BL Lac object, and is likely caused by changes in the accretion disk. In contrast, the variability in the FSRQ and BL Lac object can be associated with shocks, magnetic reconnections, and turbulence in the jet. The unprecedented availability of data, which promises to expand in the future, represents a new phase of observational astronomy that provides valuable information on changes in the jets of blazars.

The study of Active Galactic Nuclei formation and evolution is important in modern astronomy, as they represent an evolutionary stage in every galaxy, and thus are important to understand the formation and evolution of galaxies. However, there is not yet a full understanding of the physics and evolution of the population that emit brightly at radio wavelengths. In order to properly compare observations with theory, a large survey of radio-loud galaxies is required. In this project, I will be studying Double Radio sources Associated with Galactic Nuclei, as I wanted to learn more about them. I will be focusing on the Fanaroff-Riley types I and II in particular. I study their luminosities and find a large overlap between the two classes, moreso than previous studies, with no obvious ‘break’. I also study their alignment in the sky, and find significant alignment up to 4 degrees. This is to test claims that the position angles of radio galaxies are aligned over cosmic distances. Such an alignment could only have arisen at the time of formation which would perhaps be able to provide some insight into early formation of supermassive black holes. I will be using data from the Evolutionary Map of the Universe pilot survey that was obtained with CSIRO’s Australian Square Kilometre Array Pathfinder, as well as GAMA-23 Early Science. A survey of this size will allow study of a large sample of sources that are free from selection effects and biases of previous and older studies and will be a valuable training set for my colleagues who are developing machine learning algorithms to find and classify radio sources.

Active galactic nuclei (AGNs) are powered by accretion of material onto their central supermassive black holes and are unique in that they emit radiation across the entire electromagnetic spectrum. Spectral energy distributions (SEDs) allow for a comparison of the energy that is emitted in each wavelength regime. Our goal is to construct the most accurate multi-wavelength SEDs by reexamining the near-IR to X-ray SEDs of 23 AGNs with reverberation mapped (RM) black hole mass measurements. These SEDs will allow us to constrain measurements, such as bolometric luminosity and accretion rate and, in turn, these measurements can be used to better understand physical processes of AGNs and how they influence the host galaxy. To improve on previous studies, this set of SEDs consists of simultaneous optical/UV/X-ray data taken by XMM-Newton and is supplemented by ground-based near-IR data to constrain the red tail. Using two-dimensional decompositions of high-resolution Hubble Space Telescope images, combined with galaxy spectral templates, we constrain and remove the host galaxy contribution and focus the SEDs on the accretion powered AGN flux. We also consider the impacts of internal reddening and the effects of broad and narrow line region emission. Finally, given the highly variable nature of AGNs, we examine 4 objects from this sample with 200-300 simultaneous optical/UV/X-ray Neil Gehrels Swift Observatory observations and quantify how variability impacts the shape of the SEDs and the scatter in measurements of bolometric luminosity and accretion rate.

All but a few Active Galactic Nuclei are too distant for any single aperture telescope to resolve, yet the geometry of these objects is key to our understanding of the evolution of galaxies in the universe. To study the geometry of Active Galactic Nuclei, methods are leveraged using smaller telescopes to gain resolution rather than relying on larger and larger single aperture telescopes. Principally, these are via reverberation mapping which utilizes temporal and spectral resolution to gain spatial resolution and interferometry which sacrifices the light gathering power of large aperture telescopes for high angular resolution. To provide a tool for recovering geometric indicators of AGNs from reverberation mapping data, an image reconstructing algorithm is developed utilizing the alternating direction method of multipliers with compressed sensing regularization. This new algorithm is applied to the Arp 151 dataset from the Lick AGN Monitoring Project 2008. In addition to the reverberation mapping algorithm, the first calibrated extragalactic results for the CHARA Array are presented for NGC 4151. At the extreme detection limit for the instruments of the CHARA Array, the observational strategies utilized for successful observations and the results obtained are discussed.

