The identification of Active Galactic Nuclei (AGN) and their classification as obscured or unobscured are important parameters for the explanation of the X-ray background and its synthesis models. AGN detection and classification is expected to be sensitive to its environment (obscuration, host galaxy starlight, star-formation) and to its intrinsic properties such as the Eddington ratio. Using the 3XMM-DR4 X-ray catalogue, and the SDSS-DR9 spectroscopic catalogue, we have obtained a new sample of X-ray selected narrow emission line galaxies. A mismatch between the optically-based and X-ray-based classifications has been found for 6% of the X-ray selected AGN, named the elusive Seyfert 2 (Sy2). The absence of an optical signature can be understood by the intrinsic weakness of the AGN which can be associated with dilution from the host galaxy. In addition, some of the sources are good candidates to be “True Sy2" AGN as they are very likely to lack the broad line region. By extending the work to higher redshift using the Chandra-COSMOS sample, which allows fainter X-ray sources to be detected, evolutionary effects can be examined. The study of elusive Sy2 among this sample have shown similar explanations for the absence of an optical AGN signature, i.e. weak AGN with low accretion rates in addition to, in some cases, dilution by galaxy starlight. Some unobscured Sy2 have also been confirmed to intrinsically lack broad-lines. Importantly, it seems that the fraction of elusive Sy2 increases with redshift. The fraction of unobscured Sy2 among X-ray selected and/or optically selected AGN from the 3XMM-SDSS sample is about 60% which drops to 47% after subtraction of the contaminants (narrow-line Sy1 and Compton-thick AGN). The main explanations for the absence of optical broad-lines are variability, host galaxy dilution, dust reddening and intrinsic absence of the broad line region.

This thesis considers the nature of the diffuse gas in the interstellar medium. In Chapter 3, the ROSAT EUV/X-ray spectra of 1987 late-type stars are studied in detail so that the absorption along the line of sight can be determined. Using two-colour plots, it is shown that the data are best described by two temperature coronal models, although no single model provides an adequate fit for all the sources. These data are combined with Hipparcos parallaxes and a further 490 sources from other published data sets in Chapter 4 to investigate the distribution of cool gas within 400 pc of the Sun. Using a spherical harmonic analysis it is shown that the best fit is obtained for a non-spherical cavity model in which the Solar System resides in a region of space with a density typically between 0.045cm-3 -0.08 cm-3. The average radius of the cavity is 60 pc although it varies between 29-280 pc with sky direction. Chapters 5 and 6 consider the X-ray emission from hot components of the diffuse gas. It is shown that spatial fluctuations with a magnitude of 1=4% are observed across the sky and that they cannot be explained by source confusion or background subtraction errors alone. A spectral analysis indicates that they are possibly caused by 'clumps' of gas in the Galactic halo, although the possibility that they are caused by differential absorption across the field of view cannot be ruled out. In Chapter 7, the techniques used to calibrate the CCD's to be flown on the CUBIC satellite are presented. A combination of laboratory measurements and semiconductor modelling are used to produce response matrices for post launch data analysis.

Narrow Line Seyfert 1 galaxies are an interesting subset of Seyfert 1 galaxies displaying intriguing optical and X-ray spectral properties that make them particularly difficult to unify with other types of Active Galactic Nuclei in terms of the standard orientation based unified scheme. We review the literature on NSL1s and the theoretical models that have been hypothesised as an explanation of their properties, along with a brief overview of AGN unification schemes. We present data from a near IR spectroscopic study of a sample of NLS1s carried out with CGS4 on UKIRT, an NLS1 radio imaging project utilising both archival and new data from MERLIN and the VLA, and a near IR imaging study on NLS1s carried out using UFT1 on UKIRT. We also present Spectral Energy Distributions of NLS1s assembled from data mined from the literature, along with results from an ISO and SCUBA study of NLS1s examining the mid 1R to sub-mm dust emission properties of the NSL1s. Finally, in conclusion, we suggest a possible sub-division of the NLS1 class into two distinct sets of objects, one of which we believe fits in with the so called "pole-on" model, the other with models that suggest a high accretion rate onto a low mass black hole.

Studies of the extremes of behaviour observed in Active Galactic Nuclei (AGN) constrain the physics of their emission processes by pushing existing models to their limits. Narrow-Line Seyfert 1 (NLS1s) represent one extreme of the AGN phenomenon, and soft X-ray selection is an efficient way to find NLS1s. This thesis examines the X-ray and optical properties of soft X-ray bright AGN and NLS1s in particular. The X-ray spectral properties, as observed by ASCA , of a sample of 22 NLS1s are discussed. The X-ray continua show a wide range in slope, with a mean only slightly steeper than that of 'normal' Seyfert Is and most show 'soft excess' emission, often containing a significant fraction of the X-ray luminosity. In addition, some NLS1s show spectral features in the range 0.7-0.9 keV and 1.1-1.4 keV, which are described in terms of absorption in photoionised material along the line-of-sight. The X-ray properties of the bright NLS1 Ark 564 are examined in detail and the X-ray spectrum is found to be consistent with a model comprising a power-law plus 'reflection' from ionised matter. A re-analysis of the ROSAT Wide Field Camera all-sky survey data is presented. A total of 19 AGN are detected in the extreme-ultraviolet, including eight NLS1s, making this the first complete sample to contain a high fraction of NLS1s. This work is extended using a larger, complete sample of AGN selected on the basis of 0.25 keV flux. Correlations between the measured X-ray and optical properties are discussed and the sample is used to isolate objects at the extreme ends of the parameter space explored. The luminosity function of the sample is presented, along with the luminosity functions of Seyferts classified on the basis of H/3 width and X-ray spectral slope.

