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

Quasars, powered by accretion onto the super-massive black holes at their galaxies' centres, are capable of ejecting highly energetic outflows of gas, which can suppress star formation in the host galaxy, thereby impacting its evolution. Quasar-driven outflows are widely invoked in galaxy formation models in order to reproduce the observed properties of massive galaxies. However, there are still fundamental open questions regarding the origins of these outflows, and their relationship to accretion processes. This thesis covers a series of multi-wavelength studies of the outflow properties of quasars at redshifts 1.5 < z < 3.5 from the Sloan Digital Sky Survey. In particular, I focus on the CIV emission space which is often used to infer the strength of quasar winds via the CIV blueshift. A major aim of this thesis is to constrain the outflow properties of quasars by combining studies of emission lines, broad and narrow absorption lines. After an introduction to quasars and their outflows, I describe my use of independent component analysis (ICA) for reconstructing the quasar spectra. With the reconstructions, I define a sample of broad absorption line (BAL) quasars, named for the presence of broad absorption lines in their spectra. I subsequently explore the outflow properties, evident both in absorption and emission, of the high-ionisation BAL and non-BAL quasars, revealing strong systematic correlations between the emission and absorption properties. I also demonstrate that (high-ionisation) BALs and non-BALs likely represent different views of the same underlying quasar population by revealing similarities in the luminosities and emission-line outflow properties between the BAL and non-BAL quasar populations. I investigate the prevalence of associated and outflowing narrow CIV absorption lines (NALs) in quasars across the CIV emission space. I find that the velocity of the outflowing NALs increases with CIV emission-line outflow velocity in BAL and non-BAL quasars alike. I use this result to further argue that BAL and non-BAL quasars arise from the same parent population. The velocity of line-locked systems, which are evidence for radiation line-driving, is also shown to increase as the CIV blueshift increases in BAL and non-BAL quasars which indicates the importance of radiation line-driving for driving outflows in both quasar populations. Finally, I present an investigation of the low-frequency radio and ultraviolet properties. I find quasars with similar ultraviolet properties but differing radio properties suggesting, perhaps, that the radio and ultraviolet emission is tracing activity occurring on different time-scales. Additionally, opposing trends are observed between the radio-detection and radio-loud fractions as functions of CIV blueshift which alludes to multiple processes contributing to the radio emission. I explore the plausibility of winds, compact jets, and star formation as sources of the radio-quiet emission, ruling out none.

Supermassive black holes (SMBHs) at the centres of galaxies are known to actively accrete, forming so-called 'active galactic nuclei' (AGN) or 'quasars'. These AGN are believed to feed back energy into their host galaxies, regulating star formation and the growth of the SMBH itself. This thesis investigates the outflow properties in quasars at 1 < z < 3, corresponding to the peak epoch of galaxy formation and SMBH growth. First, I make use of recent improvements to atomic energy-level data and photoionisation models to constrain the properties of the Fe III emitting material in quasars. I show that this material must be dense and micro-turbulent to explain the observed strength of emission, and thus arise from the inner parts of the broad line region. The strength of this emission is shown to correlate with the outflow properties. Second, I develop an SED model capable of reproducing the median observed photometric colours in the SDSS quasar population. This model is then used to investigate the properties of the emission from the hottest, sublimation-temperature dust components in the near infrared spectra of quasars. This dust is believed to be located at the inner edge of the toroidal obscuring structure. The strength of hot dust emission is found to correlate with the quasar outflow properties, providing evidence of a link between the broad line region and the dusty structures surrounding the inner regions of the AGN. Finally, I quantify the outflow properties in a sample of heavily dust-reddened quasars. When matched in redshift and luminosity, there is no significant difference between the outflow kinematics of reddened and unobscured quasars. Assuming a paradigm in which dust-reddened quasars arise from major galaxy mergers, quasar-driven outflows must therefore persist after the obscuring dust is cleared from the line of sight.

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

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.

Strong gravitational lensing is a rare but important astrophysical phenomenon. Manifesting as dramatic arcs and multiple objects, the pattern of a lensed background source is dependent on the distribution of intervening matter: both baryonic and dark. For this reason, strong lens systems can be used to probe the invisible dark matter structure of lensing objects, placing constraints on the size of dark matter components in order to test the LambdaCDM paradigm. The magnifying power of lensing also permits greater study of the lensed source; by using lenses as 'cosmic telescopes', observers have access to the very faintest and most distant objects in space. A single lens system can yield a rich body of scientific results concerning the objects involved. A thousand lens systems, however, would provide a statically robust sample which can be used to place very tight constraints on the Hubble constant, on galaxy evolution and even on the nature of dark energy. Imminent large sky surveys will image millions of strong lenses but finding those lenses among the billions of objects observed will be impossible to do by eye. This thesis presents the development of a novel machine learning method which automates the lens finding process. The tool uses a support vector machine to classify lens candidates, and won a prize in the 2017 Strong Lens Finding Challenge. It was subsequently applied to data from the Kilo Degree Survey, finding several highly likely new lens systems. This thesis also presents results from the detailed study of three strongly lensed radio quiet quasars (RQQ). These objects are of great interest not only due to their ability both to trace dark matter substructure and to measure the expansion of space, but also since it is unknown whether their radio emission results from the AGN activity seen in the optical or whether starburst -- or more exotic -- activity is responsible. EVN and e-MERLIN observations of RQQ HS~0810+2554 have revealed brightness temperatures in excess of the limit from starburst emission, and subsequent modelling predicts a linear alignment of two highly compact components with the optical core, providing conclusive evidence for AGN jet emission. VLA observations of PG~1115+080 and J1004+4112 similarly find evidence of AGN activity, in the very long and narrow shape of the modelled source and in the variability of the source, respectively. The work advances the programme to determine once and for all the role of RQQ emission mechanisms in galaxy feedback models, and demonstrates the importance of high quality observations in understanding the phenomena associated with galaxy evolution.

