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Globular Clusters (GCs) are among the oldest gravitationally bound stellar systems, as they are characterised by stellar populations with ages of 11.5 to 12.5 Gyr. They probably formed during, or just after, the epoch of reionisation. Therefore, the study of these systems at high redshift can not only give insight into the processes and environments that governed star formation at that time, but they can also represent valuable probes of the reionisation epoch itself. However, the exact phases of GC formation are still unknown. Due to their compact sizes, the ability to directly observe the ancestors of present-day GCs at high redshift is still difficult; thus, we must turn to cosmological simulations. In this doctoral thesis, I utilised a suite of numerical simulations of a cosmological volume from the First Billion Years (FiBY) Project and I also ran a set of simulations of isolated galaxy environments down to z ∼ 2. As a first step, I performed an “agnostic exploration” of the demographics of low-mass stellar systems within the FiBY simulations. Specifically, I identified a population of dense objects with a stellar component of up to a hundred thousand solar masses and such that their fraction of baryons (over the total mass of baryons and dark matter) is close to unity; these objects are possible “infant GC candidates”. Compared to their present-day counterparts in the local Universe, these infant stellar clusters have a higher gas fraction and appear to be more extended, consistent with the properties of the few examples of proto-globular clusters currently observed at high redshift. They are compact systems, with densities higher than the average population of FiBY systems at the same stellar mass. The infant GC candidates appear to be more massive and more abundant in massive host galaxies, indicating that the assembly of galaxies via mergers may play an important role in building several GC-host scaling relations. I assessed the present-day GC mass (GC number) – halo mass relation and found it offers a satisfactory description of the behaviour of the infant GC candidates at high redshift, suggesting that such a relation may be set at formation. I also reported a new relation between the specific star formation rate of a host galaxy and the mass of the most massive GC that holds across a large range of redshifts, thus providing a way to probe both the evolution of GCs as well as their host galaxy across time. As a second step, I examined the origin of these infant GC candidates through the use of energy-angular momentum phase-space characterisation to identify bound structures within the simulations. By analysing the evolution in phase-space, I determined the redshift at which the infant GCs first became gravitationally bound. By studying the collapse of both the gaseous and stellar components of the objects, I found evidence that they undergo radial infall and that the collapse itself is driven by the internal self-gravity of the infant GC candidates. The phase-space characterisation also revealed that some infant GC candidates shared certain features in the energy-angular momentum plane, allowing them to be grouped by their behaviour in phase-space. This classification is reflected in the formation environment: infant GC candidates of the same phase-space class are found in hosts of similar morphology, the majority of which are located in clumpy, irregular proto-galaxies. Within the set of infant GC candidates studied, there are two which consist only of stars by z = 6. By analysing these particular objects, I found that supernova feedback is the main physical mechanism behind expelling the gas and halting star formation. These candidates best represent the local Universe GCs and provide a possible future for the gas-rich infant GC candidates beyond z = 6. Finally, I explored the subsequent evolution of these infant GC candidates within their host galaxies by performing a new set of isolated, cutout numerical simulations evolved to redshift z ∼ 2. These simulations were designed to focus on the three main groupings of infantGCcandidates and to investigate whether theywould survive within their hosts, without the influence of large-scale external factors. Three host galaxies were selected, containing a total of 11 infant GC candidates; most of these candidates are gas-rich, with a single object being composed only of stars by z = 6 (the time at which the previous FiBY simulations ended). I found that all gas-rich candidates are destroyed: their star formation activity peaks at z ∼ 6 and, once the supernovae release their feedback and expel the gas, the remaining stars in the candidates can not adjust to the new potential and the objects dissolve. By comparing the destruction of the gas-rich candidates with the history of the star-only candidate (which had previously lost its gas due to supernovae as well, but survived), I found that a key factor in whether a candidate would survive the feedback phase is the ratio of gas mass to stellar mass. If this factor is too large, then the survival of the candidate is unlikely. This observation is further reinforced when comparing this ratio with the timescale for dissolution: on average, objects with a lower gas to stellar mass fraction live longer. To complete this analysis, I examined the evolution of the star-only candidate and its trajectory around its host galaxy’s barycentre. I found that the object is on an infalling orbit towards the centre of the host and, eventually, it begins to merge with the central part of the galaxy, in agreement with the GC infall formation scenario for nuclear star clusters.

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.

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.

