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

Weak gravitational lensing is a powerful astronomical tool for constraining cosmological parameters that is entering its prime. Lensing occurs because gravitational fields deflect light rays and measuring this deflection through a statistic known as cosmic shear allows us to directly measure the properties of dark matter and dark energy on large scales. In principle, gravitational lensing is a clean probe of the cosmology of the Universe, as it depends on gravity alone and not on incomplete astrophysical models or approximations. In practice, however, there are several factors that limit the accuracy and precision of lensing measurements. These include accurate measurement of galaxy shapes, correctly accounting for distortions to galaxy images due to the point spread function of the telescope, the presence of intrinsic alignments (IAs) of galaxy shapes due to physical processes, and inaccuracies in commonly-used galaxy photometric redshift information. These effects may all introduce systematic errors in lensing measurements which must be carefully accounted for to ensure that cosmological constraints from lensing are unbiased and as precise as possible. The Canada-France-Hawaii-Telescope Lensing Survey (CFHTLenS) is the largest weak lensing survey completed to date, covering 154 square degrees of the sky in 5 optical bands, with photometric redshift information for every survey galaxy. With lensing measurements from more galaxies than ever before, the statistical uncertainties on parameter estimates will be the lowest ever achieved from weak lensing. If left unaccounted for, sources of systematic error would dominate over the statistical uncertainty, potentially biasing parameter estimates catastrophically. A technique known as tomography in which galaxies are sorted into bins based on their redshift can help constrain cosmological parameters more precisely. This is because utilising the redshifts of survey galaxies retains cosmological information that would otherwise be lost, such as the behaviour of dark energy and the growth of structure over time. Tomography, however, increases the demand for systematics-free galaxy catalogues as the technique is strongly sensitive to the IA signal and photometric redshift errors. Therefore, future lensing analyses will require a more sophisticated treatment of these effects to extract maximal information from the lensing signal. A thorough understanding of the error on lensing measurements is necessary in order to produce meaningful cosmological constraints. One of the key features of cosmic shear is that it is highly correlated over di erent angular scales, meaning that error estimates must take into account the covariance of the data over different angular scales, and in the case of tomography, between different redshift bins. The behaviour and size of the (inverse) covariance matrix is one of the limiting factors in such a cosmological likelihood analysis, so constructing an accurate, unbiased estimate of the covariance matrix inverse is essential to cosmic shear analysis. This thesis presents work to optimise tomographic weak lensing analysis and achieve the tightest parameter constraints possible for a CFHTLenS-like survey. N-body simulations and Gaussian shear fields incorporating an IA model (known as the `non-linear alignment' model) with a free parameter are used to estimate fully tomographic covariance matrices of cosmic shear for CFHTLenS. We simultaneously incorporate for the first time the error contribution expected from the non-linear alignment model for IAs and realistic photometric redshift uncertainties as measured from the CFHTLenS. We find that non-Gaussian simulations that incorporate nonlinearity on small scales are needed to ensure the covariance is not underestimated, and that the covariance matrix is shot-noise dominated for almost all tomographic correlations. The number of realisations of the simulations used to estimate the covariance places a hard limit on the maximum number of tomographic bins that one can use in an analysis. Given the available number of lines of sight generated from CFHTLenS-like simulations, we find that up to ~ 15 tomographic bins may be utilised in a likelihood analysis. The estimated tomographic covariance matrices are used in a least-squares likelihood analysis in order to find the combination of both angular and tomographic bins that gives the tightest constraints on some key cosmological parameters. We find that the optimum binning is somewhat degenerate, with around 6 tomographic and 8 angular bins being optimal, and limited by the available number of realisations of the simulations used to estimate the covariance. We also investigate the bias on best- t parameter estimates that occurs if IAs or photometric redshift errors are neglected. With our choice of IA model, the effect of neglecting IAs on the best- t cosmological parameters is not significant for a CFHTLenS-like survey, although this may not be true if the IA signal differs substantially from the model, or for future wide-field surveys with much smaller statistical uncertainties. Similarly, neglecting photometric redshift errors does not result in significant bias, although we apply similar caveats. Finally, we apply the results of this optimisation to the CFHTLenS cosmic shear data, performing a preliminary analysis of the shear correlation function to produce both 2D and optimal tomographic cosmological constraints. From 6-bin tomography, we constrain the matter density parameter Ωm = 0:419+0:123-0:090, the amplitude of the matter power spectrum σ8 = 0:623+0:101 -0:084 and the amplitude parameter of the non-linear alignment model, A = -1:161+1:163 -0:597. We perform this analysis to test the validity and limitations of the optimal binning on real data and find that 6-bin tomography improves parameter constraints considerably, albeit not as much as when performed on simulated data. This analysis represents an important step in the development of techniques to optimise the recovery of lensing information and hence cosmological constraints, while simultaneously accounting for potential sources of bias in shear analysis.