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

Quasars are amongst the most luminous objects known in the Universe and thus can be observed out to large distances and correspondingly early times in the history of the Universe. Luminous quasars are powered by accretion of matter onto supermassive black holes (10⁶ - 10¹⁰M⊙) and are situated in the centre of some of the most massive galaxies and are a crucial test of massive galaxy and supermassive black hole assembly and evolution models - proving hard to recreate using simulations. As well as being of intrinsic interest, the spectra of quasars contain information about the state of the intergalactic medium (IGM) in the vicinity of the quasar, and also cosmologically distributed material in the foreground, via absorption lines due to the intervening material. This allows studies of the IGM at high redshift along different sight lines, providing insight into the metallicity, temperature and homogeneity of the Universe. This thesis discusses a new method of finding high redshift quasars using new multi wavelength data from the Dark Energy Survey, the VISTA Hemisphere Survey and Wide-Field Infrared Survey Explorer. The beginning of the thesis focusses on developing an automated selection code for z > 6 quasars, including the automatic rejection of foreground contaminating sources such as instrumental artefacts, asteroids, galactic stars and lower redshift quasars and galaxies. Following on from my first discovery of a z = 6.1 quasar in the DES and VHS data, I have developed a robust selection method that allows me to go straight from candidates to spectroscopy without needing additional photometric follow up. The method uses a grid of quasar models with a range of reddening and a series of brown dwarf spectral energy distribution models to derive a X² statistical likelihood of an object being a quasar and an associated photometric redshift. This differs from previous methods in that it allows for automatic rejection of brown dwarf stars without requiring further data. My selection delivers a ranked candidate list which down weights astrophysical contaminants and imaging artefacts. The thesis then discusses extending this method to higher redshift and the discovery of two quasars at z = 6.75 and z = 6.9. Included with this is a discussion about spectroscopic reduction of near IR data and the properties which can be derived from it. The final part of the thesis uses the quasars I have found to study their environment such as the IGM which they are embedded in, I have been developing a robust method for measuring their hydrogen near zone sizes that can be compared to simulations.

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.

In recent years the formation of primordial galaxies, cosmic metal enrichment, and hydrogen reionisation have been studied using both refined observations and powerful numerical simulations. High-redshift quasars have become a ubiquitous tool in the study of this era with more than 115 quasars now spectroscopically confirmed at z > 6.0. In this thesis, I use spectra of high-redshift quasars to provide improved observational constraints through a mixture of existing and new techniques. I first investigate the claim of neutral gas around the most distant known quasar, ULASJ1120+0641(J1120), with a cosmological redshift of z=7.1. Its spectrum shows a relatively weak Lyman-α emission line, which has been interpreted as evidence of absorption by neutral gas. Attributing this to a Gunn-Peterson damping wing has led to claims that the intergalactic medium is at least 10% neutral at that redshift. However, these claims rely on a reconstruction of the unabsorbed quasar emission. Initial attempts using composite spectra of lower-redshift quasars mismatched the CIV emission line of J1120, a feature known to correlate with Lyman-α and which is strongly blueshifted in J1120. I attempt to establish whether this mismatch could explain the apparently weak Lyman-α emission line. I find that among a C IV-matched sample the Lyman-α line of J1120 is not anomalous. This raises doubts as to the interpretation of absorbed Lyman-α emission lines in the context of reionisation. I then use a high quality X-Shooter spectrum of the same z=7 quasar to measure the abundances of diffuse metals within one billion years of the Big Bang. I measure the occurrence rates of CIV, CII, SiII, FeII and MgII, producing the first measurement at z > 6 for many of these ions. I find that the incidence of CIV systems is consistent with a continuing decline in the total mass density of highly ionized metals, a trend seen at lower redshifts. The ratio CII/CIV, however, seems to remain constant or increase with redshift, in line with predictions from models which include a decline of the ionising ultraviolet background. The evolution in MgII appears somewhat more complex; while the number density of strong systems continues to decline at high redshift,the number density of weak systems remains high and may even increase. This could signal an increase with redshift in the cross-section of low-ionisation metals. Large numbers of weak MgII systems are also seen at z∼2, suggesting they were already in place when reionisation was ending. I use this X-Shooter spectrum to study metal absorbers associated with the z=7 quasar itself. I find that one such absorber shows signs of only partially covering the line-of-sight, and investigate the possible implications for the quasar’s environment. Finally, I investigate the evolution of the intergalactic medium’s Lyman-α opacity using spectra of quasars at 5.7 < z < 7.1. I assemble a sample of 92 quasar spectra, more than 3 times larger than previous samples. The sample consists of quasars drawn from DES-VHS, SDSS and SHELLQs, new reductions of archival data, and new data. I develop methods to quantify the opacity distribution, providing measurements of the distribution function up to z=6.1. I find that the Lyman-α opacity evolves strongly with redshift. The scatter may be even larger than previously appreciated, posing a serious challenge for models of reionisation.

