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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.

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

The detection of Mpc scale diffuse radio emission in galaxy clusters provides evidence that cosmic ray electrons, as well as cluster scale magnetic fields are present in clusters. As such, radio observations of clusters provide a unique opportunity to study the non-thermal populations of the intra-cluster medium. Observations of Faraday rotation in sources embedded in cluster and group environments offers an additional method for probing the cluster/group magnetic field. In this thesis I present low frequency radio observations of multiple galaxy clusters in order to investigate the nature of diffuse radio emission present in many clusters. I also present observations of the giant radio galaxy NGC 6251 and discuss both the source properties as well as the host group environment. In Chapter 1 of this thesis I review the current understanding of galaxy clusters, groups and radio galaxies. I also describe some of the astrophysical processes important to this thesis. In Chapter 2 I discuss the interferometry and the process of calibrating interferometric data. I also describe some of the techniques used later in the thesis such as QUfitting and RM synthesis. In Chapter 3 I present my observations of the massive merging galaxy cluster MACSJ2243.3-0935. I report the discovery of a radio halo in MACSJ2243.3-0935, as well as a new radio relic candidate, using the Giant Meterwave Radio Telescope and the KAT-7 telescope. The radio halo is coincident with the cluster X-ray emission and has a largest linear scale of approximately 0.9 Mpc. I measure a flux density of $10.0\pm 2.0\, \rm mJy$ at 610 MHz for the radio halo. I discuss equipartition estimates of the cluster magnetic field and constrain the value to be of the order of $1\, \rm \mu G$. The relic candidate is detected at the cluster virial radius where a filament meets the cluster. The relic candidate has a flux density of $5.2\pm 0.8\, \rm mJy$ at 610 MHz. I discuss possible origins of the relic candidate emission and conclude that the candidate is consistent with an infall relic. In Chapter 4 I present my GMRT observations at 610 MHz of 3 disturbed galaxy clusters, A07, A1235 and A2055. No diffuse emision was observed any of the three clusters. In order to place upper limits on the radio halo power in these clusters I have injected simulated halos at difffent radio powers into the uvdata. A07 has a radio halo upper limit of $P_{\rm 610MHz}=1.5\times10^{24}$ W Hz$^{-1}$. A2055 has a radio halo upper limit of $P_{\rm 610MHz}=1.8\times10^{24}$ W Hz$^{-1}$. A1235 has a radio halo upper limit of $P_{\rm 610MHz}=5.8\times10^{23}$ W Hz$^{-1}$. These limits are below the $P_{610}-L_{\rm X}$ relation and rule out bright radio halo in these clusters. I have identified these clusters as potential hosts for Ultra Steep Spectrum Radio Halo (USSRH). Observations with LOFAR should be capable of confirming whether or not these clusters host USSRH. In Chapter 5 I present observations of the giant radio galaxy NGC 6251 with LOFAR HBA. NGC 6251 is a giant radio galaxy with a borderline FRI/FRII morphology located in a poor group. The images presented in this chapter are the highest sensitivity and resolution images of NGC 6251 at these frequencies to date. Analysis of the low frequencies spectral index did not reveal any change in the low frequency spectra when compared with the higher frequency spectral index. NGC 6251 is found to be either at equilibrium or slightly electron dominated, similar to FRII sources. I calculated the ages of the low surface brightness extension of the northern lobe and the backflow of the southern lobe, which are only clearly visible at these low frequencies, to be 205 Myr$.