The past 20 years of observations have revealed correlations between supermassive black holes (SMBHs) and large scale galaxy properties. These correlations are generally interpreted to be the signature of coevolution between galaxies and black holes, therefore accurate measurements of the properties on which these correlations are constructed is crucial for understanding galaxy-black hole evolution over cosmic time. We first present HI 21 cm spectroscopy for 31 AGN host galaxies with direct black hole mass (MBH) measurements from reverberation mapping (RM), 12 of which are the first published detections at 21 cm. We measure emission-line fluxes, widths, and recessional velocities from which we derive HI mass, gas mass (MGAS), and redshift. Combining MGAS with stellar mass (MSTARS) allows the baryonic mass (MBARY) to be estimated. We then combine the emission line measurements with Hubble Space Telescope and ground-based optical and near-infrared images to measure Tully-Fisher distances to 24 of the 31 galaxies, 14 of which are the first reported distances independent of redshift. We estimate dynamical mass (MDYN) within the HI radius, and a comparison to MBARY constrains the dark matter mass (MDM). We find significant correlations between MBH and MBARY, MDYN, and MDM, suggesting black hole-galaxy connections on scales larger than commonly considered. Finally, we present preliminary results of dynamical modeling of the broad line regions (BLRs) of five AGNs, four of which are the first reported. Direct modeling of the continuum light curve and broad emission lines yields constraints on BLR geometry and kinematics in addition to constraints on MBH that do not depend on the scale factor required by traditional RM methods. This work presents measurements of fundamental properties of RM AGNs and their host galaxies that may serve as important tools for future analyses devoted to furthering our understanding of black hole-galaxy coevolution.

Quasars are among the most luminous and massive objects in the universe. They host an actively accreting supermassive black hole (SMBH) at their centre that can power energetic phenomena that outshine the host galaxy. A substantial fraction (∼ 20 − 40%) of quasars show broad absorption-line (BAL) features in their rest-ultraviolet (UV) spectra that show clear evidence for powerful outflows originating from the central SMBHs. These outflows can carry a significant amount of energy and mass from the central SMBHs to their host galaxies. Therefore, BAL quasars are prime targets for investigating luminous quasar feedback on galaxies by which the star formation rate and the evolution of the host galaxies are thought to be controlled. SimBAL provides a novel method to study the complicated spectra of BAL quasars. Tens of thousands of BAL quasar spectra available in public archives were assumed to be impossible to analyze for more than five decades. The spectral synthesis code SimBAL our group developed can analyze even the most heavily absorbed BAL quasar spectra and provide constraints on the physical properties of the outflow as a function of velocity. Using this software, I discovered the most powerful BAL quasar outflow analyzed to date in SDSS J135246.37+423923.5. The spectral analysis showed that the outflow has the velocities reaching ∼ −38000 km/s and the outflowing gas is located near the dusty torus at ∼ 10 pc from the central SMBH. The outflow has the log kinetic luminosity log LKE ∼ 48.1 [erg/s] which exceeds the bolometric luminosity of the quasar and is energetic enough to power feedback. I performed the first systematic study of a large sample of low-redshift iron low- ionization broad absorption-line (FeLoBAL) quasars and characterized the general properties of the FeLoBAL outflows. The SimBAL analysis of 50 FeLoBAL quasar spectra revealed that the FeLoBAL gas is located at a wide range of distances (0 ≲ log R ≲ 4.4 [pc]) and has a wide range of physical properties. We discovered a new type of FeLoBAL quasar, loitering outflow objects, that are characterized by low outflow velocities and high column density winds located at log R ≲ 1 [pc] from the central SMBH. Finally, I used SimBAL to analyze a spectroscopic variability for J164653.72+243942.2 that showed a variable BAL features from an extremely-high velocity outflow (v ∼ 0.17c). The series of work presented in this dissertation demonstrates only a small fraction of scientific discoveries that can be made with SimBAL. I briefly summarize the results from the series of publications led by SimBAL collaboration that make use of the properties of the low-redshift FeLoBAL quasar outflows constrained from the SimBAL analysis. Finally, some of the planned future work using SimBAL is outlined.

Active galactic nuclei (AGN) driven feedback has been proposed to be one of the most efficient ways to quench star formation and help maintain quiescence in massive galaxies. However, direct evidence of feedback in typical quiescent galaxy populations has been restricted to only a handful of sources. This thesis presents a series of evidence that large-scale winds in a new (and relatively common) class of early-type galaxy, known as `red geysers', may represent AGN radio mode feedback in action. The work is primarily based on integral field spectroscopic data from the SDSS IV- MaNGA survey that has completed observing 10,010 galaxies in the optical wavelength at $z