This thesis describes an investigation into weak gravitational lensing, a unique and powerful astronomical tool for the study of dark matter on large scales. Lensing distorts background images, inducing correlations in the observed ellipticities of galaxies, and these correlations can be used to estimate many characteristics of the Universe. Key to all weak lensing studies is a reliable and unbiased method to detect weak lensing distortions from observed galaxy images that are contaminated by Earth and telescope-based shearing and smearing distortions. A new galaxy model-fitting technique is presented that has been developed in order to satisfy this requirement, which will also permit future signal-to-noise optimised measurements of weak lensing shear. Model-fitting provides a good alternative to the standard scite{KSB} method (KSB), and comparisons between the two techniques are drawn from an analysis of deep {it R} band imaging from the COMBO-17 survey, revealing strong evidence for the presence of bias in KSB galaxy shape measurement. With the galaxy model-fitting technique, an investigation into the effectiveness of the Oxford Dartmouth Thirty degree survey (ODT) for gravitational lensing studies is presented, resulting in the detection of weak gravitational lensing by large scale structure, or `cosmic shear', in 0.7 square degrees of the best seeing ODT images. One concern for all cosmic shear studies is that the weak lensing signal, manifest in the weakly correlated ellipticities of distant galaxies, is contaminated by the intrinsic alignment of close galaxy pairs, potentially induced during galaxy formation by physical interactions such as tidal forces. This contamination is investigated theoretically, through numerical simulations, and observationally, with an analysis of the COMBO-17 survey and the study of published results from the Red-sequence Cluster survey and the VIRMOS-DESCART survey, concluding that the intrinsic alignment effect is at the lower end of the range of theoretical predictions. The impact of intrinsic galaxy alignments on cosmological parameter estimation is investigated, with an analysis of the weak lensing results from the COMBO-17 survey. When marginalising over the observationally constrained intrinsic alignment signal, the amplitude of the matter power spectrum sigma_8 is reduced by ~0.03 to sigma_8(Omega_m / 0.27)^{0.6} = 0.71 pm 0.11, where Omega_m is the matter density parameter. With distance information from either spectroscopy or photometric redshifts, the down-weighting of nearby galaxy pairs in weak lensing analysis can be optimised to virtually eliminate the systematic errors in the shear signal arising from intrinsic galaxy alignments, leaving a much smaller, largely statistical error. This method is applied to the photometric redshift sample of the COMBO-17 survey. Weak lensing measurements from the forthcoming SuperNova/Acceleration Probe weak lensing survey (SNAP), and the Canada-France-Hawaii Telescope Legacy survey, are expected to be contaminated on scales >1 arcminute by intrinsic alignments at the level of ~ 1% and ~2% respectively. Division of the SNAP survey for lensing tomography significantly increases the contamination in the lowest redshift bin to ~7% and possibly higher. Removal of the intrinsic alignment effect by the downweighting of nearby galaxy pairs will therefore be vital for the lensing tomography studies of SNAP.