There is a strong consensus within the scientific literature that the most powerful radio galaxies are the result of gas-rich, major mergers causing large amounts of material to fall on to the black hole at the centre of the resulting galaxy, thus producing an immensely powerful radio jet. However, such galaxies are extremely rare and are not representative of the general radio galaxy population. Most radio galaxies have low power, and their basic properties and origins are poorly constrained. One of the most popular suggestions is that they are bona-fide detections of jet-mode active galactic nuclei (AGN), a class of AGN thought to maintain the dearth of star formation in early-type galaxies (ETGs), after most of the fuel for star formation has been removed by an earlier event. If true, then all ETGs must regularly go through such an active phase, and low-powered radio galaxies should be much the same as their radio-quiet (quiescent phase) ETG counterparts. To test this hypothesis, a complete sample of 11 nearby low-powered radio galaxies was constructed and observed with a suite of instruments. We focus here on the spatially-resolved stellar kinematics, stellar populations, and ionized gas distribution, kinematics and ionization derived from integral-field spectroscopic (IFS) observations with the Visible Multi-object Spectrograph (VIMOS) and archival data from the Multi-unit Spectroscopic Explorer (MUSE), both on the Very Large Telescope (VLT). We compare the characteristics and properties (accessible with V-band IFS data) of our sample of low-power radio galaxies to those of ordinary (i.e. optically-selected) ETGs from the Atlas3D and MASSIVE IFS surveys. For the stellar component of our sample galaxies, we use the kinematic maps to identify regular/non-regular rotators and kinematic substructures. In our sample 4 galaxies are regular rotators and two (with a tentative third) galaxies were found to contain kinematically decoupled cores (KDCs). We use the lambdaR specific angular momentum parameter from Atlas3D to classify the galaxies according to the fast/slow rotator classification scheme. We found a similar fraction (45+/-13%) of slow rotators to the combined Atlas3D and MASSIVE samples, corrected for mass distribution. We also find that fast rotators tend to have aligned kinematic and photometric position angles, while the slow rotators can have any angle between the two position angles. The best-fitting stellar populations show our sample to be typical massive ETGs: i.e. mostly old, metal rich and alpha-element enhanced. We found and measure the radial stellar population gradients and found averages of Deltalog age = 0.014 ± 0.007 dex-1 and Delta[Fe/H] = -0.03±0.01 dex arcsec-1. The KDCs found previously were observed to be large and contain old stellar populations and are the result of galaxy mergers. Some literature suggests that if the AGN is fuelled by accreted cold gas then it might be expected to form stars within the accretion disc. However we see no evidence of a very young stellar population right at the centre of our sample galaxies. For the ionized gas components of our sample galaxies, the estimated gas masses were found to be in the range of 104 - 106 Modot, which is the upper end of the range expected from optically-selected ETGs. This was the only suggestion we find that our sample galaxies may be intrinsically different than optically-selected ETGs. We detected spatially extended gas in 4 of our sample, with a range of kinematics. Two galaxies had ionized gas kinematics misaligned to that of the stars, suggesting an external origin to the gas, while the kinematics of one was aligned with that of its stars (consistent with an internal origin). The final galaxy was unsettled and thus not clear. Using the classic Baldwin-Phillips-Terlevich (BPT) diagnostic plots, as well as similar alternatives, for galaxies with observations outside of the wavelength range required for the BPT plots, we find the dominant ionization source for each galaxy. We find that 9 of our sample galaxies are low-ionized nuclear emission region galaxies (LINERs; 5 of which can be attributed to the radiation field from the AGN), 1 is a Seyfert II and 1 a passive (line-less) galaxy. All of the properties found are consistent with that of optically selected ETGs with a similar mass distribution. We thus find no conclusive difference between the stellar and ionized gas properties all of these ETGs except for a tentative suggestion of slightly higher ionized gas masses in our radio galaxies. If confirmed, the latter may be gas built up during the quiescent phase of ETGs, ultimately triggering the active (radio) phase itself then re-removing such gas from the galaxy. Some of the sample galaxies have clear evidence of merger events (e.g. large KDCs), but others have a complete lack of evidence of a merger, suggesting that radio galaxies can form without the requirement of a merger event. Indeed, it is well known that NGC 612 harbours a large extended gas disc with kinematics that are consistent with an internal origin of the gas. As such, in rare cases, it is possible that radio galaxies can be fuelled through entirely secular processes.