Some of the most fundamental measurements we can make of the Universe are where and when stars formed in galaxies. In recent years, astronomers have converged on a picture in which the star formation rate density of the Universe peaks at approximately redshift (z) 2, when the Universe is around a quarter of its present age. There, star-forming galaxies harbour large reservoirs of molecular gas, assemble stellar mass rapidly, and typically display disordered morphologies. In this thesis, I study the evolution of galaxies on large and small scales, with a particular focus on the epoch around the peak of cosmic star formation. My overarching aim is to understand the physical processes that drive and quench star formation in galaxies over cosmic time. In the first half of this thesis, I focus on global measurements of star formation, using the High-z Emission line survey (HiZELS), a deep, near-infrared narrow-band survey, which identifies star-forming galaxies at z=0.8-2.2. I characterise the dark matter halo environments of these galaxies via a clustering analysis, along with a Halo Occupation Distribution model fitting procedure, then study the relationships between host dark matter halo environment and galaxy properties. I show that the clustering strength and the host dark matter halo masses of the HiZELS galaxies increase linearly with H-alpha luminosity (and, by implication, star formation rate) at all three redshifts. The typical galaxies in our samples are star-forming centrals, residing in dark matter haloes of mass ~10^12M_sol. I find a remarkably tight redshift-independent relation between the H-alpha luminosity scaled by the characteristic luminosity, L_H-alpha/L_H-alpha*(z), and the host dark matter halo mass of central galaxies. Simple analytic modelling suggests that this is consistent with a model in which the dark matter halo environment is a strong driver of galaxy star formation rate and therefore of the evolution of the star formation rate density in the Universe. I investigate this further by distinguishing the stellar mass and star formation rate dependencies of the clustering of HiZELS galaxies. I compare my observational results to the predictions of a pioneering cosmological hydrodynamical simulation, the Virgo Consortium's Evolution and Assembly of GaLaxies and their Environments project, known as EAGLE. In the subsequent chapters of this thesis, I focus more heavily on simulations of galaxy formation, which are important tools for constraining and understanding the physics at play in galaxies. I use EAGLE to investigate the quenching of star formation in simulated galaxies via a novel application of Principal Component Analysis. I show that the key relations between halo mass, stellar mass and star formation rate are in good agreement with observed low-redshift galaxies. Having studied the global properties of star-forming galaxies, I then turn to smaller scales, investigating what we can learn from spatially-resolved imaging. I present a detailed study of the spatially-resolved dust continuum emission of realistic simulated high-redshift galaxies. These galaxies, drawn from the FIRE-2 simulations, reach Milky Way masses by z~2. Post-processing them using radiative transfer techniques, I obtain predictions for the full rest-frame far-ultraviolet to far-infrared Spectral Energy Distributions of these simulated galaxies, as well as maps of their emission across the wavelength spectrum. As has been observed in distant galaxies, the rest-frame far-infrared emission of the simulated galaxies is compact, spanning half-light radii of ~0.5-4kpc. The derived morphologies of simulated galaxies are notably different in different wavebands; a galaxy can appear clumpy and extended in the far-ultraviolet yet compact at far-infrared wavelengths. Finally, I perform a multi-wavelength study of a single observed galaxy, SHiZELS-14 (z=2.24), drawn from the HiZELS survey and subsequently imaged at 0.15'' resolution at multiple wavelengths. The data comprise kpc-resolution imaging in three different widely used tracers of star formation: the H-alpha emission line (from SINFONI/VLT), rest-frame far-ultraviolet continuum (from HST F606W imaging), and the rest-frame far-infrared (from ALMA), as well as the rest-frame optical (from HST F140W imaging). SHiZELS-14 displays a compact, dusty centre, as well as extended emission in both H-alpha and the rest-frame FIR. The ultraviolet emission is spatially offset from the extended dust emission, and appears to trace holes in the dust distribution. I find that the dust attenuation varies across the spatial extent of the galaxy, reaching up to ~5 magnitudes of extinction at H-alpha wavelengths in the most dusty regions. Global star formation rates inferred using standard calibrations to the different tracers vary from ~10-1000M_sol, and are particularly discrepant in the galaxy's dusty centre. This galaxy highlights the biased view of galaxy evolution provided by short-wavelength data in the absence of long-wavelength data, and is in line with my simulations.