The overall aim of this thesis is to investigate the environments of AGN, in particular, the density of galaxies in the environments of radio-loud and radio-quiet AGN. This determines whether AGN trace dense environments at high redshifts and whether the environments are important in addressing the problem of radio-loud dichotomy. I extend my research by investigating whether star-formation evolves differently in high-redshift AGN environments compared to the field. I begin by investigating the environments of 169 AGN using Spitzer data at z ∼ 1. I investigate the source density of star-forming galaxies in the environments of radio galaxies, radio-loud quasars and radio- quiet quasars. I do not find any significant overdensity of star-forming galaxies in these environments, although I find tentative evidence for a diff erence in the colours of galaxies in the radio galaxy environments compared to the quasar and field environments. I next use VIDEO data to investigate the environments of the quasars out to z ∼ 3. Firstly, I use a training sample of QSOs and galaxies, which trains a neural network to detect QSOs in the VIDEO data. I detect 274 possible QSOs in the VIDEO data using this method. I am able to determine that the efficiency of the neural network clas- sification is 95 per cent using the training sample. I compare these results to a colour selection method, which detects 88 QSOs in the VIDEO data, and find that the neural network is able to detect ∼ 80 per cent of the colour selected QSOs at Ks = 21. I then investigate the source overdensity using a radial analysis on the environments of the VIDEO QSOs. I find a significant overdensity of galaxies in the environments of the whole QSO sample and in the environments of the radio-loud quasars compared to the radio-quiet quasars. I extend the density analysis by using a second density measure, called the spatial clustering amplitude technique, to compare the environments of the quasars with their radio luminosities, absolute magnitudes and redshifts. I do not fi any significant correlations between environmental density and radio luminosity, absolute magnitude or redshift for the QSOs. I extend this research to investigate the type of galaxies found in the AGN environments. However, I do not find any significant differences between the type of galaxies found in the QSO environments and the background field.

Clues about the timing of reionization and the nature of the ionizing sources responsible are imprinted in the ionization and thermal state of the IGM. In this thesis, I use high-resolution quasar spectra in conjunction with state-of-the-art hydrodynamical simulations to probe the IGM at high redshift, focusing on the ionization and thermal state of the gas. After reionization, the ionization state of the IGM is set by the intensity of the ultraviolet background(UVB), quantified by the hydrogen photoionization rate, Γ_bkg. At high redshifts this has been estimated by measuring the mean flux in the Lyα forest, and scaling Γ_bkg in simulations such that the simulated mean flux matches the observed value. In Chapter 3 I investigate whether the precision of these estimates can be improved by using the entire flux probability distribution function (PDF) instead of only the mean flux. Although I find it cannot improve the precision directly, the flux PDF can potentially be used to constrain other sources of error in observational estimates of Γ_bkg, and so may increase the precision indirectly. The ionizing output of a quasar will locally dominate over the UVB, and this leads to enhanced transmission bluewards of the quasar Lyα line, known as the proximity effect. In Chapter 4 I present the first measurements of Γ_bkg at z > 5 from the proximity effect. The UVB intensity declines smoothly with redshift over 4.6 < z < 6.4, implying a smooth evolution in the mean free path of ionizing photons. This suggests that reionization ends at z > 6.4. There is a drop in Γ_bkg by roughly a factor of five, which corresponds to a drop in the ionizing emissivity by about a factor of two. Such a redshift evolution in the emissivity cannot continue to much higher redshift without reionization failing to complete, which suggests that reionization cannot have ended much higher than z = 6.4. Estimates of Γ_bkg from the proximity effect and the mean flux are generally discrepant at z ~ 2-4, with those from the proximity effect systematically higher. This is generally attributed to effects of the quasar environment. I investigate the significance of several environmental biases on proximity effect measurements at z ~ 5-6 in Chapter 5. The biases are found to be small, and so the proximity effect is expected to give relatively unbiased estimates of Γ_bkg at z > 5, in contrast to lower redshifts. Photoionization heats the gas in the IGM, and so the thermal history of the IGM provides important constraints on reionization. The thermal state of the IGM is reflected in the level of small-scale structure in the Lyα forest. In Chapter 6 I quantify the small-scale structure using two independent statistics, the curvature and the peakiness, and convert these into a temperature by comparing with simulations. These are the first measurements of the temperature in the general IGM at z > 5. Both statistics show an increase in the temperature by a factor of roughly two from z = 4.4 to 5.6. This rise is sensitive, however, to any smoothing of the gas density distribution due to the thermal history spanning reionization. I find that this should only be a small effect, as otherwise the corrected temperatures at z ~ 4-5 are implausibly low. The temperature evolution therefore suggests a late reionization. The temperatures at z ≥ 4.8 are well fit by an adiabatic cooling curve, for which reasonable peak temperatures at the end of reionization are reached at 6 ≲ z ≲ 7. The temperatures at z ~ 4-5 are consistent with reionization being carried out by Pop II stars. In conclusion, the ionization and thermal state of the IGM at z ~ 5-6 suggest a late hydrogen reionization, driven by star-forming galaxies and ending around 6.5 ≲ z ≲ 7. This is consistent with other recent lines of observational evidence, and supports theoretical models that infer a late reionization from the observed star formation rate history.

This thesis presents the discovery of the first luminous z & 6.5 quasars in the VISTA kilo-degree Infrared Galaxy Survey (VIKING). After some basic quality control, quasar selection is investigated via use of initial data supplementedwith detailed modelling of the photometric and spatial distributions of stars of spectral type M, L and T, which are known to be the cause of significant contamination in quasar colour selection spaces. Optimised selection constraints are placed on detection significance and morphology and the performance of a traditional colour selection technique is compared to a Bayesian model comparison technique. The latter is found to offer a ∼10 per cent gain in completeness over traditional colour selection. Quasar candidates are ranked via Bayesian model comparison and a subset of the highest ranked objects are put forward for follow-up imaging. In June 2011, 44 high-z quasar candidates underwent deep optical i- and z- band imaging on the ESO NTT. Just 6 of these candidates were found to have optical colours consistent z & 6.5 quasars. Spectroscopic follow-up of these objects is ongoing, but thus far three new quasars have been discovered at redshifts of z=6.5, 6.7, 6.9. This discovery rate is consistentwith zero evolution in the rate of decline in quasar space density from z & 6.4. This differs fromthe latest results from UKIDSS. Further results expected from these and other surveys will begin to constrain the true nature of quasar space density evolution in the near future. The discovery of three z ≥ 6.5 quasars in VIKING is a significant highlight in the first year of VISTA science operations. These quasars will remain important probes of the high-z universe throughout the next decade.