My Ph.D. has been composed of work involving the use of far–IR and submm observations of AGN. During this time it has focused on the in- terplay between AGN and their host galaxies and cluster environments. Understanding the role of AGN, and how they affect the evolution of both their host galaxies and surrounding environments, is a pressing concern in cosmological models of the universe, affecting as they do the chemical makeup, star formation rate, and morphology of their host galaxies. In Chapter 2, we focus on attempting to determine whether there is an inherent physical difference between Broad Absorption Line Quasars and non–BAL QSOs using Herschel observations taken at 250, 350 and 500 μm as part of the H–ATLAS (Eales et al. 2010) survey. BAL QSOs have been considered the most visible form of AGN feedback, and therefore are a prime starting point for understanding how galaxy evolution may be affected by the presence of an AGN. By using matched samples of 50 BAL and 329 non–BAL QSOs, we create weighted stacks at each wavelength, finding similar far–IR flux–densities for each sample within the errors. By SED modelling using a simple modified black body (Hildebrand 1983) fit to Mrk 231 and IZw1, we derive likely upper and lower limits for the BAL and non–BAL QSOs in each wavelength, again finding they are consistent within the errors. A bevy of statistical tests run on either population similarly finds no evidence to reject the null hypothesis they are drawn from the same parent population. These results would imply that HiBAL QSOs can be unified with ordinary QSOs within a simple orientation dependent scheme. We cannot make the same distinction for LoBALs or FeLoBALs, which the literature suggests may well be a separate evolutionary phase. In Chapter 3, we determine whether the presence of an AGN correlates to an overdensity of star–forming galaxies in the FIR, as has been found at shorter wavelengths (Falder et al. 2010). For the SHAGs study, 171 AGN were observed and selected at z∼1. By using observations at 250 μm, we are able to trace close to the peak of the grey–body SED created by reprocessing by dust of radiation from young O and B stars. Following data reduction, we determine number counts and correct for completeness within a 1Mpc radius of the central AGN. We find an overdensity on the order of around 0.4 sources per AGN, implying a degree of activity already significantly lower than at higher redshifts. This overdensity appears to be somewhat different between RL AGN and RQQ within 1Mpc. A cor- relation is found between radio luminosity and star formation overdensity, consistent with a stronger dependence found by Falder et al. (2010) at 3.6 μm, and there also appears to be a correlation between stellar mass and star formation overdensity for radio–loud QSOs. The galaxies in the environs of the AGN have LIRG–level luminosities, and are likely the pro- genitors of modern day S0 galaxies, whose population increases steadily from z∼1 to the present day (Postman et al. 2005; Smith et al. 2005). Our work with SCUBA–2, presented in Chapter 4, follows on from a prior sample of X–ray–absorbed QSOs (Stevens et al. 2005). This new sample is composed of more highly–absorbed X–ray QSOs and covers a larger area than the initial sample, so is ideal for an analysis of source counts around AGN at high–redshift. Data from the JCMT have been reduced, and completeness corrections and flux corrections applied to catalogues to determine the number counts around AGN. A comparison background, created using data from the Cosmology Legacy Survey has been used to derive comparison counts. The AGN have been investigated, yet none are detected above 3 at 850 μm, in contrast to the original sample. This may suggest that star formation in their host galaxies has been suppressed. Upon stacking in redshift and BAL classification, no difference in flux– density is apparent and the sources studied here have a similar stacked submm output to an unabsorbed QSO sample created for the original X– ray absorbed QSOs. However, over half of the sources here are BAL QSOs in contrast to the original absorbed QSO sample which contained only 1 BAL QSO. From the work in Chapter 2, one might expect BAL and non–BAL QSOs to have similar flux–densities. We argue that the sources studied in this thesis have likely undergone rapid evolution owing to a strong outflow, and as such star formation has been suppressed sufficiently that the submm emission is below the confusion noise. BAL winds may still be present, but essentially, the show is already over. A similar mechanism may already have occurred in unabsorbed QSOs if all QSOs pass through an X–ray–absorbed phase. With regard to source counts, we find that there is tentative evidence for an overdensity of sources around these AGN. The SFRs of the companion sources have been calculated using several greybody analogues, all of which imply a high degree of activity, suggesting these fields will evolve to become some of the most massive regions at the present epoch, in keeping with current theories of SMGs and high–redshift clusters.