It is now believed that most, if not all, galaxies contain a supermassive black hole (SMBH) and that these play a crucial role in their host galaxies' evolution. Whilst accreting material, a SMBH (known as an active galactic nucleus, AGN, during this growth phase) releases energy which may have the effect of quenching star formation and constraining the growth of the galaxy. It is believed that AGNs can be divided into two broad fundamental categories, each with its own feedback mechanism. The radiative-mode of feedback occurs in gas-rich galaxies when substantial star formation is occurring and their young AGNs are growing rapidly through efficient accretion of cold gas. A fraction of the energy released by an AGN is transferred into the surrounding gas, creating a thermal "energy-driven" wind or pressure "momentum-driven" wind. Gas and dust may be expelled from the galaxy, so halting star formation but also cutting off the fuel supply to the AGN itself. The jet-mode occurs thereafter. The SMBH has now attained a large mass, but is accreting at a comparatively low level as gas slowly cools and falls back into the galaxy. The accretion process generates two-sided jets that generate shock fronts, so heating the gas surrounding the galaxy and partially offsetting the radiative cooling. This restricts the inflow of gas into the galaxy, so slowing the growth of the galaxy and SMBH. There are several convincing theoretical arguments to support the existence of these feedback mechanisms, although observational evidence has been hard to obtain. A new radio telescope - the Low Frequency Array (LOFAR) - recently started operations. LOFAR is especially suitable for investigating AGN feedback. It has been designed to allow exploration of low radio frequencies, between 10 and 240 MHz, which are particularly relevant for research into AGN activity. Also, with its large field-of-view and multi-beam capability, LOFAR is ideal for conducting extensive radio surveys. A project to image deeply the ELAIS-N1 field was started in May 2013. This thesis uses a number of surveys at different wavelengths, but particularly the low-frequency radio observations of the ELAIS-N1 field, to improve our knowledge of jet-mode AGN feedback and hence of the interplay between the complicated processes involved in galaxy formation and evolution. The more important pieces of research within the thesis are as follows: - A sample of 576 AGNs in the nearby universe was assembled and used to find a relationship between radio luminosity, X-ray luminosity and black hole mass. Moreover, the relationship is valid over at least 15 orders of magnitude in X-ray luminosity, strongly suggesting that the process responsible for the launching of radio jets is scale-invariant. - The established "Likelihood Ratio" technique was refined to incorporate colour information in order to optimally match the radio sources in the ELAIS-N1 field with their host galaxies. - The resulting catalogue was used to investigate ways in which radio sources can be matched automatically with their host galaxies (and so avoiding laborious visual examination of each source). The conclusions have helped the design of a pipeline for an extensive wide-area survey currently being conducted by the LOFAR telescope. - The catalogue was also used to investigate the evolution of jet-mode AGNs. This involved: deriving source counts; obtaining redshifts for each object; classifying the radio sources into the different populations of radiative-mode AGNs, jet-mode AGNs and star-forming galaxies; and using the above preparatory work in order to derive a luminosity function for jet-mode AGNs. - Key conclusions are that (1) feedback from jet-mode AGNs peaks at around a redshift of 0.75, (2) the space density of jet-mode AGNs declines steadily with redshift and (3) the typical luminosity of a jet-mode AGN increases steadily with redshift.

The most intensely star-forming galaxies lie at z ∼ 2 and are thought to be the progenitors of the most massive galaxies today, yet study of this important population has been hampered by vast quantities of dust, making them almost invisible in the optical and ultraviolet (UV) regimes, and by the low sensitivity and angular resolution of many infrared (IR) facilities. Chapter 2 describes the use of the flux and angular extent boost provided by strong gravitational lensing in the detailed study of individual high-redshift dusty star-forming galaxies (DSFGs). The low number density of such systems has been overcome by recent wide area far-infrared (FIR)–mm surveys, and a sample of candidate systems which are bright enough to study with single-dish FIR telescopes are assembled from these surveys. The chapter further describes spectra of these galaxies obtained using the the Spectral and Photometric Imaging REceiver (SPIRE; Griffin et al. 2010) Fourier transform spectrometer (FTS) on board the Herschel Space Observatory (Pilbratt et al. 2010), exploiting the increased flux densities to search for FIR atomic and ionic spectral lines: important coolants of warm gas surrounding star-formation regions. Chapter 3 describes the first “blind” redshift obtained using Herschel, via the detection of [C ii] 158 μm in one of our spectra. Confirmation of this redshift was provided by detection of CO lines with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the Plateau de Bure Interferometer (PdBI), and along with multi-wavelength photometric follow-up, allowed a characterisation of the galaxy, indicating both a merger-driven starburst and an active galactic nucleus (AGN) within the system. Chapter 4 describes the first detection of a massive outflow of molecular gas at high-redshift. Stacking five repeat spectra of the Cosmic Eyelash, one of the best-studied strongly lensed DSFGs, one of the massive star-forming clumps is shown to drive this outflow, albeit likely at a velocity lower than that required to become unbound from the hosting gravitational potential well. Chapter 5 describes line measurements and spectral energy distribution (SED) fitting from the full set of spectra and Herschel PACS mini scan maps. The spectra are stacked to search for faint lines, and compared to a detailed interstellar medium (ISM) model to determine average physical properties of the star-forming gas. Photodissociation region (PDR) properties are found to be similar to those derived using other models, however a cosmic ray ionisation rate of 103 times that of the Milky Way, expected in galaxies of this type cannot reproduce the observed line ratios, in particular the low [O i] 63 μm flux. Chapter 6 finally describes the conclusions drawn from the work presented in this thesis and how these data and analysis add to our knowledge and interpretation of high-redshift DSFGs.