This thesis presents results from four different studies on the diffuse interstellar bands and the local interstellar medium. Firstly, a detailed investigation into the profile of the λ6614 diffuse interstellar band (DIB) is presented, which was aimed at ascertaining the origin of the observed fine structure and overall band shape, and why both of these properties change between a number of different lines-of-sight. A new method for normalising DIBs is described, and revealed new information about how the λ6614 band evolves between lines-of-sight. Observations and modelling of the DIB profile towards HD 147889 suggests that this line-of-sight has the highest internal excitation of the carrier, which accounts for its greater width and additional structure. As a continuation of this project, a further fifteen DIB profiles were examined towards HD 179406 and HD 147889, some of which are thought to correlate with the column density of the C2 molecule. It was shown that HD 179406 and HD 147889 demonstrated almost identical band profiles for the ‘C2’ DIBs, which differed considerably to the ‘non-C2’ DIBs investigated in this study. Analysis presented in this chapter strongly supports the idea that the ‘C2’ DIBs form a distinct class, and the normalisation method used provides a new criterion for validating whether a DIB may be classed as a ‘C2’ DIB. A study of small-scale structure in the ultraviolet (UV) region towards three stars within the ρ Ophiuchus system was undertaken using observations from the Hubble Space Telescope, to deduce column densities of atomic and molecular species, and to obtain physical parameters such as density and kinetic temperature through chemical modelling. The biggest absorption differences were found within the C2 molecule profile, and it was found that the medium in which C2 molecules reside is denser towards ρ Oph A and ρ Oph B than ρ Oph D. Modelling additional species, such as Fe I and Fe II which are also thought to be present in the data, may help to further characterise the ISM towards these three stars. Optical and near-infrared (IR) observations of Herschel 36 were undertaken using the Southern African Large Telescope (SALT) and Gemini North. The aim of this project was to investigate the full DIB spectrum along this unusual sight-line, and determine how many DIBs were present and how many demonstrate the behaviour previously observed. A number of issues arose with the optical observations and the data reduction processes, meaning that the overall aim was not fully realised. However, observations from Gemini North showed more promise but did not detect either near-IR DIB, although higher S/N observations are recommended to verify their absence.

Galaxies in the redshift range 1 < z < 3 existed during the most vigorous period of star formation in the history of the Universe. In the past 15 years, large rest-frame UV spectroscopic samples of z ~ 3 star-forming galaxies have been assembled. However, this particular redshift range, the so-called Redshift Desert, has only begun to be characterized. Most studies involve low resolution, low signal-to-noise spectra because the small angular size (δ ≤ 1") and faintness (RAB = 24 - 25.5) of high redshift galaxies limit what can be accomplished with a reasonable investment of observing time, even using the world's largest optical telescopes. One way to circumvent these two issues is to study gravitationally lensed galaxies. The magnification boost (up to a factor of 30x) and morphological distortion of a high redshift galaxy by an intervening mass concentration allow for the study of the high redshift Universe in unprecedented detail. I present a detailed analysis of the rest-UV spectrum of two gravitationally lensed galaxies: the 'Cosmic Horseshoe' (zsys = 2.38115) and the 'Cosmic Eye' (zsys = 3.07331). The characterization of the stellar populations and the interstellar gas geometry, kinematics, and composition which I achieve is a preview of the type of information that will be available for unlensed high redshift galaxies with the next generation of optical telescopes. I probe the lower redshift end of the Redshift Desert with a study of Fe ii and Mg ii features in the rest-frame near-UV spectrum of 96 star-forming galaxies in the redshift range 1 < z < 2. Stacked spectra are used to explore average outflow and line profile trends with stellar mass and reddening. I also investigate the phenomenon of emission filling of absorption lines which has implications for the line strength and velocity offset of interstellar absorption lines. Individual galaxies are used to assess the range of outflow velocities as well as the prevalence of emission filling in galaxies from this epoch. This is the first large scale study of fine-structure emission from Feii in high redshift galaxies, both in stacked and individual galaxy spectra. An alternative to investigating galaxies by collecting their light is to study them as seen in absorption against a cosmic backlight, such as a quasar. The Sloan Digital Sky Survey, an imaging and spectroscopic survey which covers about one-quarter of the night sky, has collected many thousands of quasar spectra. I search ~ 44,600 of these spectra, up through Data Release 4, for Mg ii λλ2796,2803 absorption doublets. The final catalog includes ~ 16700 Mgii absorption line systems in the redshift range 0.36 ≤ z ≤ 2.28. Measurements of the absorption redshift and rest equivalent widths of the Mg ii doublet as well as select metal lines are available in the catalog. This is the largest publicly available catalog of its kind and its combination of large size and well understood statistics make it ideal for precision studies of the low-ionization and neutral gas regions of galaxies. I conclude this thesis by suggesting several avenues for extending the studies of high redshift star-forming galaxies presented herein.