In this thesis, I use the new generation of radio interferometer along with X-ray observations to investigate the dynamics and energetics of radio-loud active galaxies which are key to understanding AGN feedback and the evolution of galaxies as a whole. I present new JVLA observations of powerful radio source and use innovative techniques to undertake a detailed analysis of JVLA observations of powerful radio galaxies. I compare two of the most widely used models of spectral ageing, the Kardashev-Pacholczyk and Jaffe-Perola models and also results of the more complex, but potentially more realistic, Tribble model. I find that the Tribble model provides both a good fit to observations as well as providing a physically realistic description of the source. I present the first high-resolution spectral maps of the sources and find that the best-fitting injection indices across all models take higher values than has previously been assumed. I present characteristic hot spot advance speeds and compare them to those derived from dynamical ages, confirming that the previously known discrepancy in speed remains present in older radio sources even when ages are determined at high spectral and spatial resolutions. I show that some previously common assumptions made in determining spectral ages with narrow-band radio telescopes may not always hold. I present results from a study of the powerful radio galaxy 3C223 at low frequencies with LOFAR to determine its spectrum on spatially small scales and tightly constrain the injection index, which I find to be consistent with the high values found at GHz frequencies. Applying this new knowledge of the low energy electron population, I perform synchrotron / inverse-Compton model fitting and find that the total energy content of the radio galaxy lobes increases by a factor greater than 2 compared to previous studies. Using this result to provide revised estimates of the internal pressure, I find the northern lobe to be in pressure balance with the external medium and the southern lobe to be overpressured. I go on to present the first large sample investigation of the properties of jets in Fanaroff and Riley type I radio galaxies (FR-I) at X-ray energies based on data from the Chandra archive. I explore relations between the properties of the jets and the properties of host galaxies in which they reside. I find previously unknown correlations to exist, relating photon index, volume emissivity, jet volume and luminosity, and find that the previously held assumption of a relationship between luminosities at radio and X-ray wavelengths is linear in nature when bona fide FR-I radio galaxies are considered. In addition, I attempt to constrain properties which may play a key role in determination of the diffuse emission process. I test a simple model in which large-scale magnetic field variations are primarily responsible for determining jet properties; however, we find that this model is inconsistent with our best estimates of the relative magnetic field strengths in my sample.

In this thesis I used a sample of radio galaxies with 1.4GHz radio luminosity, S1.4 > 10mJy, with the aim of finding high-redshift radio galaxies in the Lockman Hole, ELAIS-N1, ELAISN2, XMM-LSS and CDFS using near-infrared data from the Spitzer Space Telescope, and in the VIMOS4/SSA22 field using near-infrared data from the UKIRT Infrared Deep Sky Survey Deep Extragalactic Survey (UKIDSS-DXS). I used these near-infrared data to filter out low redshift (z < 2) radio galaxies by only including sources with either a very faint identification or non-detection in the K−band or at 3.6μm. I then applied a radio selection criteria based on the compactness of the radio structure in the FIRST survey, to ensure that it was possible to correctly associate a near-infrared identification if detected. Using these filtering critera I reduced the sample from 498 to 220, and I obtained spectroscopic observations for 46 of these. I successfully measured redshifts for 22 of these. Four of these sources were found to have redshifts at z > 3, thus proving the efficiency of the selection. Using these spectroscopically identified sources I investigated the correlation between radio luminosity and Lyα emission line luminosity and showed that these radio sources lie on the same correlations as their more radio-luminous counterparts, suggesting that accretion rate on to the central supermassive black hole determines the power of the AGN, both in terms of photoionizing radiation and radio power. I also investigated the near infrared properties of the host galaxy for these sources. Studying the 3.6μm magnitude versus redshift, I found that many of the radio galaxies in our sample lie on a similar relation as other powerful radio sources studied by different methods, although in my selection there is a bias to the fainter end of the distribution, likely due to the selection bias that I impose. The selection criteria has led us to report on the discovery of the highest redshift radio galaxy ever found, however the robustness of the redshift should be confirmed by follow up spectroscopy as the source has KAB magnitude of ∼ 20.7 which is in an area of the K − z diagram which makes it consistent with both z∼ 1 (in the case of [Oii]) and z ∼ 5 (in the case of Lyα). Finally I determined the number density of high-redshift sources in my sample at different epochs and show that the SKADS simulation is consistent with our observed radio galaxy sample. Using this knowledge I go on to predict the number of sources for current and future near-infrared surveys, where the future appears very bright for this kind of work.

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

In this thesis I investigate the hosts and cluster environments of a sample of 41 radio galaxies between z = 0.4 and z = 0.6. I use spectroscopic data for a 24 object subsample to investigate their star formation histories via the strength of the 4000A break. I find that the higher radio luminosity or high excitation objects in the sample have evidence for young stellar populations, but the lower radio luminosity or low excitation objects do not. My investigations into the Fundamental Plane (FP) of 18 of the radio galaxies, using the same spectroscopic data as well as data from the literature, show that the Fanaroff- Riley type I objects (FRIs) lie on the FP of local radio galaxies once corrected for passive evolution but the Fanaroff-Riley type II objects (FRIIs) do not. I suggest that an evolution in the size of the host galaxies, aided by a combination of passive evolution and a mass-dependent evolution in the mass-to-light ratios, may explain the observed offsets. Finally, I use wide field multi-band imaging to investigate the cluster environments of the full z ~ 0.5 sample. I find that the environmental overdensity is positively correlated with the radio luminosity and observe a greater number of close companions around the FRIIs than the FRIs (albeit with only nine FRIs in the sample). The cluster environments of the radio galaxies with the greatest host luminosities show tentative evidence for an alignment between the major axis of a galaxy and that of its cluster, whilst there are hints that the objects with the highest radio luminosities have clusters whose major axis is aligned with the position angle of the radio jet. My results suggest a picture in which FRII type radio sources reside in particularly rich cluster environments at z ~ 0.5 but FRI type radio sources in less rich environments. The environment plays a key role in determining both the radio properties of the galaxy and the evolution of its host. The effect of the environment on the emission line properties and star formation histories of the galaxies leads to the overlap seen in the morphological and spectral properties of radio galaxies.