Supermassive black hole binaries (SMBHBs) can lurk, often unseen, in the centers of post-merger galaxies, and pulsar timing arrays (PTAs) are rapidly approaching the sensitivities required to detect nanohertz gravitational waves (GWs) from these giant pairs. Independently, numerous electromagnetic surveys are seeking evidence of these dynamic duos’ effects on their host galaxies by searching for periodicities in time-domain observations. Combining these two methods to use multi-messenger techniques allows us to learn more about these binaries than using one messenger alone. In this thesis, we have created Bayesian methods to search for SMBHBs using electromagnetic observations of quasars and through GW emission in PTA data. By using electromagnetic observations to identify an SMBHB candidate, we gain numerous pieces of information that also define the source's GW emission, including the location of and distance to the SMBHB's host galaxy and the orbital period of the SMBHB. In our study, we developed the first multi-messenger techniques used by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and applied them to a well-known supermassive black hole binary candidate, 3C 66B. We placed the lowest chirp mass limit to date on an SMBHB within 3C 66B of M Next, we analyzed the capabilities of Bayesian methods to search for electromagnetic signatures of these binaries in simulated time-domain data sets from next-generation surveys. We developed a Bayesian model selection technique to identify periodicities induced into a quasar light curve by the orbital motion of an SMBHB from within intrinsic red noise. We discovered that future surveys, such as the Legacy Survey of Space and Time (LSST), will identify more robust SMBHB candidates than current surveys, such as the Catalina Real-time Transient Survey (CRTS). Finally, we present the results of searches for bright continuous GWs (CWs) from individual SMBHBs in NANOGrav's 12.5-year data set. A red noise process, which could be the first signs of an emerging stochastic GW background (GWB), was previously detected for the first time in this data set. In our work, we searched for CWs alongside this common noise process for the first time in real PTA data, and developed necessary data-handling techniques which will be critical for the detection of a CW, which may come soon after the potentially imminent detection of the GWB.

It is now well established that a substantial fraction of all galaxy assembly occurs in intense bursts of star formation and black hole accretion, but the role of these two modes and how much they affect one another remains unclear. We thus investigate this in three complementary studies. In the first, we assemble a sample of 513 quasars identified by the Sloan Digital Sky Survey with detections by Herschel. These objects span a redshift range of 0 < z < 4, and their SEDs give a mean SFR of ~1000M☉/year. When comparing these SFRs to the intrinsic properties of the quasars, we find no clear connections between the quasars and the ongoing star formation events in their hosts. We then look for evidence of AGN feedback in broad absorption line (BAL) quasars, as such features are indicative of outflowing material. We find that high-ionization BAL quasars have indistinguishable properties to those of classical quasars. In our second study, which describes an iron low-ionization BAL quasar, SDSS J121441.42-000137.8, our results are again consistent with no feedback. Thus, it seems unlikely that feedback plays a dominant role in quenching star formation at the extreme SFRs seen in our BAL objects. We lastly study the host of an optically-bright quasar, SDSS J160705.16+355358.6, with evidence of an ongoing merger. We create the Point Spread Function (PSF) using a star that is in the same part of the field as our object, a method which is relatively unexplored. By subtracting the PSF, we are able to extract some of the host properties. We compare two PSF creation methods and find the empirical approach to be superior. Fits to the SEDs of the two galaxies are consistent with both falling on or above the main sequence of star formation. It is additionally plausible that these two galaxies could coalesce into a single massive quiescent galaxy by z ~ 2, and thus serve as progenitors to this class of galaxy that has proven challenging to our understanding of galaxy assembly.