This DPhil dissertation presents two new and independent samples of faint radio sources. The first sample is the 37 SXDS radio sources with flux densities at 1.4-GHz above 2 mJy, a spectroscopic completeness of 65% and a median redshift z_med ≈ 1.1. The second sample is the 47 TOOT00 radio sources with flux densities at 151 MHz above 100 mJy, a spectroscopic completeness of 85% and z_med ∼ 1.25. Optical, near- and mid-IR photometry, optical spectroscopy, and radio observations are used in the analysis and comparison of the two samples. The quasar fraction in the TOOT00 radio sources is 0.13 < f_q < 0.25 above the FRI/FRII break in radio luminosity, while use of 24 μm data reveals objects with significant but sometimes obscured accretion and gives quasar-mode fraction of 0.5 → 0.9 above the FRI/FRII break. The FRI/FRII divide seen at z < ∼ 0.5 is also observed at z ∼ 1 for FRII objects in the TOOT00 and SXDS samples, but examples of FRI radio sources above the FRI/FRII break do exist. The total number of the TOOT00 objects and their distribution are consistent with simulations based on extrapolations from previous work, while for the SXDS objects the results are only broadly similar. Based on that comparison, the redshift spikes seen at z ∼ 1.3 in TOOT00 and at z ∼ 0.65 & 2.7 in SXDS appear to be significant, and might be due to Large-Scale Structure. A V/Vmax test suggests the cosmic evolution of the TOOT00 and SXDS samples, is different. The TOOT00 radio sources are 2-times more luminous in host-galaxy starlight than the SXDS radio sources. The almost proportionality between radio luminosity at 1.4 GHz and 24 μm luminosity suggests that L_rad traces accretion luminosity and L[OII] ∝ L_rad^0.7 may reflect imperfections in the L[OII]-accretion luminosity scaling. Mid-IR 24 μm observations in the SXDS sample suggest that 30% of the light from the nucleus is absorbed by the torus and re-emitted in the mid-IR, while ∼ 1% of the light is scattered above and below the torus.

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.

This thesis presents a study of high-redshift obscured (type-2) quasars, selected at mid-infrared and radio wavelengths. This population had remained elusive, even to hard X-ray surveys, and in Chapter 2 I compare the selection of type-2 quasars in X-ray and mid-infrared surveys, as well as explaining the criteria I will use to search for these objects at z ~ 2, around the peak in the unobscured (type-1) quasar activity. Chapter 3, presents a sample of radio-intermediate type-2 quasars selected from the criteria de- scribed in Chapter 2. Optical spectroscopy shows indeed that at least half of the objects have the characteristic narrow emission lines, and lie around the expected redshift of z = 2. The other half of the objects are consistent with also being type-2 quasars at similar redshifts, although no emission lines are visible. In Chapter 4,1 discuss the possibility of two types of obscured quasars, some obscured by a dusty torus and some by a dusty host galaxy, to explain the lack of emission lines in half of the sample. I model the number of type-1 quasars expected to follow similar selection criteria and at the same redshifts as our type-2 quasars, and find that the obscured quasars outnumber the unobscured by a ~2-3:1 ratio. I conclude that most supermassive black hole growth is obscured by dust. When comparing this to predictions from unified schemes, I find that this result is only consistent with the schemes provided host-obscuration is indeed happening. The lower ratio of type-2 to type-1 quasars inferred from X-ray surveys (~1:1) suggests that some of the type-2 quasars in this sample might be Compton thick. Radio data taken at three frequencies, are presented in Chapter 5, to study the spectral properties and intrinsic luminosities of our sample. I show that some type-2 quasars have flat radio spectra, which is inconsistent with obscuration by the torus, but consistent with host-obscuration. Some gigahertz-peaked spectra, characteristic of young radio jets, are present, but the majority of the sources have very steep spectra. These steep spectral indices can be explained by active developed jets in which continuous injection of electrons is accompanied by inverse-Compton losses against the cosmic microwave background. In Chapter 6, I select a similar sample of type-2 quasars in a different field, where X-ray data are available. The selection criteria are kept identical, except for the radio flux density cut, which is lowered. This is expected to introduce significant numbers of starburst contaminants. To filter these out, and due to a lack of spectroscopy, I use a bayesian method to fit the spectral energy distributions, obtain photometric redshifts, and select between a quasar and a starburst model. I measure the X-ray properties for the resultant sample of type-2 quasars. The entire sample is found to be Compton-thick, and repeating the modelling of Chapter 4, I find that the population of Compton-thick quasars is at least comparable to the population of unobscured quasars, and probably larger.