The work contained within this thesis Is made up primarily of two pieces Both address active galactic nuclei And the galaxies that live nearby The obscured fraction of the population Is the topic of one publication And the type-II fraction in the optical regime In chapter four this is the theme I research the vicinity overdensity Around radio galaxies in chapter three, you’ll see I reduce some spectra at redshift one But not all observations in the end got done With the spectra I have I do what I can As if all target observations had actually ran In the end I conclude with results and the theme of research to be done further downstream. The works contained herein addresses two major topics in extragalactic astrophysics, namely the Type-II AGN fraction and the Overdensity-Radio power relation. Quantifying the Type-II AGN fraction has been attempted by many works in many different observational regimes, finding rather contrasting results. Accretion onto supermassive black holes contributes between 5 per cent and 20 per cent of the luminosity of the Universe, and seems to be closely linked to star formation processes. The large uncertainty on this value is due to the ill-determined contribution from obscured accretion, namely the Type-II fraction. In Chapters 3 and 4 I address this issue from a theoretical standpoint in the X-ray regime and an observational standpoint in the optical regime respectively. In Chapter 3 I show how crude X-ray spectroscopy of partially obscured AGN can lead to catastrophic underestimations of the intrinsic X-ray luminosity of these sources. Acting over an entire population, these partial obscurers can produce an obscured AGN fraction which decreases as a function of observed luminosity. The results are consistent with observations in the X-ray vs. IR luminosity of AGN classes. In Chapter 4 I select a statistically significant sample of AGN from an unbiased 250μm galaxy sample. After spectroscopic classification I find the optical Type- II AGN fraction to be consistent across several decades in [OIII] luminosity, a common proxy for intrinsic AGN luminosity. I also investigate the relation of AGN activity to host galaxy mass, as well as star formation activity and star formation history. Probing the environments of protoclusters will help to constrain the models of structure formation in the Universe. Until now, no dataset has been big enough to probe the environments of high redshift radio galaxies at a statistical level; While many believe that the feedback processes of high luminosity radio jets will have a direct impact on star formation in the surrounding medium it has not been tested. In Chapter 2 I investigate this on an statistical level, finding no meaningful correlation between radio galaxy radio power and source overdensity in the vicinities of these sources. In Chapter 5 I discuss the reduction of a 24μm sample at redshift z ∼ 1 for direct comparison with a local 12μm sample. With only a fraction of the target sample being observed, no statistically significant results could be derived, but the objects are spectroscopically classified and spectroscopic redshifts are measured where possible. Correlations in the data set are investigated and the limitations of the sample selection strategy are discussed.

A significant fraction of the star formation density between z = 1–3 has been traced to luminous and ultra-luminous infrared galaxies. Among the most extreme objects seen are those identified via surveys at submillimetre wavelengths; the high infrared luminosities (LIR = 10¹¹−¹³ L⊙) of these “submillimetre galaxies” (SMGs) arise due to the reprocessing of UV and optical light from massive, young stars by interstellar dust, and imply star formation rates SFR= 50–2000M⊙ yr−¹. Such SFRs, combined with the observed increase in number density of SMGs by a factor ∼ 20× out to z = 2 make them candidates to be the progenitors of the most massive “red and dead” elliptical galaxies which dominate the cores of clusters in the present day, yet limitations in the technical capabilities of radio and infrared telescopes have long hindered a detailed understanding of these galaxies, and in particular, the physics of the interstellar medium (ISM). Hence the following key questions remain unanswered: (i) how much molecular gas (H2) do SMGs contain, and what are the likely upper-limits on how long they can sustain their present level of activity?; (ii) do the prodigious luminosities of SMGs stem from a common origin, or does the selection criterion return a “mixed bag” of galaxies?; (iii) what are internal kinematics of SMGs, and to what extent do these influence the global and local star-formation (Schmidt-Kennicutt) law?; (iv) is star-formation in distant SMGs distributed across the entire galaxy, or does it occur in isolated clumps?; (v) what are the typical densities and temperatures of star-forming regions in SMGs, and do they adhere to the observed correlation between far-infrared and radio emission in star-forming galaxies? Recent upgrades to the Karl G. Jansky Very Large Array (VLA), the inauguration of the Atacama Large Millimeter Array (ALMA), and the superlative legacy of the Herschel Space Observatory have jointly provided the toolkit to study the gas and dust emission in distant galaxies in unprecedented detail, and thus to begin to address these fundamental questions. In this thesis, I present ∼120 hours of new VLA observations of ¹²CO J =1−0 emission – the best tracer of molecular gas – in a sample of four lensed SMGs. The combination of high angular resolution with the VLA and the magnifying effects of gravitational lensing allow the ISM properties of these z ∼ 2.5 star-forming galaxies to be seen for the first time on sub-galactic scales, implying gas