This thesis presents a characterisation of the broadband behaviour of TeV gamma-ray emitting blazars that belong to the family of active galactic nuclei. Their high bolometric luminosity as well as their bright non-thermal emission up to at least TeV energies make them among the most energetic persistent sources in the universe. Blazars are also identified by large relativistic plasma jets that are aligned at a small angle to the observer’s line of sight. These jets have a size of order ∼kpc-Mpc and radiate non-thermal emission. The emission is strongly variable over the entire spectrum on all time scales and one can observe flux changes by an order of magnitude on sub-hour time scale in the most extreme cases. The origins of these phenomena are still poorly understood and the underlying mechanisms for the gamma-ray emission in blazars are still debated. These violent and extreme properties make blazars candidate sources of ultra-high-energy cosmic rays (E > 10^18 eV), whose origins are still unknown. Blazars are also potential factories of PeV neutrinos, which diffuse flux has been detected by the IceCube detector. The goal of this thesis is to provide a detailed characterisation of the broadband emission properties of blazars with the ultimate aim to understand their emission mechanisms. This work focuses on two TeV emitting blazars, belonging to the category of BL Lac type objects: 1ES 2344+514 and Mrk 421. These two objects exhibit on a temporary basis a synchrotron energy flux peaking at energies around or above 1 keV. Only a handful objects detected in the TeV band are known to exhibit such peculiar and extreme properties; they are dubbed as extreme high-frequency BL Lacs (EHBL). 1ES 2344+514 and Mrk 421 are also among the brightest and the closest TeV gamma-ray emitting blazars. Thanks to their brightness, their broadband evolution can be characterised on relatively short time scales by current instruments. Thanks to their proximity, their gamma-ray emission is not strongly absorbed by the extragalactic background light. Therefore, 1ES 2344+514 and Mrk 421 are ideal targets to better understand the most extreme phenomena observed in the jets of blazars. In order to achieve an optimal energy and temporal coverage from radio to TeV, the present work makes use of data from a large number of instruments: MAGIC, ACT, Fermi-LAT, Swift-BAT, Swift-XRT, Swift-UVOT, instruments from the Whole Earth Blazar Telescope network, NOT, KAIT, OVRO, Metsähovi and Medicina. This thesis also makes use of observations from the NuSTAR instrument, which is currently the most sensitive telescope in the hard X-ray regime (> 10 keV) and the only instrument capable of precisely measuring the flux on a sub-hour time scale at these energies. The NuSTAR data are crucial to constrain models given that within leptonic scenarios, the hard X-ray emission is expected to originate from electrons with similar energies than those responsible for the TeV emission measured by MAGIC and FACT. Based on this extensive multiwavelength coverage, several atypical results are reported and the complex behaviour of 1ES 2344+514 and Mrk 421 are characterised with a high precision. The emission is interpreted using leptonic scenarios set up for this work. The blazar 1ES 2344+514 was observed during a period of strong flux activity in summer 2016. The measured very-high-energy (VHE; E > 100 GeV) flux reached a similar level as the historical maximum. A high activity was also measured in the X-rays. The spectral properties during this peculiar event are investigated in detail. One finds that the X-ray spectrum is hard, indicating a low-energy component of the spectral energy distribution (SED) peaking around or above 10 18 Hz (∼4 keV). This value is about two orders of magnitude higher than during archival observations of 1ES 2344+514. 1ES 2344+514 showed an EHBL behaviour on a temporary basis in August 2016. The SED is interpreted with a leptonic and a proton-synchrotron scenario for the gamma-ray emission. Thanks to the wealth of multiwavelength data that has been collected, these models can be better constrained compared to previous works. One finds that both theoretical models are able to describe the SED but require a system out of equipartition by several orders of magnitude, challenging current acceleration process thought to occur in relativistic plasma jets. Mrk 421 was observed from radio to TeV over a ∼7-month period between November 2016 and June 2017. Based on the dense temporal and energy coverage, the correlation patterns of the source are characterised in order to better constrain the emission mechanisms. The VHE versus X-ray flux correlation is studied by binning the data into several sub-energy bands. The correlation is characterised up to the hard X-ray band. The fluxes are strongly correlated, in agreement with leptonic models, which predict that the X-ray and VHE fluxes are emitted by a single electron population. Nonetheless, the present work unveils that the correlation characteristics strongly vary across the sub-energy bands: the correlation ranges from a linear to a cubic trend. Over the multiwavelength campaign, several nights appear as outliers and follow different correlation patterns. They show an enhanced gamma-ray activity without a strong X-ray emission. Using simultaneous MAGIC/NuSTAR/Swift-XRT observations, this peculiar behaviour can be explained by an emitting region in the jet that is changing its size over time. The correlation between the X-ray and UV/optical emission is also studied. An anti-correlation at the significance level above 3σ is found. A hint of this pattern was measured only once until now and only at a marginal significance. The second occurrence of an anti-correlation reported in this work is an important result as it strongly supports the hypothesis that this behaviour has a real physical origin. It may be indicative of changes in the cooling and acceleration efficiencies of electrons in the jet. Finally, a short and bright VHE flare of Mrk 421 is investigated in detail. Interestingly, this VHE flare is identified by an absence of clear X-ray flux increase. This particular day appears as an evident outlier in the VHE versus X-ray correlation study. Using time-resolved SEDs and a leptonic modelling, this work proposes that such a behaviour may be caused by the appearance of a second electron population (which adds up to the electron population responsible for the quiescent state) with a narrow energy distribution located in a compact blob.

