In this thesis I re-analyse the SCUBA archive data for the L1688 main cloud of Ophiuchus, incorporating all available scan-map and jiggle-map data. I create a new core mass function (CMF) for L1688 using updated values for the distance to this region, as well as new estimates for the temperatures and masses of the cores. I show that the CMF for LI688 is consistent with a three part power-law, with slopes the same as those seen in the stellar IMF. The deeper maps allows the discovery of a turnover in the CMF at 0.7A/, which shows that the core mass function appears to mimic the stellar initial mass function. This concordance is indicative that the stellar IMF is determined at the prestellar core phase. I also present HCO* (J=4 > 3) spectral line observations from HARP on the JCMT. Data are presented for 59 of the prestellar cores mapped using SCUBA. Using these data. I present a proposed evolutionary diagram for prestellar cores in the form of a radius-mass plot. I hypothesise that a core is formed in the low-mass, low-radius region of the plot. It then accretes quasi-statically, increasing in both mass and radius. When it crosses the limit of gravitational instability it begins to collapse, decreasing in radius, towards the region of the diagram where protostellar cores are seen. My predictions are borne out when I plot the collapsing cores on this diagram. I outline an analytical model, created by Whitworth & Ward-Thompson (2001), describing the collapse of a starless core with a Plummer-like density profile. I describe my addition of a simple radiative transfer code, which allows simulated spectral line profiles to be created for such cores. The model is shown to be consistent with previous models, and with the observed physical properties of prestellar cores. This model is applied to 20 of the spectral line profiles from the HCO+ (J=4 + 3) data. These 20 modelled cores are placed onto the proposed evolutionary diagram. Their modelled physical states are found to be consistent with the proposed evolutionary track. In conjunction with the SCUBA and HARP data, these fits allow a potential timeline for the LI688 cloud to be established for the first time.

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This thesis presents new results from the SCUBA Local Universe Galaxy Survey (SLUGS) the optically-selected sample. SLUGS is the first large systematic submillimetre survey of the local Universe, and was carried out with the SCUBA camera on the James Clerk Maxwell Telescope. Since SCUBA is sensitive to the 90% of dust too cold to radiate significantly in the IRAS bands this new sample, taken from the Center for Astrophysics (CfA) optical redshift survey (Huchra et al. 1983) and hence selected on the basis of optical emission alone, represents the first unbiased survey of dust in galaxies along the whole length of the Hubble sequence. There is little change found in the properties of dust in galaxies along the Hubble sequence, except a marginally significant trend for early-type galaxies to be less luminous submillimetre sources than late-types. Nevertheless, 6 out of 11 elliptical galaxies were detected, although some of the emission may possibly be synchrotron rather than dust emission. The IRAS and submillimetre fluxes are found to be well-fitted by a two-component dust model with dust emissivity index fi=2. The ratio of the mass of cold dust to the mass of warm dust is found to be much higher for the optically-selected galaxies than found for the previous IRAS-selected SLUGS sample (Dunne et al. 2000 Dunne & Eales 2001), and can reach values of 1000. Comparison of the results for the IRAS- and optically-selected samples shows that there is a population of galaxies containing a large proportion of cold dust that is unrepresented in the IRAS sample. Local submillimetre luminosity and dust mass functions are derived, both directly from the optically-selected SLUGS sample, and by extrapolation from the IRAS PSCz survey using the method of Serjeant & Harrison (2005) (by extrapolating the spectral energy distributions of the IRAS PSCz survey galaxies out to 850 /zm we probe a wider range of luminosities than probed directly by the SLUGS samples), and excellent agreement is found between the two. They are found to be well-fitted by Schechter functions except at the highest luminosities. As a consequence of the omission of cold galaxies from the IRAS sample the previous //i45-selected luminosity function is found to be too low by a factor of 2, reducing the amount of cosmic evolution required between the low-z and high-z universe. Finally, the results of the OS SLUGS are used to assess the reliability of the Carilli & Yun (1999, 2000a) radio-submillimetre redshift estimator technique, for the first time using a sample of 'normal' local galaxies.