masses of 10⁹−¹ºM⊙ (subject to the 12CO-luminosity-to-H2-mass conversion factor, αCO), and demonstrating the presence of an extended, low-excitation gas reservoir. In conjunction with observations of the excited gas phase from the Plateau de Bure Interferometer (PdBI), these new data point to variations in the densities and temperatures of H2 throughout each galaxy. The wide bandwidth and phenomenal sensitivity of the VLA yields the first detections of 115-GHz continuum emission at high redshift, which I use in conjunction with well-sampled dust spectra from Herschel and data at longer wavelengths from the VLA to decompose the radio spectra of two galaxies into contributions arising from thermal dust emission, optically-thin free-free emission from Hii regions, and non-thermal synchrotron emission. From these measurements, estimates of SFRradio are made, providing an independent check on SFRIR, and enabling the degeneracy between the heating of dust due to star formation and that due to hidden AGN activity to be broken. Via this spectral index de-convolution, I find L[AGN]/LIR fractions of 35% and 55% for the two SMGs, in broad agreement with previously published estimates for these sources based on their mid-IR spectral properties. In the exceptional case of SMMJ23152–0102, magnification by the foreground cluster is so extreme (∼ 32.5×) that the VLA synthesised beam traces regions of order ∼ 130 pc in the source plane, and identifies a series of cool, dense clumps (Tk = 30–70 k; log[n(H2)/cm−3]= 3.6–3.9) within the gas reservoir, which contain between them 10–60% of the total molecular gas of the system. These clumps are offset from the far-infrared/radio correlation, which I argue has implications for their ages.

Supermassive black holes are leading candidates for the regulation of galaxy growth and evolution over cosmic time, via ‘feedback’ processes, whereby outflows from the Active Galactic Nuclei (AGN) halt star formation within the galaxy. AGN feedback is generally thought to occur in two modes, high-excitation (HERG, or ‘quasar-mode’) and low-excitation (LERG or ’radio-mode’) each having a different effect on the host galaxy. LERGs curtail the growth of the most massive galaxies, whereas HERGs are thought to be activated by mergers/interactions, switching off star formation at high redshift. A critical problem in current extragalactic astrophysics lies in understanding the precise physical mechanisms by which these feedback processes operate, and how they evolve over cosmic time. Radio-loud AGN are an essential tool for studying major feedback mechanisms, as they are found within the largest ellipticals, and hence are beacons for the most massive black holes across the bulk of cosmic time. In this thesis I develop and study existing complete radio samples with extensive new multi-wavelength data in the radio, optical and infrared, aiming to investigate the evolution of AGN feedback modes, and methods to locate and study such systems at the very highest redshifts. This will serve to inform further studies of radio-AGN planned with next generation radio instruments such as the LOw Frequency ARray (LOFAR). Very few radio-loud AGN systems are currently known at high redshifts, and the effectiveness of traditional high redshift selection techniques, such as selection based on steep spectral index, have not been well quantified. A purely evidence-based approach to determining the efficiency of various high redshift selection techniques is presented, using nine highly spectroscopically complete radio samples; although weak correlations are confirmed between spectral index and linear size and redshift, selection first of infrared-faint radio sources remains by far the most efficient method of selecting high-z radio galaxies from complete samples. Radio spectral curvature in four of the complete samples is analysed and the effect of radio spectral shape on the measurement of the radio luminosity function (RLF) of steep-spectrum radio sources is investigated. Below z=1, curvature has negligible effect on the measurement of the RLF, however at higher redshifts, where source numbers are low, the shape of the radio spectrum should be taken into account, as individual source luminosities can change up to 0.1-0.2 dex, and this can in some cases introduce errors in space density measurements of up to a factor of 2-3 where source numbers are low. Building upon these samples, the very first independent determinations of the separate RLFs for high and low excitation radio sources across the bulk of cosmic time are made, out to z=1. Here it is shown that HERGs show very clear signs of strong evolution, in line with theoretical predictions. LERGs also show some very weak evolution with redshift, showing increases in space density of typically around a factor of 2. These measurements are also used to estimate the contribution of LERGs, which typically show weak or no emission lines to the ‘missing redshift’ population, which are sources within the complete samples not identifiable spectroscopically. Complementary to this, a pilot study is presented in selecting ‘missing redshift’ sources which are classed as infra-red faint (IFRS), which show no optical or near-IR identification, and are compact in the radio. Follow up spectroscopy on these candidate high z sources detected no line emission. Finally, work carried out towards the testing and commissioning of the new LOFAR telescope is presented. The findings from this thesis will serve to both streamline and inform high redshift radio-AGN searches and studies planned to be carried out with LOFAR and other multi-wavelength complementary surveys in the near future, and help to open up an as yet unexplored epoch in radio-AGN formation and evolution.