The goal of this thesis is to develop an obscured AGN model for the X-ray background (XRB), and then to test the model via its predictions for the intensity of the cosmic background in the far infra-red, and by searching for obscured nuclei in individual X-ray sources in the near infra-red. A model invoking populations of obscured QSOs with a flat distribution of absorbing columns is found to give a good fit to the X-ray source counts and to the XRB spectrum. The model also accommodates the recent detection of a population of narrow-line X-ray galaxies, identifying them as low-luminosity obscured AGN. We investigate the selection effects present in optical QSO samples, and find that absorption in both the X-ray and optical wavebands combine to produce the tight observed X-ray/optical correlation, while still allowing a wide range of intrinsic column densities. X-ray sources with no obvious optical counterparts are predicted by this model, with a frequency in agreement with the numbers of these 'blank field' sources found in deep ROSAT surveys. We predict that in forthcoming AXAF surveys, ~ 50 per cent of the sources detected will be optically blank and lie at z (^>))(_~) 1. Obscured QSOs are estimated to outnumber unobscured QSOs by a factor of at least four to one. At longer wavelengths, we make a conservative prediction that obscured QSOs can account for approximately ten per cent of the sub-mm number counts and the far infra-red background, which is a modest, but non-negligible fraction. We then make observations to determine the extent of dust reddening in X-ray galaxies and QSOs, in the light of predictions of our obscured QSO model. We present K-band imaging of ROSAT and ASCA X-ray luminous galaxies and blank field sources. A trend is observed between the hardness of the X-ray spectra and the occurrence of red, unresolved counterparts, consistent with an obscured QSO origin for the X-ray emission. Near infra-red spectroscopy of X-ray luminous galaxies is then presented, searching for broad Pa(_a) emission lines. A broad line is possibly detected in one of six galaxies observed, which would indicate AGN activity. From far infra-red ISO photometry of X-ray galaxies and QSOs, we detect emission at ~ lOOμm from six narrow- line galaxies at low redshift, and from a Type 2 QSO at higher redshift. For at least two of the low redshift galaxies, there is clear proof of AGN activity on the grounds of X-ray variability and near infra-red spectroscopy. This, together with the 100μm detections, forms conclusive proof of the presence of dust-enshrouded AGN in at least some X-ray galaxies, supporting the obscured QSO explanation for the cosmic X-ray background.

Unified schemes for radio-loud active galactic nuclei (AGN) propose that the different observed classes arise from the same basic phenomenon. The luminosity and orientation of the relativistic jet then determines the type of object seen. The luminosity division has been suggested in which low luminosity FRI radio galaxies are unified with BL Lacertae objects, and high luminosity FRII radio galaxies are unified with both non-OVV and OVV quasars. Orientation independent parameters, such as extended radio power, cluster environment and host galaxy properties, can be used to test the validity of the proposed schemes. Optical and near-infrared data were obtained of twelve blazars, nine BL Lacertae objects (BL Lacs) and three Optically Violently Variable (OVV) quasars, in order to investigate the magnitudes and morphologies of their host galaxies. The results from the analyses conducted have been presented in the two papers Wright, McHardy & Abraham (1997a, MNRAS, accepted) and Wright et al. (1997b, MNRAS accepted). One- and two-dimensional numerical modelling routines were developed to complete the analysis of the data. Host galaxies were resolved in all three of the OVV quasars but a morphology was derived for only one object, PKS 7036+017. From modelling, the emission of the host in PKS 0736+017 was found to be best described by an elliptical morphology. For the BL Lacs, host galaxies were resolved in five of the sources. Morphological classification was not possible for the majority of the data. However tentative morphologies were determined for two sources; B2 0752+258 appears to be best described by a disc model, while S4 0954+65 appears to be an elliptical galaxy. The host galaxy of TEX 0836+182 appears elongated, which together with the blue colour derived, is probably indicative of interaction in this source. Comparison of the host galaxy properties derived for the OVV quasars and BL Lacs in the present study with those of the respective putative parent populations show them to be similar, adding weight to the unification scenario. However, the number of objects is small and larger samples are required to test unification more rigorously.

Quasars strongly lensed by galaxy clusters are inherently rare, yet they offer rich insights into astrophysics and cosmology. Cluster-lensed quasars can be used to measure the current expansion rate of the Universe, the Hubble constant H0. A measurement of H0 from cluster-lensed quasars is particularly important given the claimed Hubble tension: a 5-sigma tension between Cosmic Microwave Background-based and distance ladder-based measurements of H0. Time-delay cosmography, the technique of using a lensed variable source such as a quasar to measure the Hubble constant, relies on the inverse scaling of H0 with the delay between the arrival times of at least two images of the strongly-lensed variable source. In addition to an observational measurement of the time delay between the images, time-delay cosmography also requires an accurate mapping of the projected mass density of the lens. Although the theory was originally proposed by Refsdal in 1964, an actual measurement of H0 from cluster-lensed quasars only recently became possible as the first systems acquired all the necessary data. My thesis presents the first measurement of the Hubble constant from SDSS J1004+4112, SDSS J1029+2623, and SDSS J2222+2745, the only three cluster-lensed quasars that have high-resolution imaging from the Hubble Space Telescope, spectroscopic redshift measurements of the lensed sources, and time delay measurements from years-long monitoring. I measured H0 = 74.1 +/- 8.0 km/s/Mpc. Although the uncertainty is too high to discriminate between Cosmic Microwave Background-based and distance ladder-based measurements of H0, I demonstrate that a competitive 1% measurement of H0 from cluster-lensed quasars can be achievable if a sufficiently large number of systems is discovered and we decrease the systematic uncertainties arising from modeling the projected mass density of the lenses by a factor of 2.5. My thesis also presents the discovery of two new cluster-lensed quasars, COOL J0335-1927 and COOL J0542-2125. While the observational follow-up of these systems is not yet mature enough for a measurement of the Hubble constant, my collaborators and I are in the process of acquiring all the necessary data. We have ongoing photometric monitoring campaigns with the 2.56-meter Nordic Optical Telescope and the 6.5-meter Magellan Telescopes to measure the time delays between the quasar images. My thesis presents the most up-to-date light curves. Additionally, we recently observed COOL J0542-2125 with the Chandra X-ray Observatory and the Hubble Space Telescope. Utilizing the Hubble imaging data, I present a revised lens model that builds on the one published in Martinez et al. (2023) which I computed using only ground-based imaging. Combined with a precise time delay measurement, an H0 measurement from COOL J0542-2125 will be possible by 2027. The work of this thesis helps lay the groundwork for the exciting era of time-delay cosmography we are about to enter. Euclid, the Vera C. Rubin Observatory, and the Nancy Grace Roman Space Telescope will discover tens of new cluster-lensed quasars, making a 1% measurement of the Hubble constant from cluster-lensed quasars within reach. A 1% measurement of H0 from cluster-lensed quasars can help solve whether the Hubble tension is real, implying that our model of the Universe is incomplete, or a result of unaccounted for systematics.