In this dissertation, I present my work on the behaviour of different features of radio-loud active galaxies to investigate how energy is transferred from their jets to the environment. To this end, I have studied the knots in the jet in Centaurus A, the radio and X-ray emission from the lobes of the FRII radio galaxies 3C 353 and Pictor A, and the gas properties of a sample of galaxy groups some of which host radio-loud AGN. Using new and archival multi-frequency radio and X-ray data for Centaurus A obtained over almost 20 years with the Very Large Array and with Chandra, I have measured the X-ray and radio spectral indices, flux density variations, polarisation variations and proper motions of the jet knots. I used these measurements to constrain the likely knot formation mechanisms and particle acceleration processes within this jet and compared my results with the variations detected in the properties of the knots in M87. I found that none of the knots are a result of impulsive particle acceleration and that those knots that are detected in both X-ray and radio are likely due to collisions between the jet and an obstacle, while the radio only knots, the majority of which are moving, are likely due to compressions of the fluid flow. Using six frequencies of new and archival radio data and new XMM-Newton observations of 3C 353, I show that inverse-Compton emission is detected in the lobes of this source. By combining this X-ray emission with the radio synchrotron emission, I have constrained the electron population and the magnetic field energy density in the lobes of this radio galaxy. I have argued that the variations in the X-ray/radio ratio in the brighter lobe requires a changing magnetic field strength. I have extended this work using a statistical analysis of the X-ray and radio emission to show that the observed small-scale variation in surface brightness cannot be reproduced by simple combinations of the electron energy distribution and the magnetic field strength. I therefore suggest that the changes in surface brightness that give rise to the filamentary structures seen in the lobes are probably due to strong spatial variations of the magnetic field strength. Finally, I present a study of galaxy groups observed with XMM-Newton in which I measure temperature and surface brightness profiles to examine whether radio-source heating makes radioloud groups hotter and more luminous than radio-quiet groups. I compare my measurements with previous luminosity-temperature relationships and conclude that there is a significant difference in the gas properties of radio-loud and radio-quiet groups.

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.

In this thesis, I explore the dynamics of radio galaxies in galaxy clusters by studying the statistical influence of the intracluster medium on the morphology of radio galaxies, and the evolution of the electron populations within different galaxies and radio relics in Abell 2255. In Chapter 2, I study the impact of cluster environment on the morphology of a sample of 4304 extended radio galaxies from Radio Galaxy Zoo. A total of 87% of the sample lies within a projected 15 Mpc of an optically identified cluster. Brightest cluster galaxies (BCGs) are more likely than other cluster members to be radio sources, and are also moderately bent. The surface density as a function of separation from cluster center of non-BCG radio galaxies follows a power law with index -1.10+-0.03 out to 10 r_500 (~7 Mpc), which is steeper than the corresponding distribution for optically selected galaxies. Non-BCG radio galaxies are statistically more bent the closer they are to the cluster center. Within the inner 1.5 r_500 (~1 Mpc) of a cluster, non-BCG radio galaxies are statistically more bent in high-mass clusters than in low-mass clusters. Together, I find that non-BCG sources are statistically more bent in environments that exert greater ram pressure. I use the orientation of bent radio galaxies as an indicator of galaxy orbits and find that they are preferentially in radial orbits. Away from clusters, there is a large population of bent radio galaxies, limiting their use as cluster locators; however, they are still located within statistically overdense regions. I investigate the asymmetry in the tail length of sources that have their tails aligned along the radius vector from the cluster center, and find that the length of the inward-pointing tail is weakly suppressed for sources close to the center of the cluster. In Chapter 3, I present a deep and high resolution P-band (370 MHz) VLA observation of the central radio galaxies and relics in Abell 2255. Using L-band (1.5 GHz) data analyzed in Lame'e (2017), I generate maps of spectral index, alpha_PL, calculated between the two frequencies. I detect two new extended sources of very steep spectrum (alpha_PL < -2) radio emission near the center of the cluster. I analyze the spectral curvature of the Original Tailed Radio Galaxy (TRG), the Goldfish galaxy, and the NE radio relic. The spectrum of the TRG and Goldfish both exhibit curvature intermediate between a power law spectrum and the exponential cut-off spectrum of the Jaffe-Perola model (Jaffe & Perola, 1973). There are two thin filaments at the end of the TRG; filament F1 only has a reliable measurement of the spectrum at one position, and is consistent with having the same spectral shape as the tail itself; the spectrum of filament F2 has less curvature than the tail, and is consistent with a power law. This suggests that F2 has been reaccelerated by a shock in the ICM, which would also explain its morphology, although that does not work for F1. The southern component of the NE relic has values of alpha_PL ranging between -0.4 and -1.4, consistent with previous observations across similar frequencies. Systematic problems with the in-band spectral index at 1.5 GHz make quantitative claims about the spectral curvature difficult, but the extremely steep values of alpha_LL are not consistent with a shock reaccelerated power law spectrum.