The first stars, galaxies and black holes in the universe produced large quantities of ionising UV radiation; forming H II regions in the neutral gas before the Epoch of Reionisation (EoR). These ionisation fronts will have come into contact with overdensities in the surrounding Intergalactic Medium (IGM), including haloes which were in the process of collapse. Previous studies have shown that the feedback processes on these secondary haloes can either disrupt the gas, or induce further cooling from the formation of molecular hydrogen. The ionising source will eventually die and create a defunct H II region, which expands into the surrounding neutral IGM. Minihaloes at the edge of these defunct H II regions are particularly susceptible to positive feedback due to not having been photoevaporated like their counterparts further inside the ionised volume. In this thesis, numerical simulations of minihaloes at the edges of H II regions formed by the first luminous objects before the EoR are presented. A methodology of including secondary ionisations from high energy photons is also implemented into the radiation hydrodynamical code ZEUS-RT. The interaction of differing spectral index sources with minihaloes including secondary ionisation is discussed. The secondary ionisations show the greatest effect for hard spectral sources with a large fraction of high energy photons; where a decrease in photoheating and an increase in ionisation rate is found at the outer reaches of the ionisation front (I-front). The increased ionisation rate lowers the optical depth of the gas for subsequent photons, and thus reduces the trapping of I-fronts in high densities found in the minihalo cores. The ratio of the free electron fraction to the temperature in the core of the minihaloes is found to constrain the resulting evolution. A high ratio is correlated with the creation of molecular hydrogen, which can then induce further cooling and the collapse of the system. A large parameter suite of 3780 two-dimensional minihalo models utilising radiative hydrodynamical simulations with 12 species and a coupled reaction network, including H2 cooling, HD cooling, Lyman-Werner radiation and secondary ionisation is performed. The parameter space includes: the spectral index representing different sources such as quasars or galaxies, the mass of the minihaloes from 10⁵ - 10⁶ M⨀, the redshift of ionisation from z ~ 10 - 30, and other factors which denote the position of the minihalo relative to the boundary of the H II region. Minihaloes with average core densities of n₀ = 2 - 10 cm⁻³ show almost unanimous positive feedback, while the majority of minihaloes under this average density are disrupted. Minihaloes with core densities above this value generally would have collapsed in isolation anyway, but are found to not be delayed by the I-front. The H₂ fraction in the minihalo gas is also increased in models with no blowout by factors between 2 - 100 times that of an isolated minihalo. This is especially significant for lower redshift, z ≤ 15, minihaloes. Finally, a parameter suite of larger 10⁶ - 10⁷ M⨀ minihaloes results in the creation of self-gravitating clumps of gas moving out of the dark matter potential. The gas core is compressed by the I-front, enriched with molecular hydrogen, and ejected by the pressure front after the source dies. These "baryon bullets" could be progenitors of primordial globular clusters found in the haloes of galaxies today. Properties such as old stellar populations and the lack of dark matter haloes can be explained by this radiative ejection method. The dynamical nature of these interacting systems and diversity of subsequent evolution suggest that minihaloes less than 10⁸ M⨀ are important in the early formation history of the universe, and in determining the constraining parameters of the EoR. The feedback mechanisms investigated, and secondary ionisation physics, should be included in astrophysical simulations and analytical calculations determining the evolution of the universe at this critical epoch.