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A key result of the Two Micron All Sky Survey (2MASS) was the discovery of a popula­ tion of AGN that appear redder than their traditional UV/optical selected counterparts at IR wavelengths. In order to investigate the nature of 2MASS­selected AGN, in this thesis I present optical spectra and near­ to mid­IR photometry for a representative sam­ ple of 29 nearby (z<0.28) 2MASS­selected AGN with red near­IR colours (J­K S # > 2.0), comparing my findings with those obtained for comparison samples of UV/optical se­ lected AGN. The spectra show a remarkable variety, including moderately reddened type 1 objects (58%), type 1 objects that appear similar to traditional UV/optically selected AGN, narrow­line Seyfert 1 AGN (14%), type 2 AGN (21%) and HII/composite objects (7%). Although the median Balmer decrement suggests significant optical reddening compared to the comparison samples, some objects show relatively little (if any) reddening in their optical spectra. In addition, the near­ to mid­IR photometric results reveal that a fraction of the broad­line objects in the 2MASS sample (26%) have unusually blue 2.17­3.4µm colours when compared to the rest of the sample, suggesting that their red near­IR colours are due, at least in part, to emission by a component of hot dust with an unusually high covering factor. Overall, a variety of mechanisms is required to explain the red near­IR colours of the 2MASS objects: although some are moderately reddened type 1 objects, or `genuine' type 2 objects that are highly obscured by the central torus structure, others show unusually strong emission from dust close to the AGN. This is confirmed by simple modelling of the colours of the 2MASS and comparison samples However, I find no clear evidence that the 2MASS objects are young, dust enshrouded AGN. In addition to the analysis of the full 2MASS sample, I present a detailed study of one of the red 2MASS AGN (J1131+16) which shows unusually strong forbidden high ionization lines. This study demonstrates the potential of optical spectroscopic studies of the FHILs for understanding the nature of the obscuring dust structure of AGN.

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In this thesis I examine some extensions to the concordance model of cosmology and their observational consequences. In particular, I look at how vector perturbations sourced by cosmic strings could generate a B-mode signal in weak lensing surveys. For a string network close to the current observational upper bound, this signal could be detected within the next 5-10 years. I also examine model independent modified gravity and forecast the constraints that could be obtained with the combination CMB+weak lensing+galaxy cluster counts. For the specific example of Planck (CMB+cluster catalogue) and DES(weak lensing), deviations from General Relativity could be constrained to within 5% in the next 3-4 years. The forecast for cluster counts relies on mass functions calibrated for ∧CDM+GR cosmologies, so I then investigate how reliable the predictions are in the case of modified gravity. At first glance, it appears that the mass function formalism is reliable and, with a little cheating, can even work in the case where modified gravity has a screening scale.

I model the chemical effects on dark clouds of their being composed of small (<0.1 pc), transient (order of 1 Myr), dense cores (which are only observed with high-resolution interferometry) within which most of the clouds' mass resides, surrounded by a diffuse background gas. I investigate the chemical evolution of the cores, and the resultant chemical enhancement of the background gas by cyclical processing through core formation and dissipation. I approximate the MHD mechanism of Falleh Hartquist (2002), by which large transient density inhomogeneities may be produced by slow-mode waves in a cold plasma, into a multi-point 1-D chemical code. Molecular freeze-out onto dust grains and subsequent re-injection into the gas phase are switched on or off according to the attainment of a critical visual extinction at individual points. I explore a grid of parameter space in five physical and chemical variables. Among a number of conclusions, I find firstly that multi-point chemical codes are highly necessary for modelling dark cloud regions, due to the large chemical variation over time and space. I find that the core chemistry is young at all times, in keeping with observations, and that the abundance and spatial extent of several important species is significantly enhanced by the cycling process. Further to this, I construct (non-interacting) assemblies of such cores, producing convolved maps mimicking observational constraints, and present evidence that such collections of transient dense cores may reproduce the morphologies observed in both low- and high-resolution molecular line studies of dark clouds. I further modify the convolution method in a number of ways, using this mapping procedure to test hypotheses derived from the chemical analysis. Hence I obtain observational diagnostics for the determination of physical and chemical conditions in dark clouds.