There is overwhelming evidence for a link between supermassive black holes (BHs) and their host galaxies, yet the physical mechanisms behind the connection are poorly understood. Discerning between possible co-evolutionary scenarios requires precise measurements of BH masses and host properties across cosmic time. BH mass measurements outside the local universe require bright active galactic nuclei (AGNs), but the AGN light overwhelms the light of the host, making host property measurements difficult. Further, BH mass estimates rely on the unresolvable broad emission line region (BLR), and the unknown nature of the BLR limits their precision.In this dissertation, I use a unique combination of gravitational lensing and reverberation mapping techniques to tackle both issues. First, I present a semi-supervised classification approach for identifying rare lensed quasars based solely on photometry in wide-field surveys. Lenses are powerful tools for measuring host properties, but studies are limited by the small sample size, so discovery efforts are crucial. Using this technique, I successfully discovered three new lenses in the Sloan Digital Sky Survey, with several other candidates awaiting further follow-up. This approach is flexible and can easily be modified to target specific types of lenses as well as lenses in surveys covering different portions of the sky.Next, I use a modeling framework to measure precise BH masses for seven AGNs and infer the structural and kinematic properties of the Hβ BLR. This nearly doubles the sample size of modeled BLRs, allowing for a correlation analysis between BLR properties and AGN observables that can be used to improve BH mass measurements for all AGNs. I then model Hβ, CIV, and Lyα in NGC 5548 using data from 85 Hubble Space Telescope orbits and multiple ground-based telescopes to understand the similarities and differences between the optical and UV BLRs. High-z BH mass measurements rely on UV lines, but our current understanding of the UV BLR is severely lacking, so understanding their connection is critical for decreasing high-z BH mass uncertainties. Finally, I tie the previous projects together with the most precise high-z BH mass measurement to date, using data from a 4.5 year monitoring campaign of a strongly lensed quasar at z=2.805. Accounting for gravitational time delays, the campaign exceeds 6 years in duration. Understanding the co-evolution of BHs and their hosts will require measurements of this precision at high-z, and this research demonstrates that gravitational lensing makes this level of precision obtainable.

Mergers play a vital role in galaxy evolution, having the potential to trigger Active Galactic Nucleus (AGN) activity, star formation, or changes in morphology. In this work, we investigate whether galaxy mergers have an effect on AGN activity in the galaxies involved. We used a visual classification scheme to classify the morphologies of nearly 50,000 galaxies and identify galaxy mergers in the CANDELS fields, a Multi-Cycle Treasury Program using the Hubble Space Telescope. We use IR and X-ray selection criteria to identify IR and X-ray AGN, and matched controls to both our AGN subsample and merger subsample. We investigated the fraction of mergers that host AGN, as well as the fraction of AGN that are found in merging systems, to study the AGN-merger connection from multiple angles. We also investigated the properties of mergers, such as the mass ratios of early-stage mergers, and their star forming properties. We find that galaxies involved in a merger or interaction are more likely than non-merging galaxies to host AGN. The fraction of merging systems that host AGN is 0.034±0.002,compared to an AGN fraction of 0.023±0.001 in non-merging systems. We also find that IR AGN are more likely to be found in merging systems, with a merger fraction of 0.353+0.021−0.019, compared to a merger fraction of 0.225+0.007−0.006 in control galaxies without AGN activity. This is likely because these are more dusty and obscured, and are expected to be observed earlier in the merger process, when merger signatures are more visible. The AGN detected in either the X-ray or both the IR and X-ray are even less likely to be found in merging systems, as the merger signatures are expected to fade later in the merging process. These results show that mergers play an important role in driving AGN activity.