We present the results of three-dimensional magnetohydrodynamic (MHD) simulations of radio galaxy (RG) outflows interacting with active galaxy cluster environments. The goal of these simulations was to analyze the consequences of these interactions such that observations of cluster radio emission can provide a more effective proxy for understanding the dynamical states of intracluster media (ICM). We conducted the first high-resolution numerical study dedicated to the evolution of a narrow-angle tail (NAT) RG utilizing 3D MHD and energy-dependent transport of cosmic ray electrons (CRe). We verify existing theoretical models of the jet bending process and, in an appendix, extend them into an empirical formalism that allows us to model the jet trajectories that develop from arbitrary jet-wind orientations. We observe that the early development of our NAT consists of an extended formation period, during which the evolving RG goes through a transient phase where it is more reminiscent of a more gently bent wide-angle tail RG. Once the jets are fully bent, we note the jet trajectories do not remain static, but episodically "flap" in response to instabilities that develop in the surrounding ICM wind. This occasional disruption of the jets enhances magnetic fields via dynamical stretching, and is critical to the state of the plasma in the tails that develops. Synthetic observations show that once plasma released early in the NAT's evolution has faded, the observed morphology of the NAT is very nearly steady-state with a roughly constant brightness distribution and a steady, self-similar, curved integrated spectrum. We then used the evolved NAT we created to run multiple simulations to explore the interactions between the tail plasma and shock fronts characteristic of those generated by a major cluster merger. Our primary motivation in conducting these simulations is to explore the possibility that CRe populations recently released from a NAT could be seed relativistic electrons necessary to explain observations of radio relics. We analyze the character the shock front develops within our simulated tails with a theoretical treatment based on a Riemann problem analysis. We note the development of vortical motions induced by the shock passage through the low-density plasma of the tails, and the resulting magnetic field amplification over and above amplification from shock compression. We examine synthetic observations of our shocked tails and compare to common properties of emission from radio relics.