The centrepiece of this thesis is a deep, new, high-resolution 1.4-GHz image covering the United Kingdom Infrared (IR) Telescope IR Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS) legacy field. Deep pseudo-continuum observations were made using the Very Large Array, prior to its recent upgrade, in its A, B and DnC configurations. The resulting mosaic has a full-width-at-half-maximum synthesised beam width of ≈ 1.7 arcsec and a point-source sensitivity of ≈ 60μJy (6σ ) across the central 0.6 deg2, while conserving flux from sources of extended emission. The full image covers 1.3 deg2. I also present a catalogue containing over 1,000 radio emitters, having chosen the 6-σ threshold by maximising the number of radio sources with secure optical/near-IR counterparts. Most of the sources in the catalogue (≈ 90 per cent) lie in the sub-mJy flux density regime. Deep, complementary data covering a wide range of wavelengths was used to explore this faint radio population, whose nature remains controversial. It was found that 53 per cent of the sample comprise active galactic nuclei (AGN). AGN dominate at & 0.2mJy and remain a significant population down to 0.1mJy; at lower fluxes – the so-called μJy radio population – star-forming galaxies become dominant. The radio sample presented here was also matched to Hubble Space Telescope imaging of the UDS field (which is part of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey – CANDELS) to classify the faint radio population morphologically. These classifications were done using the Gini–M20 method. It was found that a low fraction of AGN and SFGs are undergoing interactions and mergers, 33 ± 9 and 13 ± 7 per cent, respectively. The merger fraction does not appear to have evolved significantly since z ∼ 3. This suggests that mergers have played a relatively minor role in the assembly of galaxies and super-massive black holes – certainly less significant than previously thought. Finally, I present a study of cosmic star-formation activity as a function of stellar mass and redshift, exploiting panchromatic stacking. Mid-IR–through–radio images, including new data from Herschel, are stacked at the positions of a K-selected (i.e. an approximately mass-selected) sample in the UDS field. Specific star-formation rates (SSFR, i.e. star-formation rate per stellar mass, or the rate at which a galaxy is converting its gas into stars) were derived from UDS radio luminosities measured here and stellar masses from the literature. The SSFR was found to be poorly correlated with stellar mass; it decreases with decreasing redshift; at a given mass, SSFR rises with redshift. These results indicate that at early epochs, galaxies were forming stars more efficiently and at a higher rate.

Determining the cosmic star formation history of the Universe is fundamental for our understanding of galaxy formation and evolution. While surveys now suggest that the "epoch" of galaxy formation occurred more than 6 billion years ago, our measurements still suff er from signi ficant scatter and uncertainties due to the use of diff erent indicators, dust extinction and the e ffects of cosmic variance in the current samples. Furthermore, understanding galaxy formation and evolution require us to go much beyond simply determining the star formation history of the Universe with high accuracy: what are the physical mechanisms driving the strong evolution that we observe? How does star formation depend on stellar mass and environment and how does that change with cosmic time? This thesis presents both a completely self-consistent determination of the star formation history of the Universe (based on a single, sensitive and well-calibrated star formation indicator up to redshift z ~ 2:3: the H α luminosity) and investigates its drivers by exploring large area surveys (probing a range of environments and overcoming cosmic variance) obtained with the High-redshift Emission Line Survey (HiZELS). HiZELS is a panoramic extragalactic survey using the WFCAM instrument on the 3.8-m UK Infrared Telescope (UKIRT) which utilizes a set of existing and custom-made narrow-band filters in the J, H and K bands to detect emission line galaxies (main targets are H α emitters at z = 0:84, z = 1:47 and z = 2:23) up to z ~ 9 over square degree areas of extragalactic sky. Detailed measurements of the H α luminosity function and its evolution with redshift are presented, revealing a signi ficant luminosity evolution. The clustering properties of H α emitters at high-redshift are quantifi ed and investigated for the first time, revealing that these distant galaxies reside in Milky-Way type dark matter haloes at z ~ 1. Mass and environment are found to have important and inter-dependent roles on star formation at high-z and the results are able to reconcile previously contradictory results in the literature. Furthermore, by conducting a novel double-narrow band survey at z = 1:47, the relationship between the [Oii]3727 and H α emission lines is studied in detail and directly compared to z ~ 0, showing no signifi cant evolution in the dust properties of star-forming galaxies, despite the very strong luminosity evolution. Finally, this thesis also presents the widest search for very distant Ly α emitters at z ~ 9.