Understanding the reionization of the intergalactic medium (IGM) by the first luminous sources remains an important open problem in cosmology. During Reionization ionization fronts propagate through the IGM, heating the reionized gas. This heat injection can be observed over a redshift interval of ∆z ∼ 1 due to the long cooling times in the low-density IGM. Simultaneously, the mean free path of ionizing photons (λmfp) describing the ultraviolet background (UVB) rapidly evolves as bubbles of reionized gas, where the UVB is stronger, merge. Thus, constraining the thermal state of the IGM and the evolution of λmfp can, in turn, be used to constrain reionization. Transmission in the Lyman-α (Lyα) forest, the ubiquitous Lyα absorption lines produced by residual neutral hydrogen in the IGM along quasar sightlines, offers a powerful tool to investigate these phenomena.Here I present studies that look into utilizing the z > 5 Lyα forest to constrain Reionization via the thermal state of the IGM and λmfp. First, I use wavelet analysis as an initial clustering statistic and shows improvement on simulated temperature constraints when compared to measurements from the 1D Lyα forest power spectrum. This work also notes the importance of careful accounting of the correlations between datasets to ensure observational constraints are not over-confident. The next two studies use the Lyα forest auto-correlation function at z ≥ 5.4 as the clustering statistic. The second study demonstrates the potential in constraining λmfp from simulated data while the third does the same for the temperature. Both of these do careful statistical inference tests to ensure the results are not over-confident and describe a method to guarantee statistical robustness of measurements. The final study makes measurements of the Lyα forest auto-correlation functions at z > 5 from the XQR-30 data set. Preliminary comparisons of these measurements to the models from the second study are done, showing agreement with previous measurements of λmfp. Each of these contribute to our understanding of the high-z Lyα forest and how it can be used to constrain Reionization.

Galaxies evolve over cosmic times in tandem with their larger environments (O(10 - 100 kpc)), which are influenced by galactic processes such as star formation and outbursts from an accreting supermassive black hole, also known as an Active Galactic Nucleus (AGN). The circumgalactic medium (CGM) is a reservoir of gas that surrounds all galaxies and is the recipient of galactic scale outflows, driven by either star formation or AGN activity, but also supplies the gas needed by the host galaxy to sustain star formation. The larger environment also impacts the CGM brightness, kinematics and metallicity, due to companion galaxy subhalos and mergers, thus offering a unique opportunity to observe the interplay between galactic and environmental processes. Our multi-wavelength dataset of seven distant (z~2) radio-loud quasars allows us to investigate these interactions. Using data from W. M. Keck Observatory and archival data from the Very Large Array and ALMA radio facilities, and Hubble and Spitzer Space Telescopes, we observe outflows driven by the AGN, spatially resolved host galaxy emissions, CGM Ly-alpha, He II, and C IV emission and the influence of companion galaxies. We find evidence for a two component CGM model, where the inner CGM is directly influenced by the quasar host galaxy, whereas the outer CGM is influenced tidal interactions with companion galaxies and the turbulent motion of the gas.Adaptive Optics (AO) has enabled higher angular resolution observations from the ground. Understanding the Point Spread Function (PSF) of the telescope limits our ability to disentangle phenomena at small angular resolution. A forward modelled PSF reconstruction represents the "pinnacle" of ground-based observational astronomy, since true knowledge of the observational PSF will reduce uncertainties in photometry and astrometry for all science cases. We investigate a PSF model for the NIRC2 instrument using AO telemetry data on Keck telescope and validate its performance using on-sky data. As a special application, we also investigate the PSF of the Corner Cube Retroreflectors left on the moon by Apollo astronauts, to investigate the performance degradation of these devices over the last 50 years due to dust deposition on their surfaces.

I present spectroscopic measurements for 260 sources from the Gemini Near Infrared Spectrograph–Distant Quasar Survey (GNIRS-DQS). Being the largest uniform, homogeneous survey of its kind, it represents a flux-limited sample of Sloan Digital Sky Survey (SDSS) quasars at 1.5 < z < 3.5. A combination of the GNIRS and SDSS spectra covers principal quasar diagnostic features, chiefly the C IV λ1549, Mg II λλ2798, 2803, Hβ λ4861, and [O III] λλ4959, 5007 emission lines, in each source. The spectral inventory is utilized primarily to develop prescriptions for obtaining more accurate and precise redshifts, black hole masses, and accretion rates for all quasars. Additionally, the measurements facilitate an understanding of the dependence of rest-frame ultraviolet–optical spectral properties of quasars on redshift, luminosity, and Eddington ratio, and test whether the physical properties of the quasar central engine evolve over cosmic time.

In this thesis, we designed an algorithm to provide robust selection criteria in the parameterspace of measured properties of quasars. Our method combines the prior knowledge of an expert observer with what unsupervised machine learning understands about the underlying structures in the data to get a data-driven boundary in the multi-dimensional parameter space of quasar physical properties. We did that by quantifying the dissimilarity of our target group to the majority of the quasars in our data set. Our versatile method can select a cluster of similar data points that are located in statistically significant lower-density regions of the parameter space. We could find more quasars in the class of extremely red quasars and show our new sample has even more exotic outflow behavior. Our final selection produces three times more quasars with visually verified CIV broad absorption line feature, which is the signature of outflow, than the previous extremely red quasar sample. Our method is very useful in selecting the most important follow-up targets for observing red quasars. In the second project, we could assemble the largest CIV absorption line catalogue to date. By providing a probability for the existence of absorption systems in a quasar spectrum that viis a by-product of our Bayesian model selection and Gaussian Processes methods, we removed the need for visual inspection which is essential in dealing with the upcoming surveys with millions of spectra. After carefully validating our method by comparing a subset of the spectra inspected in the largest visually inspected CIV catalog to what our method predicts, we could find 113,775 CIV absorption systems with at least 95% confidence among 185,425 selected quasar spectra from SDSS DR12. We obtain a posterior distribution for column density, velocity dispersion, and absorption redshift for each investigated spectrum which can be used to get the maximum a posteriori value and the credible interval. Our method is specifically useful when we want to obtain information from low signal-to-noise ratio data.