Observations of galaxy environments have revealed numerous correlations associated with their intrinsic properties. It is therefore clear that if we are to understand the processes by which galaxies form and evolve, we have to consider the role of their immediate environment and how these trends change across cosmic time. In this thesis, I investigate the relationship between the environmental densities of galaxies and their associated properties by developing and implementing a novel approach to measuring galaxy environments on individual galaxy scales with Voronoi tessellations. Using optical spectroscopy and photometry from GAMA and SDSS, with 250μm far-infrared observations from the Herschel-ATLAS SDP and Phase-One fields, the environmental and star formation properties of far-infrared detected and non–far-infrared detected galaxies are compared out to z ∼ 0.5. Applying statistical analyses to colour, magnitude and redshift-matched samples, I show there to be significant differences between the normalised density distributions of the optical and far-infrared selected samples, at the 3.5σ level for the SDP increasing to > 5σ when combined with the Phase-One data. This is such that infrared emission (a tracer of star formation activity) favours underdense regions, in agreement with previous studies that have proposed such a correlation. I then apply my method to synthetic light cones generated from semianalytic models (SAMs), finding that over the whole redshift distribution the same correlations between star-formation rate and environmental density are found. However, as the SAMs restrict the role of ram-pressure stripping, the fact that we find the same qualitative results may preclude ram-pressure as a key mechanism in truncating star formation. I also find significant correlations between isothermal dust temperature and environment, such that the coldest sources reside in the densest regions at the 3.9σ level, indicating that the observed far-infrared emission in these densest regions is the product of ISM heating by the older stellar populations. I then extend my analysis to a deeper sample of galaxies out to z ∼ 2.2, combining near-infrared and optical photometry from the VIDEO and CFHTLS-D1 observations, cross-matched in colour, magnitude and redshift against 1.4 GHz VLA radio observations. Across the entire radio sample, galaxies with radio detected emission are found to reside in more overdense environments at a 4.0σ significance level. I then divide my radio sample to investigate environmental dependence on both radio detected star-forming galaxies and radio detected AGN individually, based upon a luminosity selection defined as L = 1023 W Hz−1. The same trends with environment are shown by my Radio-AGN sample (L > 1023 W Hz−1) which favour overdense regions at the 4.5σ level, suggestive of the interaction processes (i.e. major mergers) that are believed to trigger accretion, in agreement with earlier work that has suggested such a relationship. At lower radio luminosities, my Radio-SF sample (L < 1023 W Hz−1) also display a significant trend towards overdense regions in comparison to my nonradio detected sample, at the less significant level of 2.7σ. This is suggestive of the low overall bolometric luminosity of radio emission in star forming galaxies, leading to only the brightest radio emitting star forming galaxies being observed and a bias towards overdense regions. This is in addition to the fact that the luminosity selection used to separate AGN from star forming galaxies is not a perfect selection and open to AGN contamination in the low-luminosity sample. I conclude that the next generation of deep radio surveys, which are expected to reach many orders of magnitude deeper than current observations, will remove radio-loud AGN contamination and allow for the detection of low-luminosity star forming galaxies via radio emission out to high redshifts. This work has allowed for the environments of galaxies to be probed on smallerscales and across both wider and deeper samples than previous studies. With significant environmental correlations being returned, this indicates that the established processes responsible for such trends must have influence on the most local of scales.