In this work a large collection of VLBI (Very Long Baseline Interferometry) maps are presented based upon data from the VLBA (Very Long Baseline Array) for 29 sources. The maps are the result of three epochs of imaging. A kinematic study was undertaken with a view to understanding some interesting structure seen in the quasars 3C273 and 3C345. Fortunately, these two sources present a large number of components and it was possible to perform model fitting to pinpoint their positions. In addition, model fitting was performed upon all of the sources that were successfully imaged. The aim of this was to eventually derive component velocities for as many blazars studied as possible. The outcomes of the work are presented in tables in the main thesis. In order to further understand the 3C273 and 3C345 jet structures seen a physical model was developed with A. Papageorgiou. With this kinematic model it was possible to trace out a variety of helical jet structures. The free parameters are the injection velocity for new components, the period of jet precession, the viewing angle and the jet half angle. Using the Levenberg- Marquardt algorithm (references given below) it was possible to fit jet trajectories to my model fitted component data. The algorithm produced quantitative output for some of the free parameters in my physical model. Therefore I report average jet half angles for 3C273 and 3C345 of 2.968 0.153 degrees and 2.519 0.573 degrees respectively. In ad dition, I find the precessional periods of 3C273 and 3C345 to be 71.161 19.066 years and 48.478 3.385 years respectively.

The thesis presented here describes the design and construction of a 200-um photometer, THUMPER, a seven-element array of stressed Ge:Ga photoconductors. The photometer has been designed to observe at the James Clerk Maxwell Telescope (JCMT) in conjunction with SCUBA, with an angular resolution of 14". A model for the expected atmospheric transmission at high-altitude, ground-based sites is developed. The model is validated through comparison with site-testing measurements of the atmosphere near to the 200-um window. It has been found that THUMPER will be able to undertake useful observations with levels of precipitable water vapour less than around 0.5 mm, occurring under the most favourable of weather conditions. The opportunities for THUMPER provided by the sites at Mauna Kea, Atacama and Antarctica are reviewed. Details concerning the manufacture and testing of stressed Ge:Ga photoconductors are presented. This includes work carried out on prototypes, followed by a description of the detectors used in the THUMPER arrays. The THUMPER detectors were tested individually and found to have values of detective quantum efficiency ranging from 6 to 12%. The basic design parameters for THUMPER are presented along with considerations for the commissioning of our instrument. In addition to plans for the integration of THUMPER at the JCMT, flux calculations have been presented indicating both Mars and Jupiter will provide sufficient fluxes for initial calibration.

The High Energy Stereoscopic System (H.E.S.S.) consists of an array of four ground-based imaging atmospheric Cherenkov telescopes. The aim of the system is to detect Cherenkov radiation emitted from extensive air showers caused by very high energy gamma-ray radiation interacting with the atmosphere. The implementation of more than one telescope allows multiple views of a single extensive air shower, providing a more accurate reconstruction of the event and offering greater cosmic ray background suppression, allowing better analysis of gamma rays. Unlike charged particles, gamma rays reach the Earth in the direction in which they were emitted thus allowing us to investigate the emission mechanisms and properties of various gamma-ray sources. Since Cherenkov radiation is produced and propagates through the atmosphere, the atmosphere plays a large role in the detection of gamma rays and must be monitored carefully. This thesis will explore the effects of the atmosphere on the H.E.S.S. telescopes and gamma-ray analysis and is structured as follows: The first chapter highlights all the important aspects of very high energy (VHE) gamma-ray astronomy focusing on the H.E.S.S. system. Chapter 2 gives a general overview of active galactic nuclei (AGN) and the extragalactic background light (EBL) as a precursor to data analysis in Chapter 4. Chapter 3 discusses the atmosphere and atmospheric monitoring performed at the H.E.S.S. site. I present data analysis of the four atmospheric monitoring instruments revealing seasonal trends and cross-correlations between instruments. Chapter 4 contains the heart of the thesis, where all aspects from the previous chapters are integrated to show the effects of the atmosphere on the H.E.ร.ร. system and subsequent AGN and EBL data analysis. I first compare trigger rate data from the telescopes and data from the atmospheric monitoring instruments. It is shown that backscatter, sky radiance, and transmissivity are correlated to the trigger rate to various degrees. It is then shown that gamma-ray data is less affected by varying atmospheric conditions, especially for the higher energy gamma rays. Active atmospheric calibration is then presented as a way to correct gamma-ray data when there is a large amount of low-lying aerosols in the atmosphere. This method is subsequently used to correct the gamma-ray fluxes for 2 AGN, PKS 2155-304 and H 2356-309. It is shown that the correction works best for the higher flux source PKS 2155-304 rather than H 2356-309 whose flux is often consistent with zero. These AGN are then used to calculate upper limits to the EBL using a simple monoenergetic model. The upper limits to the EBL in the wavelength range of 0.4-1 μm are shown to be consistent with the results from the most exhaustive study of the EBL testing over 8 million EBL shapes. Chapter 5 is the concluding chapter discussing future work to be done at H.E.S.S. site, with the addition of a new LIDAR and VHE telescope, and beyond.