This PhD dissertation investigates the phenomenon of extreme outflows in quasars and its implications for galaxy evolution and quasar feedback. The study focuses on two distinct quasar populations: extremely red quasars (ERQs) and normal quasars, both exhibiting powerful outflows, but in different contexts. The first part of the dissertation explores a population of extremely red quasars (ERQs) with exceptionally fast outflows, likely representing a young stage of galaxy evolution. The ERQs, situated at a median redshift of ? ≈ 2.6, are studied through Keck/KCWI integral field spectra of 11 quasars. Analysis of their properties, such as median color (?–?3 = 5.9 mag), median bolometric luminosity (⟨?bol⟩ ≈ 5 × 10^47 erg/s), Ly? halo luminosity (⟨?halo⟩ = 5 × 10^43 erg/s), and maximum linear size (>128 kpc), reveals similarities to blue quasars while exhibiting unique characteristics. The ERQ halos have compact and circularly symmetric inner regions, and they are kinematically quiet. Despite their powerful outflows, there is no clear evidence for feedback on circumgalactic scales, potentially due to long outflow travel times. The dissertation also confirms the importance of narrow Ly? emission spikes in ERQ aperture spectra for systemic redshift determination and measuring outflow speeds. In the second part, the dissertation investigates the driving factors behind fast outflows in normal quasars using a large sample of 39,249 quasars at a median redshift of ⟨?⟩ ≈ 2.17. Redshifts based on the Mg ii emission line are re-measured, enabling the exploration of unprecedented outflow velocities (>6000 km s−1) while ensuring statistical significance and uniformity. Analysis reveals significant correlations, indicating that higher Eddington ratios and softer far-UV continua (h? >54.4 eV) are primary contributors to faster outflows. Supporting evidence suggests that radiative line-driving may generate extreme outflow velocities, influenced by multiple factors. This study emphasizes the importance of considering a multi-dimensional parameter space to understand the fundamental causes of extreme outflows in quasars. In conclusion, this comprehensive dissertation provides valuable insights into extreme outflows in quasars, encompassing both ERQs and normal quasars. The findings contribute to our understanding of the early stages of galaxy evolution and the impact of quasar feedback on host galaxies. The study highlights the complexities of quasar outflows and their driving mechanisms, underscoring the significance of multi-dimensional analyses in future studies.

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.

We investigate the relation between AGN and star formation (SF) activity at 0.5 < z < 3 by analyzing 898 galaxies with X-ray luminous AGN (L [subscript x] > 10⁴⁴ erg s⁻¹) and a large comparison sample of ~ 320,000 galaxies without X-ray luminous AGN. Our samples are selected from a large (11.8 deg²) area in Stripe 82 that has multi-wavelength (X-ray to far-IR) data. The enormous comoving volume (~ 0.3 Gpc³) at 0.5 < z < 3 minimizes the effects of cosmic variance and captures a large number of massive galaxies (~ 30,000 galaxies with M [subscript *] > 10¹¹ \ [solar mass]) and X-ray luminous AGN. While many galaxy studies discard AGN hosts, we fit the SED of galaxies with and without X-ray luminous AGN with Code Investigating GALaxy Emission (CIGALE) and include AGN emission templates. We find that without this inclusion, stellar masses and star formation rates (SFRs) in AGN host galaxies can be overestimated, on average, by factors of up to ~ 5 and ~ 10, respectively. The average SFR of galaxies with X-ray luminous AGN is higher by a factor of ~ 3 to 10 compared to galaxies without X-ray luminous AGN at fixed stellar mass and redshift, suggesting that high SFRs and high AGN X-ray luminosities may be fueled by common mechanisms. The vast majority (> 95%) of galaxies with X-ray luminous AGN at z = 0.5 - 3 do not show quenched SF: this suggests that if AGN feedback quenches SF, the associated quenching process takes a significant time to act and the quenched phase sets in after the highly luminous phases of AGN activity.

We revisit the possibility of redshift evolution in the MBH-σ relation with a sample of 22 Seyfert 1 galaxies with black holes (BHs) in the mass range 6.3-8.3 (log10 MBH) and redshift range 0.03 < z < 0.57 with spectra obtained from spatially resolved Keck Low-Resolution Imaging Spectrometer observations. Stellar velocity dispersions were measured directly from the MgIb region, taking into consideration the effect of FeII contamination, active galactic nucleus (AGN) dilution, and host-galaxy morphology on our measurements. BH masses are estimated using the H-beta line width, and the luminosity at 5100 Å is estimated from surface brightness decomposition of the AGN from the host galaxy using high-resolution imaging from the Hubble Space Telescope. Our selection criteria allowed us to probe lower-luminosity AGNs and lower-mass BHs in the non-local universe than those measured in previous single-epoch studies. We find that any offset in the MBH-σ relation up to z~0.6 is consistent with the scatter of local BH masses, and address the sources of biases and uncertainties that contribute to this scatter.We also present Bayesian AGN Decomposition Analysis for SDSS Spectra (BADASS), an open source spectral analysis code designed for automatic detailed deconvolution of AGN and host galaxy spectra, implemented in Python, and designed for the next generation of large scale surveys. BADASS simultaneously fits all spectral components, including power-law continuum, stellar line-of-sight velocity distribution, FeII emission, as well as narrow, broad, and outflow emission line features, all performed using Markov Chain Monte Carlo to obtain robust parameter uncertainties and autocorrelation analysis to assess parameter convergence.We use BADASS to perform correlation analysis of 63 SDSS Seyfert 1 galaxies with evidence of strong non-gravitational outflow kinematics in the [OIII]λ5007 emission line. We confirm findings from previous studies that show the core of the [OIII] profile is a suitable surrogate for $\sigma_*$, however there is evidence that the core experiences broadening that scales with outflow velocity. We find sufficient evidence that σ, [OIII] core dispersion, and the non-gravitational outflow dispersion of the [OIII] profile form a plane whose fit results in a scatter of ~0.1 dex. Finally, we discuss the implications, caveats, and recommendations when using the [OIII] dispersion as a surrogate for sigma* for the MBH-σ relation.