I present the interferometry visibility catalogue for > 94 % of sources in the Australia Telescope 20 GHz (AT20G) survey covering all de- clinations of the Southern hemisphere. The catalogue was obtained utilizing the additional dataset from the sixth antenna (with long ∼ 4500 m baselines) of the Australia Telescope Compact Array (ATCA) in the main AT20G survey follow-up observations at 18.8 GHz. It provides angular size information at subarcsec (∼ 0.1 arcsec) resolution. Applications of the visibility catalogue in different fields - study of radio spectra of AGNs and AGN powered radio sources; the effect of spectral curvature at high radio frequencies on redshift cut-off of compact AGNs and search for gravitational lenses in the southern hemisphere - are presented in detail. The visibility amplitudes were calculated using a non-standard method using scalar averaging of the visibility amplitudes. The missing visibilities, due to observational logistics, do not introduce bias and, hence, the catalogue has the same completeness and reliability level corresponding to the AT20G survey. Most sources have visibilities measured at two widely separated hour angles and are presented as independent numbers, providing limited morphological information about the sources. The visibility corresponding to 0.14 arcsec was used to separate the sources into compact and extended extragalactic sources. 77% of the AT20G sources are found to be compact AGNs. Cross matches with lower frequency radio surveys the 1.4 GHz NRAO VLA Sky Survey (NVSS) and the 0.8 GHz Sydney University Molonglo Sky Survey (SUMSS) combined with the AT20G observations at 5 and 8 GHz are used to study the spectra of the sources from lower frequencies to 20 GHz. The limitations of spectral index at higher radio frequencies as a diagnostic for separating compact and extended sources is discussed. A visibility-spectra diagnostic plot that is capable of cleanly separating different classes of extragalactic sources is introduced. Discussion of properties of sources from cross matches with optical, ROSAT X- ray and Fermi gamma-ray catalogues is presented. Galactic free-free thermal planetary nebulae that were easily detected in this visibility-spectra plot are listed in the appendix. A physical basis for the use of spectral index to select compact and extended extragalactic sources at lower frequencies is provided, with recommendations for the use of ν−0.5 as the limit in future studies to maintain consistency. A moustache plot for compact AGNs showing spectral curvature at higher rest frame frequencies is presented. Discussions supporting the validity of studies concentrating on redshift cut-off of flat-spectrum compact AGNs at lower frequencies is presented. The results showing redshift cut-off is affected by spectral steepening at higher observed frequencies is also presented. A search for gravitational lensing in the southern hemisphere using the visibility data and follow-up imaging observations at 7 mm and with the Gemini South 8 m optical telescope are presented. The motivation, the strategy pushing the boundaries of the available instrument, the data reduction steps and the results are presented. Awaiting further data processing, one source is identified as most likely case of gravitational lens system and three are listed as candidates, result consistent with the successful northern hemisphere Cosmic Lens All Sky Survey (CLASS) results.

Though active galaxies have been studied since the inception of radio astronomy, there remain several large gaps in our understanding. Chief among these, is how the jets possessed by the radio loud variety are ignited. A sub-population of radio galaxies known as compact symmetric objects (CSOs) seem to possess newly formed radio jets (100s to 1000s years old). Selecting these sources from the VLBA Imaging and Polarimetry Survey (VIPS), a survey of over 1000 compact radio sources, yields the opportunity to establish an understanding of these sources and what might have caused their recent radio activity. Starting with the VIPS survey we identified 120 CSO candidates, and then performed multi-frequency followup observations to either confirm or deny them as CSOs. We have confirmed 24 CSOs within VIPS and 33 candidates remain. Two of the newly identified CSOs exhibit linearly polarized emission, a rare property within the class. The statistics of the VIPS CSO sample are also investigated to shed light on these nascent radio galaxies. Detailed analysis was performed on two particular sources, the newly classified CSO J11584+2450 and complicated core-jet J16021+3326, in identifying these atypical sources. The observations of CSOs are interpreted within the AGN unification paradigm. The observations of HI absorption toward their cores and polarization of CSOs both tentatively support unification.

The question as to whether the distribution of radio loudness in active galactic nuclei (AGN) is actually bimodal has been discussed extensively in the literature. Furthermore, there have been claims that radio loudness depends on black hole mass and Eddington ratio. We investigate these claims using the low redshift broad line AGN sample of Greene & Ho 2007, which consists of 8434 objects at z < 0.35 from the Sloan Digital Sky Survey (SDSS) Fourth Data Release. We obtained radio fluxes from the Very Large Array Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey for the SDSS AGN. Out of the 8434 SDSS AGN, 846 have radio emission within 4" of the optical counterpart and are considered to be core emission. We also perform a systematic search for extended emission in FIRST that can be positively associated with the optical counterparts and find 51 out of the 846 previously detected core sources have extended emission that must be taken into account when calculating the total radio luminosity. Further, we find an additional 12 objects that have extended radio emission but no detectable radio core and have classic FR II type morphologies. Using these data, the question of radio bimodality and the dependence of radio-loudness on physical parameters are investigated for different subsets of the total sample. We find modest trends in the radio-loud fraction as a function of black hole mass and Eddington ratio, where the fraction of RL AGN increases for the largest black hole mass group and decreases with increasing Eddington ratio. With extended emission taken into account, we find strong evidence for a bimodal distribution in radio-loudness, where the lower radio luminosity core-only sources appear as a population separate from the extended sources with a dividing line at log(R) = 1.75. This dividing line is interesting in that it requires the radio luminosity to be 50 times the optical luminosity, ensuring that these are indeed the most RL AGN, which may have different or extreme physical conditions in their central engines when compared to the more numerous radio quiet AGN in this sample.

