There have been a multitude of observational findings supporting a co-evolution of the central BH and its host galaxy. This co-evolution is expected to be driven by a connection between the two growing mechanisms of active galactic nuclei (AGN; accretion onto the central super-massive black hole), and star formation. In an effort to find more direct evidence for a connection between the two mechanisms of AGN and star formation I investigate the star formation rates (SFRs) of galaxies hosting X-ray, optical, and radio AGN. For the analysis of these samples I have developed methods to calculate the mean and individual SFRs of distant AGN and improve on previous work by simultaneously taking into account redshift and mass when interpreting the results. I use infrared photometry from WISE, Spitzer and Herschel to decompose the infrared (IR) spectral en- ergy distributions (SEDs) into AGN and star formation components and undertake careful treatment of the upper limits in the SED analyses and average SFR calculations. Using these methods I have calculated the mean SFRs of ~2000 X-ray and ∼3000 optically selected AGN spanning the AGN luminosity range of 10^43 <L(AGN)< 5 × 10^47 erg/s at redshifts of 0.2< z <2.5. I have established that the mean SFR as a function of AGN luminosity shows a flat relationship with a slight positive trend at the highest AGN luminosities. Crucially, I show that the positive trends observed here, and in previous work, are strongly influenced by the internal redshift and mass dependencies of average SFRs observed in the global galaxy population. Indeed, the AGN studied in this thesis show mean SFRs consistent to the main sequence of star-forming galaxies, indicating that on average they tend to live in star-forming galaxies. Using new sensitive sub-mm photometry from the ALMA interferometer for a sample ~100 X–ray AGN I obtain up to 10 times improvement on the SFR constraints compared to those obtained using the archival photometry. Furthermore, I demonstrate that combining deep 870μm observations with mid-infrared photometry is a useful diagnostic for AGN identification.

Phase-dependent line-profile variations of Wolf-Rayet + O-star binaries provide an opportunity to probe the physical and ionization structure of WR winds and the shock zone between the two stars. A model is developed to calculate the phase-dependent variation in the absorption by the wind, which takes into account the structure of the shock zone, the cavity caused by the O-star wind, and the changing ionization balance of the wind. Observations of the line-profile variations of 2 Vel and HD 152270 are discussed in detail and then compared with the model. The variations of the weaker absorption lines, particularly the FeIV pseudo continuum in 2 Vel are shown to be quantitatively consistent with the expectations of selective absorption. The emission lines on the other hand are shown to vary by more than can be explained by selective absorption alone, and in a way which is inconsistent with selective absorption. These variations are broadly consistent with the scenario of excess emission from the shock zone, fueled by the colliding winds. In both stars the ratios of the emission line strengths relative to single WCL stars suggest unusually strong lines which provides additional support to the idea of excess emission. The level of the excess emission in the lines is probably very large, approaching the intensity of the O star in the lines. The fit to the FeIV pseudo-continuum lines leads to an iron abundance which is approximately 1 - 2 times solar in both 2 Vel and HD 152270. By modelling the region around the N IV 1718 line it is possible to reduce the upper limit of the nitrogen abundance in WC stars by about 2 orders of magnitude, giving C/N > 1000 and > 2000, in 2 Vel and HD 152270 respectively.

This thesis describes several open cluster searches for very low mass stars (VLM) and brown dwarfs (BD) (M/M? < 0.08). With recent developments in charged coupled devices, it has become possible to survey open clusters to well into the BD regime. The principal aim of this work is to employ these new instruments to conduct such open cluster surveys, to investigate the properties of VLM and BD populations. The thesis begins with a short introduction, which describes the fundamental properties of brown dwarfs, relates research in this field to other areas of astronomy, and summarises the project aims and thesis structure. The second chapter gives a review of the current theoretical and observational status of the field, and the third chapter describes the principles and methods used to carry out the photometry presented in this thesis. The results are presented in chapters (4), (5), (6) and (7). An RIZ survey of Praesepe produces a sample of 26 candidate VLM and BDs. K band photometric follow up of 11 of these highlights one as a binary system that almost certainly contains at least one brown dwarf. K photometry is still needed for the remaining 15. The RIZ survey also finds a possible steepening of Praesepe's mass function into the BD regime. The Praesepe infrared sequence compares very well with the latest non gray model predictions, and the binary fraction in Praesepe is found to be essentially the same as for The Pleiades. A catalogue of Pleiades stars is used to model the cluster. King profiles are plotted for 4 mass bins and these profiles suggest that the cluster is relaxed. Virial analysis is then used to show that the cluster contains not more than 131M of dark matter (BDs) to add to the observed mass of 735M. Further deep surveys in The Pleiades, Praesepe and the Hyades result in 88 new Pleiades BD candidates, 141 Praesepe VLM candidates and 33 Hyades BD candidates. These new candidate lists, as well as finder charts are given in the appendices.