Currently in the design stage, the Cherenkov Telescope Array (CTA) is an advanced facility for ground-based high-energy gamma-ray astronomy. This research presents the projected system performance of CTA calculated as part of a large Monte Carlo simulation effort including air shower and telescope response simulations. The integral sensitivity of CTA’s baseline Subarray-E is found to be nearly an order of magnitude more sensitive between 100 GeV and 10 TeV compared to existing ground-based Cherenkov telescope systems. This research finds that this particular subarray achieves CTA’s goal of milli-Crab sensitivity at 1 TeV. In addition, this work uses a multi-layer perceptron neural network to separate the cosmic-ray background from the gamma-ray signal. This includes employing 5 different methods for estimating the neural network response cut, with the energy-scaled signifi- cance method being found to provide the best and most stable performance. Performance measures calculated include: energy resolution, angular resolution, effective collecting area as well as flux sensitivity. In addition, the results of three small studies are also presented. The first includes the performance results of a high-altitude (3700 m) subarray. This re- search confirms a gain in sensitivity at the lowest detectable energies under ∼100 GeV. The second and third are small technical studies on CTA’s dynamic range performance. Finally, to assess CTA’s prospects for detecting pulsed gamma-ray emission data for the Crab and Vela pulsars taken from Fermi Gamma-Ray Space Telescope observations are analysed and compared with the CTA sensitivity performance derived in this research. Results for CTA Subarray-E and Subarray-B suggest these arrays will both have sufficient sensitivity perfor- mance for detecting the Vela pulsar if it behaves as expected on the basis of the published spectra.

A study of kaon production in high energy nucleon-air nucleus collisions in the atmosphere; has been made based on the muon momentum spectrum at sea level. Starting from the measured vertical momentum spectrum the spectra at zenith angles between 75º and 90º have been predicted for various admixtures: of kaons and pions, taking into account all decay modes: of both charged and neutral kaons. The predicted values are compared with the intensities measured using the Durham horizontal muon spectrographs Although the method of computation allows the spectra to be predicted to an accuracy of 1% the present statistical errors on both the vertical and horizontal intensities are such that only an upper limit to R, the ratio of intensities at production of kaons to pions of all charge states, can be given; this has a value of about kOfo over a range of mesoa energies from about 70 G-eV to 1000 GeV. In order to extend the determination of the value of R to lower and higher energies respectively, reviews have been made of measurements of muon polarization at sea level and of the intensity of the electro magnetic cascades at various heights in the atmosphere. Combining the three methods an estimate is given of R over a range of meson energies from. 3 GeV tc 5000 GeV. If R is taken to be independent of energy a value of 20% would fit the data best over the whole energy range, but the error on this value approaches ± 20%.Comparing R with Ri, the observed ratio of numbers of non-pions to pions produced in individual interactions, it is concluded that, provided that the majority of non-pions are kaons, there is no evidence for the effective mean energies of pions and kaons from individual nuclear interactions being different in the range of primary energies from 10 GeV to 10(^5) GeV. Using the measured vertical spectrum of muons the energy spectra and angular distribution of neutrinos at sea level are calculated. The information is used in the interpretation of the preliminary results from, the Tata Institute-Osaka-Durham joint neutrino experiment in progress at the Kolar Gold Field Comparison of observed rates of events with rates predicted from extrapolation to higher energies of machine results on neutrino-nucleus collisions indicates that the cross-section for inelastic interactions continues to rise above 10 GeV Support for this conclusion is provided by the first results of the Case Institute-Witwatersrand University neutrino experiment. The possibility is discussed of detecting the hypothetical Intermediate Boson of weak interactions.