"Quasar-mode feedback" occurs when momentum and energy from the environment of accreting supermassive black hole couple to the host galaxy. One mechanism for such a coupling is by high-velocity (up to ~0.2c) quasar-driven ionized outflows, appearing as blue-shifted absorption and emission lines in quasar spectra. Given enough energy and momentum, these outflows are capable of affecting the evolution of their host galaxies. This dissertation presents the studies of emission and absorption quasar outflows from different perspectives. (1). By conducting large broad absorption line (BAL) quasar surveys in both Sloan Digital Sky Survey and Very Large Telescopes (VLT), we determined various physics properties of quasar absorption outflows, e.g., the electron number density ((ne), the distance of outflows to the central quasar (), and the kinetic energy carried by the outflow (̇k). We demonstrated that half of the typical BAL outflows are situated at > 100 pc, i.e., having the potential to affect the host galaxies. (2). Our group carried out a Hubble Space Telescope program (PI: Arav) for studying the outflows in the Extreme-UV, collaborating with Dr. Gerard Kriss from Space Telescope Science Institute (STScI). We developed a novel method to fit the multitude of quasar absorption troughs efficiently and accurately. We have identified the most energetic quasar-driven outflows on record and discovered the largest acceleration and velocity-shift for a quasar absorption outflow. (3). By using the VLT data, Xu led the project to study the relationships between BAL outflows and emission line outflows. We found possible connections between these two types of quasar outflows, e.g., the luminosity of the [III λ5007 emission profile decreases with increasing ne derived from the BAL outflow in the same quasar. These findings are consistent with BAL and emission outflows being different manifestations of the same wind, and the observed relationships are likely a reflection of the outflow density distribution.

The center of every large galaxy is believed to host a supermassive black hole (SMBH). In some of these galaxies, the nuclear region is extremely luminous and far outshines the entire stellar population. Called active galactic nuclei (AGN), these central sources are equally fascinating and perplexing. Due to their immense distance and small size, modern observation techniques are unable to resolve the structure of these regions. However, the polarization of scattered light carries the imprint of the geometry and kinematics of both the sources and the scattering medium. Here, we present a study of the broad H$\alpha$ lines in a sample of 46 AGN. Following the work of Smith et al. 2002, 2004, and 2005, we show supporting evidence for the proposed two-component scattering model of AGN using the largest sample of spectropolarimetric observations of broad-line AGN assembled to date. Objects of differing polarization classifications (equatorial or polar) are shown to have significant differences in their distributions of both total flux and polarized flux inter-percentile velocity values. Furthermore, clear distinctions between these classifications are found in polarization position angle and core polarization percentage. These results are supported by Kolmogorov-Smirnov tests between the classifications and are in accordance with those suggested by previous studies. We also explore the validity of a spectropolarimetric method of estimating SMBH masses. In general, SMBH mass estimates require information on the inclination of the object in the sky, but we can avoid requiring this information by using polarized inter-percentile velocity measurements instead. Using this method, we achieve estimates on the order of 10^6 to 10^8 solar masses -- estimations consistent with both the expected values and those in the literature.

Although the first supermassive black hole (SMBH) was observed over 100 years ago, the details regarding how they form, evolve, and impact their surroundings remain active topics of research. In fact, only over the past 5 years has the existence of black holes been confirmed via the advent of detectors such as the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) and the Event Horizon Telescope (EHT). From an observational perspective, although all massive galaxies are thought to harbor nuclear SMBHs, the observability is dependent on the accretion activity. The activity of accreting SMBHs, or Active Galactic Nuclei (AGN), is expected to evolve with time and is likely a function of the ability to efficiently channel gas inflow to the galactic center. Many black hole feeding mechanisms exist, such as gravitational instabilities in galaxies that are barred or nucleated, gas dynamic processes involving multiple black holes, and major galaxy interactions. To further complicate the matter, it is possible that more than one mechanism plays a role at any given time. However, analyzing which SMBHs are active, and why, is vital to understanding various feedback processes and how the growth of a SMBH and its host galaxy are tied. In this dissertation, I present various analyses that are all focused on studying the activity of SMBHs in various environments. Specifically, I have studied a population of AGN in nucleated galaxies to analyze how nuclear star clusters affect SMBH activity (Chapter 2), and a binary AGN candidate (two SMBHs that are gravitationally bound; Chapter 3) that did not appear to be undergoing any major merger. However, the majority of my dissertation focuses on detecting and analyzing AGN pairs, (``dual AGN", if the SMBHs are not yet gravitationally bound). Despite the importance of dual AGN to wide-ranging astrophysical fields such as galaxy formation and gravitational waves, the rate of dual AGN has yet to be accurately measured. Yet, the rate of dual AGN can inform us of the role galaxy mergers play in triggering AGN, timescales for post-merger SMBHs to sink to the center of the potential well, as well as merger-related feedback physics. Dual AGN that are widely separated relative to the instrument PSF and have near unity flux ratios are easy to identify, however dual AGN with small separations and/or flux ratios can only be distinguished from a single AGN with advanced statistical analysis. As a result, very few dual AGN have been confirmed. Thus, I've developed a tool called BAYMAX (Bayesian Analysis of Multiple AGN in X-rays), that quantitatively evaluates whether a given source in a Chandra observation is composed of a single or multiple point sources, using a Bayesian framework. With BAYMAX, I am methodically expanding the known population of multiple AGN systems (Chapters 5 and 6), while learning more about their preferential environments via multi-wavelength (Optical, IR, and X-ray) analyses.