We present an analysis of radio data for 24 radio galaxies observed with the Very Large Array (VLA) in at least 3 different frequencies. These systems possess prominent X-ray surface brightness depressions associated with cavities or bubbles that were created by interactions between powerful radio sources and the surrounding hot gas. We use the X-ray cavities to measure the mechanical jet power of radio sources over six decades of radio luminosity, independently of the radio properties themselves, which we measure from the VLA data. Using these measurements, we examine the ratio between radio power and total jet power (the radiative efficiency). We find that the jet power increases with increasing radio power and that the radio sources in our sample are generally poor radiators, with radiative efficiencies ranging from 1 (Cygnus A) to 0.0001 (HCG 62). We argue that radio aging may be partly responsible for this wide range in the radiative efficiencies, in combination with intrinsic differences in the magnetic field strengths and the particle content (k, the ratio between the heavy-particle energy and electron energy). We place limits on the magnetic field strengths and on the particle content of the radio lobes. We find that the magnetic field strengths derived under different assumptions (equipartition, pressure equilibrium, and by requiring the synchrotron age to be equal with the buoyancy age) are larger than the cluster wide magnetic field strengths, with the ones derived under pressure equilibrium being much larger than the others. In order for pressure-equilibrium magnetic field strengths to be equal with the equipartition magnetic field, the particle content (k) needs to vary from ~ 1 to 35000. Such high values of k support the idea of heavy jets. By comparing the synchrotron ages with the X-ray-derived ages, we find that the two ages are uncoupled, with the youngest systems, which may be in a driving stage, being better estimated by the synchrotron age. Having radio images in at least 3 frequencies, we are better able to classify a cavity as active, intermediate, or ghost based on the break frequency. However, because of the continuous range in break frequencies that we find, the separation of our sample into different categories is somewhat arbitrary. Lastly, we find that 327 MHz radio maps are typically better suited for tracing the cavity sizes than 1.4 GHz radio maps, and they are a promising proxy for X-ray cavities in systems where it is difficult to image the cavities directly in X-rays.

During the quasar era (redshifts between 1 and 3) Radio Galaxies (RGs) have been claimed to have substantially influenced the growth and evolution of large scale structures in the universe. In this dissertation I test the robustness of these exciting claims. In order to probe the impacts in more detail, good theoretical models for such RG systems are required. With this motivation, I seek to develop an essentially analytical model for the evolution of Fanaroff-Riley Class II radio galaxies both as they age individually and as their numbers vary with cosmological epoch. To do so, I first compare three sophisticated semi-analytical models for the dynamical and radio lobe power evolution of FR II galaxies, those given by Kaiser, Dennett-Thorpe & Alexander (1997, KDA), Blundell, Rawlings, & Willott (1999, BRW) and Manolakou & Kirk (2002, MK). I perform multi-dimensional Monte Carlo simulations leading to virtual radio surveys. The predictions of each model for redshift, radio power (at 151 MHz), linear size and spectral index are then compared with data. The observational samples are the low frequency radio surveys, 3CRR, 6CE and 7CRS, which are flux-limited and redshift complete. I next perform extensive statistical tests to compare the distributions of model radio source parameters and those of the observational samples. The statistics used are the 1-Dimensional and 2-Dimensional Kolmogorov-Smirnov (K-S) tests and the 4-variable Spearman partial rank correlation coefficient. I search for and describe the "best" parameters for each model. I then produced modifications to each of the three original models, and extensively compare the original and the modified model performances in fitting the data. The key result of my dissertation is that using the Radio Luminosity Function of Willott et al. (2001) as the redshift birth function of radio sources, the KDA and MK models perform better than the BRW models in fitting the 3CRR, 6CE and 7CRS survey data when using K-S based statistical tests, and the KDA model provides the best fits to the correlation coefficients. However, no pre-existing or modified model can provide adequate fits for the spectral indices. I also calculate the volume fraction of the relevant universe filled by the generations of radio galaxies over the quasar era. This volume filling factor is not as large as estimated earlier. Nonetheless, the allowed ranges of various model parameters produce a rather wide range of astrophysically interesting relevant volume fraction values. I conclude that the expanding RGs born during the quasar era may still play significant roles in the cosmological history of the universe.