In this thesis, I investigate the spatial distribution of radio sources, and quantify their clustering strength over a range of redshifts, up to z _ 2:2, using various forms of the correlation function measured with data from several multi-wavelength surveys. I present the optical spectra of 30 radio AGN (S1:4 > 100 mJy) in the GAMA/H-ATLAS fields, for which emission line redshifts could be deduced, from observations of 79 target sources with the EFOSC2 spectrograph on the NTT. The mean redshift of these sources is z = 1:2; 12 were identified as quasars (40 per cent), and 6 redshifts (out of 24 targets) were found for AGN hosts to multiple radio components. While obtaining spectra for hosts of these multi-component sources is possible, their lower success rate highlights the difficulty in acheiving a redshift-complete radio sample. Taking an existing spectroscopic redshift survey (GAMA) and radio sources from the FIRST survey (S1:4 > 1 mJy), I then present a cross-matched radio sample with 1,635 spectroscopic redshifts with a median value of z = 0:34. The spatial correlation function of this sample is used to find the redshiftspace (s0) and real-space correlation lengths (r0 _ 8:2 h 1Mpc), and a mass bias of _1.9. Insight into the redshift-dependence of these quantities is gained by using the angular correlation function and Limber inversion to measure the same spatial clustering parameters. Photometric redshifts from SDSS/UKIDSS are incorporated to produce a larger matched radio sample at z ' 0:48 (and low- and high-redshift subsamples at z ' 0:30 and z ' 0:65), while their redshift distribution is subtracted from that taken from the SKADS radio simulations to estimate the redshift distribution of the remaining unmatched sources (z ' 1:55). The observed bias evolution over this redshift range is compared with model predictions based on the SKADS simulations, with good agreement at low redshift. The bias found at high redshift significantly exceeds these predictions, however, suggesting a more massive population of galaxies than expected, either due to the relative proportions of different radio sources, or a greater typical halo mass for the high-redshift sources. Finally, the reliance on a model redshift distribution to reach to higher redshifts is removed, as the angular cross-correlation function is used with deep VLA data (S1:4 > 90 _Jy) and optical/IR data from VIDEO/CFHTLS (Ks < 23:5) over 1 square degree. With high-quality photometric redshifts up to z _ 4, and a high signal-to-noise clustering measurement (due to the _100,000 Ks-selected galaxies), I am able to find the bias of a matched sample of only 766 radio sources (as well as of v vi the VIDEO sources), divided into 4 redshift bins reaching a median bias at z ' 2:15. Again, at high redshift, the measured bias appears to exceed the prediction made from the SKADS simulations. Applying luminosity cuts to the radio sample at L > 1023 WHz 1 and higher (removing any non-AGN sources), I find a bias of 8–10 at z _ 1:5, considerably higher than for the full sample, and consistent with the more numerous FRI AGN having similar mass to the FRIIs (M _ 1014 M_), contrary to the assumptions made in the SKADS simulations. Applying this adjustment to the model bias produces a better fit to the observations for the FIRST radio sources cross-matched with GAMA/SDSS/UKIDSS, as well as for the high-redshift radio sources in VIDEO. Therefore, I have shown that we require a more robust model of the evolution of AGN, and their relation to the underlying dark matter distribution. In particular, understanding these quantities for the abundant FRI population is crucial if we are to use such sources to probe the cosmological model as has been suggested by a number of authors (e.g. Raccanelli et al., 2012; Camera et al., 2012; Ferramacho et al., 2014).

The search for life beyond Earth is the driving force behind several future missions to Mars. An essential task in the lead-up to these missions is a critical assessment of the habitability for, and feasibility of, life. However, little research has been conducted on this issue, and our understanding of the plausibility for life on Mars remains unconstrained. Owing to the anoxic and iron-rich nature of Mars, microbial iron reduction (MIR) represents a compelling candidate metabolism to operate in the Martian subsurface, past and present. The objectives of this thesis are to address the feasibility of MIR on Mars by i) better defining the habitability of MIR on Earth, and ii) assessing the range and availability of organic electron donors in the subsurface of Earth and Mars. Samples collected from Mars-relevant environments on Earth were used to initiate MIR enrichment cultures at 4°C, 15°C and 30°C. Results indicate MIR is widespread in riverbed and subglacial sediments but not sediments from desert or recent volcanic plains. The iron-reducing microorganisms in subglacial enrichments are at least psychrotolerant and in some cases psychrophilc. Culture-independent methods highlighted the changes in diversity between temperature conditions for subglacial sediments, and indicated that members of the prolific MIR Geobacteraceae family are common. The genera