It has long been assumed that Active Galactic Nuclei (AGN) reside at the centers of their host galaxies, but is this really true? A galaxy merger is expected to lead to the formation of a supermassive black hole (SMBH) binary, which can shrink through dynamical processes until it eventually coalesces through the emission of gravitational waves. Numerical relativity simulations show that, depending on the initial spin-orbit configuration of the binary, the merged SMBH receives a gravitational recoil kick that may reach several 1000 km s−1. The kick causes the merged SMBH to oscillate for up to ∼ 1 Gyr in the gravitational potential well of the galaxy, during which, the recoiling SMBH may be observed as a ‘displaced’ AGN. Displacements ∼ 10-100pc may be expected even in nearby elliptical galaxies and can be measured as spatial offsets in high resolution optical/infrared images. In this dissertation, I present the results of a study of ∼100 nearby elliptical galaxies, that host AGN, using Hubble Space Telescope archival and new optical/infrared images, to analyze spatial offsets between the photo- center and AGN position. Evidence for significant spatial offsets has been found in about 20% of the sample of which six are considered to be robust displacements (∼5 - 40pc). Three other galaxies display a dual nucleus structure. These results are discussed in the context of the gravitational recoil hypothesis and alternative displacement mechanisms, including acceleration by radio jets, are considered. A second study described in this dissertation focusses on the luminous quasar E1821+643 which has previously been identified as an SMBH recoil candidate based on the large Doppler shifts of emission lines attributed to the retained gas. Follow-up spectroastrometry revealed a spatial displacement between the nucleus and the gas emitting the [OIII] lines. Hubble Space Telescope (HST) images were used to map the distribution of narrow-line emission on sub-arcsecond spatial scales which show that the [OIII] emission is intrinsically asymmetric. Spectroastrometry simulations reveal that the asymmetric [OIII] emission can account for only ∼25% of the observed displacement, which strengthens the case for a recoiling SMBH.

The mass of a supermassive black hole (MBH) is a fundamental property that can be obtained through observational methods. Constraining MBH through multiple methods for an individual galaxy is important for verifying the accuracy of different techniques, and for investigating the assumptions inherent in each method. However, there exist only a few galaxies where multiple MBH determination techniques can be applied. NGC 4151 is one of these rare galaxies for which multiple methods can be used, stellar and gas dynamical modeling because of its proximity and reverberation mapping because of its active accretion. In this work, we reduced the integral field unit spectroscopy of the nucleus of NGC 4151 from Onken et al., observed in the H band with Gemini North NIFS, improving the process itself as well as the analysis of the spatially-resolved spectra. We also improved on the methods for constraining the line of sight velocity distribution as a function of position within the nucleus. Stellar dynamical modeling was then performed over a range of choices of MBH and mass-to-light ratio, as well as the de-projected luminosity density for various inclinations. The best reproduction of the observed kinematics and luminosity density was found with MBH = 2.44 +/- 0.16 x 10^7 MSun and mass-to-light ratio = 0.318 +/- 0.003 MSun/LSun and an inclination of 45 degrees. This measurement falls within the range of values in the literature; it is below the reverberation mapping mass of Bentz et al. (3.59 +0.45 -0.37 x 10^7 MSun) and above the reverberation mapping mass of De Rosa et al. (1.97 +/- 0.04 x 10^7 MSun), and within the uncertainties on both the gas dynamical modeling mass of Hicks & Malkan (3.0 +0.8 -2.2 x 10^7 MSun) and the stellar dynamical modeling mass of Onken et al. (3.60 +/- 1.10 x 10^7 MSun). This work also represents a preparatory step in the application of a new bar-optimized stellar dynamical modeling code.