This thesis studies actively accreting supermassive black holes (SMBH) known as quasars and their impact on the formation and evolution of massive galaxies. Theoretical work predicts that quasars are responsible for regulating the growth of the SMBH and its host galaxy. Winds driven by radiation pressure from the accretion disk and quasar jets are predicted to remove a substantial amount of gas from the host galaxy. Massive galaxies are observed to form the majority of their stars when the Universe was only 3.5 billion years old. During this epoch, the most massive SMBH already grew a substantial fraction of their mass. The SMBH is predicted to regulate the growth of the host galaxy during this co-evolutionary phase.We have undertaken an extensive adaptive optics (AO) assisted integral field spectroscopy (IFS) survey of distant quasar host galaxies when the Universe was only 2.2-4.6 Gyr old. We conducted observations using the 10-meter W.M. Keck Observatory optical and infrared telescope. The survey aims to measure the gas-phase properties of quasar host galaxies such as metallicities, star formation rates, photoionization mechanisms, and search for galaxy-wide outflows potentially driven by the quasar. We detect outflows in all objects on scales from

Bright high-redshift quasars (z > 6), hosting supermassive black holes (M [subscript BH] > 10⁸M [subscript circled dot]), are expected to reside in massive host galaxies embedded within some of the earliest, most massive galaxy overdensities. We analyze 1.2mm ALMA dust continuum maps of 35 bright quasars at 6 < z < 7 and search the primary beam for excess continuum emission as evidence for early protoclusters. We compare the detection rates of continuum sources at [greater than or equal to] 5σ significance in the fields surrounding the quasars (A [subscript eff] = 4.3 arcmin²) with millimeter number counts in blank field surveys. We discover 15 sources in the fields (excluding the quasars themselves), corresponding to an overdensity δ [subscript gal] [triple bar] (N [subscript gal] – N [subscript exp]) / N [subscript exp] = -0.07 ± 0.56, consistent with no detected overdensity of dusty galaxies within 140 physical kpc of the quasars. However, the apparent lack of continuum overdensity does not negate the hypothesis that quasars live in overdense environments, as evidenced by strong [CII] overdensities found on the same scales to similarly-selected quasars. The small field of view of ALMA could miss a true overdensity if it exists on scales larger than 1 cMpc, if the quasar is not centered in the overdensity, or if quasar feedback plays a role close to the quasar, but it is most likely that the large line of sight volume probed by a continuum survey will wash out a true overdensity signal.

I present the initial results of characterizing moderate redshift (z > 2) quasars in the Dark Energy Survey (DES) by their variability. As the scales associated with supermassive black holes (SMBHs) are too small to be resolved directly, alternative methods are necessary to learn about their structure, how they grow, and how they impact their environments. One such method is through variability. Quasars are naturally variable objects, and by measuring the time delay to different emitting regions as they respond to changes in the extreme ionizing UV photons produced at the inner edge of the disk, we can estimate normally inaccessible size scales associated with SMBHs. This technique is known as reverberation mapping. I first apply this technique to probe accretion disk sizes. Only a handful of such measurements exist in the literature, sampled through reverberation mapping or gravitational microlensing. I measure time delays between the DES photometric bands to place constraints on accretion disk sizes, and then present a software extension to the JAVELIN code that provides a Bayesian framework for fitting a thin accretion disk model directly to the data rather than the individual lags themselves. This is tested on fake data as well as the highest quality dataset available for a local active galaxy, NGC 5548, before being applied to a sample of DES quasars. This new framework, under our thin disk assumption, gives competitive accretion disk sizes for quasars with our survey quality data alone, and adds over a dozen objects to the relatively small number of quasars with measured disk properties. Next, I present the serendipitous discovery of a z=0.65 low-ionization broad absorption line (LoBAL) quasar in a post-starburst galaxy in the DES data, spectroscopically confirmed with the Australian Dark Energy Survey (OzDES) project. LoBAL quasars are a minority of all BALs, and rarer still is that this object also exhibits broad FeII (an FeLoBAL) and Balmer absorption. This is the first BAL quasar that has signatures of recently truncated star formation, which we estimate ended about 40 Myr ago. The characteristic signatures of an FeLoBAL require high column densities, which could be explained by the emergence of a young quasar from an early, dust-enshrouded phase, or by clouds compressed by a blast wave. The age of the starburst component is comparable to estimates of the lifetime of quasars, so if we assume the quasar activity is related to the truncation of the star formation, this object is better explained by the blast wave scenario. Finally, I conclude with describing our efforts for the spectroscopic reverberation mapping campaign with DES and OzDES. As with the accretion disks, the goal is to measure a time delay, this time between the quasar continuum emission and the response of the broad emission lines from the broad line region. Coupled with the gas velocity dispersion from the spectra, this enables us to estimate the mass of the SMBHs, on which we will calibrate relationships to make future mass measurements far less resource intensive.