Geobacter and Desulfosporosinus are responsible for MIR in the majority of enrichments. Long-term anoxia and the availability of redox constituents are the major factors controlling MIR in these environments. A MIR enrichment culture was unable to use shales and kerogens as the sole source of electron donors for MIR, despite the presence of known electron donors. Furthermore, MIR was inhibited by the presence of certain kerogens. The causes of inhibition are unknown, and are likely to be a combination of chemical and physical factors. Experiments were conducted to assess the ability of three pure strains and a MIR enrichment to use non-proteinogenic amino acids common to carbonaceous meteorites as electron donors for MIR. Results demonstrate that γ-aminobutyric acid served as an electron donor for the enrichment culture, but no other amino acids supported MIR by this or other iron-reducing cultures. The D-form of chiral amino acids was found to exert a strong inhibitory effect, which decreased in line with concentration. Theoretical calculations using published meteoritic accretion rates onto the surface of Mars indicate that the build up inhibitory amino acids may place important constrains on habitability over geologic time scales. Contamination of a pure strain of Geobacter metallireducens with a strain of Clostridium revealed a syntrophic relationship between these microorganisms. Anaerobic heterotrophs are likely to play an important role in maintaining an available supply of electron donors for MIR and similar chemoorganic metabolisms operating in the subsurface. This research indicates that MIR remains a feasible metabolism to operate on Mars providing a readily available redox couple is present. However, given the observed inhibition in the presence of bulk carbonaceous material and certain amino acids found in meteorites, the use of extraterrestrial carbonaceous material in the Martian subsurface for microbial iron reduction is questionable, and should be the focus of future research.

Observations of neutral hydrogen (HI) absorption in the central regions of eight active galaxies are reported. These observations represent the most detailed studies of the neutral hydrogen component so far made of these sources, providing information on the HI gas distribution and kinematics on scales of 0.2 arcsec, corresponding to linear scales of tens of parsecs in the nearest galaxies observed. The sample of active galaxies presented here consists of the radio galaxy 3C293, two Seyfert galaxies NGC7674 and NGC7469, and five luminous infrared galaxies; NGC6240, IIIZw35, UGC2369, IR0335+1523 and Zw049.0. In each of these galaxies MERLIN has been used to provide the highest angular resolution observations of the neutral hydrogen content in these sources made to date. In addition to these high resolution HI studies the newly operational Giant Meterwavelength Radio Telescope has also been used to provide arcsecond resolution observations of the HI gas content in two of the MERLIN sources, 3C293 and NGC7674.Against the central few kiloparsecs of the radio galaxy 3C293 both the neutral hydrogen distribution and the radio continuum emission is imaged on linear scales of 150pc. The distribution and velocity structure of the HI gas against this radio galaxy is attributed to two gas structures. One of these gas structures is indicative of quiescent gas lying 10kpc from the galaxy centre and is physically associated with both ionised gas and dust lanes in the galaxy. The second HI velocity structure is situated within 600pc of the active galactic nucleus (AGN) and is observed to form a circumnuclear disc with a velocity gradient of 179km/s/arcsec. This circumnuclear disc of gas is shown to enclose a mass of M_dyn<600pc)leq3.9*10^9(sin^{-2}i) Msolar, where i represents the inclination of the disc. Extremely localised HI absorption is observed against the unresolved nuclei of the Seyfert galaxies NGC7674 and NGC7469. In the case of the Seyfert-2 galaxy NGC7674, it is shown that the HI is distributed within a 100pc of the AGN. Against the unresolved Seyfert-1 nucleus of NGC7469 the HI absorption is thought to either result from quiescent gas in the outer parts of this galaxy or from gas close to the nucleus. The HI absorption distribution in both of these sources is consistent with the absorption arising in circumnuclear dusty tori. Of the five luminous infrared galaxies observed, HI absorption is detected in four cases. The observations of IIIZw35, IR0335+015 and Zw49.0 all revealed unresolved 1.4GHz radio continuum structure against which HI absorption is detected. However, against the prototypical luminous infrared merger NGC6240 extensive radio continuum is resolved and imaged. It is concluded that the radio continuum structure of NGC6240 is consistent with the radio emission arising from both a starburst region and one or two AGN existing in the remnant nuclei of the merging galaxies. Extremely broad HI absorption is detected against the two dominant radio continuum components. The HI gas structure is consistent with a neutral gas disc forming between the two merging nuclei as they orbit around each other.

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.