EUV spectral observations provide an excellent means with which to examine the composition and physical state of gas along the line of sight to and in the atmospheres of hot hydrogen rich white dwarfs. Here I present the results of an analysis of a large number of spectra of hot DA type white dwarfs in the temperature range 25000K &lsim; Teff &lsim; 55000K obtained with the Extreme Ultraviolet Explorer satellite. No evidence for the presence of photospheric helium is found in the spectra of any of the sample objects, adding to the growing body of observations which argues against the one channel evolutionary hypothesis of Fontaine & Wesemael (1987). Furthermore, it is found that contrary to theoretical predictions and the results of earlier photometric studies, the majority of the opacity of EUV radiation observed towards those DAs with 40000K &lsim; Teff &lsim; 50000K is provided by HI, HeI and HeII along the line of sight as opposed to photospheric heavy metals. The 228A Lyman edge of HeII is detected in the spectra of 6 of the sample objects allowing a direct measurement of the line of sight averaged ionization fractions of hydrogen and helium. The ionization fractions towards these 6 stars can be consistent with a conclusion of a uniformly ionized ISM with weighted means of fH = 0.37 +/- 0.1 fHe = 0.28 +/- 0.04. The limits placed on the fractions towards the remaining sample objects do not contradict such a conclusion. The observed high level of helium ionization can be consistent with a model in which the blast from a nearby supernova shock ionized the gas of the LISM some million or so years ago. The most peculiar line of sight characteristics towards the DAO+dM binary RE J0720-318 revealed by the unusually large HeI edge at 504A(e.g. fH ~ 0.9; Burleigh, Barstow & Dobbie 1997), has been attributed to the presence of an ionized cloud lying along this line of sight at a distance of 123-170 pc.

This thesis contains observations and multiwavelength modelling of Active Galactic Nuclei (AGN) and Starburst (SB) galaxies. The drive behind this work is the identification of the powering mechanism of high luminosity galaxies, where obscuration due to gas and dust hide the central regions from direct view. I present near infrared (NIR) observations of Ultraluminous Infrared galaxies (ULIRGs) and X-ray luminous starburst galaxies (XLSBs). Observations in the NIR are particularly useful in the case of dust obscuration since much less attenuation is found at these wavelengths than in the optical (by a factor of around 10). ULIRGs are defined as galaxies with extremely high infrared luminosities, attributed variously to a hidden AGN, SB region, or combination of both. We examine the dominant power source in the sample observed, finding evidence for both star formation and active nuclei. XLSBs are a group of objects, with X-ray luminosities much higher than those of normal galaxies, thought to originate only from star formation. We investigate the properties of the SB regions and rule out hidden AGN as potential power sources, providing more evidence for the existence of this group. In addition to NIR based work, I detail the building and application of a computer model which aims to deconvolve multiwavelength spectral energy distributions (SEDs), encompassing radio to X-ray frequencies, in order to quantify the relative strength of the principal components, and hence better understand the physical processes involved. Emission components used include synchrotron emission, thermal emission at various temperatures from dust, stellar emission, recombination emission and accretion disk emission (when relevant). Preliminary results indicating that reasonable fits can be achieved, highlighting distinctions between stellar and non-stellar activity.

This thesis describes the search for brown dwarfs in Hyades and Pleiades open clusters and in the Taurus star forming region. Initial surveys were carried out with deep CCD observations. Infrared follow up work was then used confirm membership status. A first chapter provides a brief introduction to the fundamental properties of brown dwarfs and highlights the reasons why it is important to study them. The second chapter gives a current review of the theory of brown dwarfs and describes recent searches for brown dwarfs and their results. Chapter 3 then describes the basic principles of photometry, reviews the instrumentation in use and describes the data reduction techniques necessary for analysis. The results of the three projects carried out throughout the course of this Ph.D. are presented in chapters 4, 5 and 6. Chapter 4 presents a deep, large area, optical IZ survey of the Hyades in which no candidate brown dwarf members remain after follow up infrared JHK photometry, optical spectra and proper motion measurements. Chapter 5 presents the results of a deep CCD IZ survey of the Pleiades with follow up infrared K photometry implemented to confirm cluster membership. Luminosity and mass functions for the cluster are then derived with the mass function found to be slightly rising across the stellar/sub-stellar boundary as has been evident in recent work (Bouvier et al. 1998, Martin et al. 1998, Hodgkin & Jameson 2000). Chapter 6 presents the very preliminary results of a partial area reduction of a deep CCD RIZ survey of the Taurus star forming region with candidate member finder charts provided in Appendix A. Some 88 isolated brown dwarf candidates are found with masses down to 7Mjupiter. Further work will be needed to verify the status of these objects. Chapter 7 summarises the important conclusions from chapters 4, 5 and 6 and also give suggestions for future work.

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