The Durham 'horizontal spectrograph' Mark II has been used to measure the momentum spectrum and the charge ratio of Cosmic-ray muons in the zenith angle range 82.5 - 90 to about 2000 GeV and the results are based on a sample of 3918 particles. By comparing the measured spectra with those predicted from the measured vertical spectrum, for different values of the K/π ratio, a value of 0.42 ± 0.20 has been derived for the K/π ratio in the muon energy range at production, 100 - 2000 GeV, corresponding to3 5primary energy range of ~3.10(^3) – 10(^5) GeV. When comparison is made with the k/π ratios determined by other indirect methods it is concluded that there is no evidence against a near-constant ratio over the wide primary energy range 10(^2) – 10(^6) covered by all methods and that the average value is 0.35 ± 0.20 over this energy range. The present results confirm the results of other authors on the existence of an appreciable charge excess up to energies of at least a few hundred GeV. The present results suggest that at the largest zenith angles there is a maximum in the charge ratio in the energy region 50 - 100 GeV, a feature which does not confirm the minimum in the same energy region, but at somewhat smaller zenith angles, reported by some other workers. The reason for the appearance of the maximum is not known despite strenuous efforts to explain it. At higher energies, one cannot say, sue to the considerable errors in all the data, that there is disagreement between the present result and other data and neither an increase or decrease in the ration is ruled out. The interaction of the incident particles (hitherto assumed to be muons) has been studied from the point of view of identification of the particles. It is shown that the results can be interpreted in terms of the known electromagnetic interactions of muons and the conclusion arrived at is that there is no evidence for the existence of an appreciable flux of the so-called X – particles that could be contaminating the incident muon beam.

Ultraluminous X-ray Sources (ULXs) are point like X-ray sources situated external to the nucleus of their host galaxy, with inferred X-ray luminosities in excess of 10^{39} erg s^{−1} . Although first observed ∼ 30 years ago, these sources are yet to be fully understood. Some have suggested that these fascinating objects may contain intermediate mass black holes, whilst others have proposed they are stellar mass black holes residing in a new extreme accretion state. This thesis works towards the conclusion of this debate, by developing our understanding of these systems and their environments. This work begins with a photometric survey of the optical counterparts of ULXs. The main aim of this survey is to find plausible candidates to gain radial velocity measurements and therefore mass functions of these systems. However, the collation of this sample also provides the opportunity to classify the stellar objects held within these systems. From this work, we find seven good candidates for optical spectroscopic follow-up. Our results also show that many of our sample are consistent with OB type stars, while some contain later type bright giants/supergiants. Possibly our best chance to gain precise measurements of M_{BH}, and settling the debate over the nature of these systems, is by using radial velocity curves of their optical counterparts to calculate a mass function of ULXs. We are currently undertaking a programme to pursue mass function measurements for these systems. To date, we have received the pilot spectra of three optical counterparts. We discuss the progress of this programme to date and perform analysis on both the absorption/emission features and the continuum of these spectra. Initial analysis reveals the presence of the He ii 4686 A line in two of our pilot spectra. This line may be associated with the accretion disc of these systems, and could therefore be used in our pursuit of the mass function. We also find the presence of both low and high ionization lines, with some evidence for shock ionisation, and electron temperature of 7,000 – 10,000 K. This Balmer decrement also indicates that the extinction can be highly variable across ULX field. This combination may suggest a ‘patchy’ environment with separate shock and photoionisation emission regions. While the continuum emission of one of our sample can be explained by either the spectra of an OB star or of a standard accretion disc, the steep slopes of two of our sample indicates non-stellar origins that could represent the optical spectrum of a super-Eddington accretion disc. Finally, this work highlights the need for further observations of these sources in order to unlock their nature. We present results of X-ray spectral variability studies of the ULX population contained within NGC 4485 & 4490. We collate Chandra and XMM-Newton observations of the interacting galaxy pair, to analyse the emission from the six ULXs previously identified, and one additional source observed in the a recent exposure. This provides us with an opportunity to study variability on both short and longer time scales. The spectral variability is generally characterised by a hardening of the source spectra as their luminosities increase. The sources show a variety of long-term light curves; however, short-term (intra-observational) temporal variability is conspicuous by its absence. This survey also reveals the detection of a possible change in accretion state that could be used to gain crude mass estimates of the compact objects. Finally, we explore further the variability of these systems with the aid of two new proprietary observations. Finally, analysis of some of the best quality X-ray spectral data publicly available on these sources has provided the opportunity to explore the nature of these systems. We apply phenomenological models to characterise the spectra of these objects and more physically motivated models in order to explore the physical processes underlying these characteristics. Results show that the spectra of these sources are fundamentally different to that of Galactic X-ray binaries, whilst the application of physical models indicates a more extreme version of the highest known luminosity state, the very high state. We therefore speculate that in observing ULXs we are observing stellar mass black holes residing in a new ‘ultraluminous’ state.