Stars of all ages and evolutionary stages are seen to be surrounded by discs of matieral. during the formation of a stellar system the stars are orbited by a massive protoplanetary disc composed of interstellar gas and dust, in which planet formation occurs. Betewwen 1 and 10 Myr the protoplanetary disc disperses, leaving behind the newly formed system of planets and smaller bodies. The remaining material which has not formed into planets is referred to as a debris disc. Even though the interstellar dust grains from the protoplanetary disc have long been removed from the system, debris discs can contain large quantities of dust due to collisions between larger bodies and cometary activity. such dust can be detected by its thermal emission. This thesis focuses on observational studies at far-infrared and sub-millimetre wavelengths of debris discs and the late stages of protoplanetary disc evolution. An overview of surveys for debris discs performed to date is presented, highlighting the limitations and statistical biases. the motivation, design and sample selection for two large surveys for debris discs around nearby stars, with the Hershel space observatory and the SCUBA-2 sub-millimetre camera on the James Clerk Maxwell Telescope, are described. The combination of a uniform obstevational strategy, longer wavelengths than previous surveys, and a large, clearly chosen sample - unbiased by stellar properties - will allow robust statistical conclusions of how the incidence and properties of debris discs depend on system parameters such as stellar mass, age, metallicity, binarity and the presence of planets. As a precursor to the Hershel and SCUBA-2 surveys, a volume-limited ample of 130 A type star systems was surveyed using observations at 24 and 70 μm, which were required to determine the presence of emission from dust, were predicted by fitting model flux distributions to optical and near-infrared photometry. Debris discs were detected around 46 systems, 12 of which including the system with the largest dust mass - are new discoveries. This survey adds to the results of previous studies which show that debris disc incidence is not correleated with host star metallicity despite the wll known giant planet - metallicity correlation, This is in accordance with what is predicted from the core accretion theory of planet formation. The most signigicant result from this survey is that, contrary to results reported in a previous work, debris discs are oberall less common around binary stars. Further investigation shows that systems with separations of ~3-150 AU are especially deficient of debris, while closer binaries and the primaries of wider binaries show debris detection rates consisten with those for single stars. A sample of circumstellar discs around 29 young stellar systems with ages of 5-30 Myr were observed with the LABOCA sub-millimetre instrument on the APEX telescope at 870μm, to provide disc masses or mass upper limits in support of a large Hershel programme. These targets included the η Chamaeleontis cluster and four bright Herbig Ae/Be stars which have not previously been observed at this wavelength. All but the Herbig Ae/Be stars were not detected, and 3σ dust mass upper limits of ~ 0.1-3 M are determined, with corresponding total disc masses of ~0.03-1Mjup. These mass limits indicate that there is insufficient remaining material in these discs to form gas giant planets, and add to the prevailing view that protoplanetary discs typically disperse within 10 Myr and that gas giant planet formation must be completed before this time. A search for cold dust emimission from two of the Solar System's nearest neighbours - α Centauri AB and ε Indi - was also performed with LABOCA. In both cases no debris disc emission was detected. A bright resolved feature was detected near α Centauri AB, nowever, follow-up observations at a second epoch, two years after the initial observations, showed that the feature is not co-moving with the stars. It is argued that the feature is most likely a pre-stellar core. The stars α Centauri A and B are detected, which is one of only very few detections of main sequence stellar photospheres at sub-millimetre wavelengths.

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.

The results are presented here of multi-wavelength observations centred on two fields of the Leiden-Berkeley Deep Survey, which form the basis for a study of the population and cosmic evolution of the high redshift, low power, Fanaroff & Riley class I (FRI) radio galaxies. These fields, Hercules.1 and Lynx.2, contain a complete sample of 81 radio sources with S_1.4GHz > 0.5 mJy within 0.6 square degrees. Wide-field, ~1.5 arcsec resolution, radio observations, along with near infra-red and optical imaging, and some multi-object spectroscopy, are used to select the best high-redshift FRI candidates, giving 37 in total. Currently, the host galaxy identification fraction is 86% with 11 sources remaining unidentified at a level of r > 25.2 (Hercules; 4 sources) or r > 24.4 (Lynx; 7 sources) and K > 20. Spectroscopic redshifts are determined for 49% of the sample and photometric redshift estimates are presented for the sample sources without spectra or previously published results. 95% of the 37 best FRI high-redshift candidate sources were then observed using sub-arcsecond radio resolution, with the aim of detecting extended emission with respect to compact core features - vital for unambiguous morphological classification. The nature of the radio observations meant that 10 extra sample sources could also be included in the data. Lower resolution data were also taken for the Lynx.2 field sources to provide a comparison with the 1.5 arcsec data. The classification of the entire radio sample is done in two stages. Sources which showed clear extension are classified by morphology alone, whereas sources with no obvious or weak extension were classified using a combination of morphology and flux density loss in the higher resolution data indicative of resolved out extended emission. Five groups are used for this - `Certain FRIs', `Likely FRIs', `Possible FRIs', `Unclassifiable sources' (for those not included in the higher resolution observations) and `Not FRIs'. The final group numbers are 8, 10, 24, 33 and 6 for groups 1-5 respectively. The space densities of the maximum, probable and minimum FRIs are then calculated and compared to two previously published measurements of the local value, and with the behaviour of the strongest FRII sources. The results for all three groups show density enhancements of factors of 5-9 at z ~ 1.0 which implies cosmic evolution of the FRI population; this enhancement is also in very good agreement with that predicted by previous models. The behaviour of the FRI/FRII dividing luminosity, as a function of host galaxy absolute magnitude, at the different cosmic epochs of the sample, and for two different star formation histories, is also investigated. A shift to brighter absolute magnitudes is found to be inconsistent with the data but this conclusion is weak due to the lack of knowledge of the host galaxy stellar populations, and the small number of sources in the sample.