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No description available.

The X-ray spectra of active galaxies are, to a first approximation, power-law-like. Upon more detailed investigation, however, features become apparent: in particular, an emission line corresponding to iron, around 6.4 keV, and the so-called 'soft-excess' below ~ 2 keV, whereby an extrapolation of a power-law fit to the 2-10 keV energy band under-predicts the observed counts. In this thesis, the soft X-ray excesses in samples of both low- and high-luminosity Active Galactic Nuclei (Seyfert galaxies and QSOs respectively) are analysed. This allows the parameters of the soft excess, such as strength and temperature, to be compared with X-ray luminosity. It is found, however, that there are very few differences between Seyfert galaxies and QSOs, leading to the idea that the soft excess is not strongly linked to the luminosity of the object. Time variability of the soft excess is studied, by investigating three objects (3C 273, PKS 0558-504 and 1H 0419-577) which have each been observed a number of times in the X-ray band. The soft excess is found to vary in each, with the changes possibly due to the Compton cooling of the corona above the accretion disc, brought on by an increase in accretion rate. Another sample, of high-redshift objects, is also discussed, finding that there is no evidence for spectral evolution or intrinsic absorption in these radio-quiet QSOs. When considering this sample as a whole, there is an indication of spectral flattening at higher energies, suggesting the presence of a Compton reflection hump. Finally, the existence and strength of the neutral, narrow iron line is investigated, finding that, in the more luminous objects, the equivalent width is much lower, this is the X-ray equivalent of the Baldwin effect found in the optical and UV emission lines.

Over the years, substantial effort has gone into calculating models of the intrinsic population of galactic Cataclysmic Variables (CVs). However, comparison with the observed properties of the known CV sample has consistently failed to match the predictions put forward by the ‘disrupted magnetic braking’ model of CV evolution. Some of the discrepancies have been blamed on the heterogeneous set of known CVs, which are not well-suited for a quantitative test of the population models. The Sloan Digital Sky Survey (SDSS) has dramatically improved the observational side of CV population studies. Sampling a large volume in ugriz colour space and extending deeper than any previous large-scale survey, SDSS provides the most homogeneous and complete sample of CVs to date. At the time of writing, the sample of SDSS CVs contains 213 systems, of which 177 are new discoveries. Establishing the detailed properties of these CV is a major task. The work presented in this thesis is part of a larger effort to derive the orbital period distribution of SDSS CV sample. In this thesis, I present my contribution and results of time-series photometric and spectroscopic observations of a total of 29 SDSS CVs. I provide a discussion of their properties and determine orbital periods for 20 systems. I also provide a quantitative analysis of the new sample of CVs from SDSS and compare their intrinsic qualities with the previously known population of CVs. The results show that the period distribution of the SDSS CVs differs from that of the previously known CVs at a 3-σ level. A substantially larger fraction of below-the-gap to above the-gap systems is observed, accompanied by a prominent accumulation of CVs at the orbital period minimum. This result is important, as the accumulation of systems at short orbital periods has been long theorised but never proved observationally. More specifically, the origin of the 80–86min period spike is entirely due to the new CVs identified in SDSS. The systems in the period spike also differ in spectral morphology and accretion activity from the longer-period CVs. Indeed ∼ 20% of the CVs identified by SDSS have white-dwarf dominated spectra indicating low mass transfer rates and late spectral type donors, the majority of which have been found close to the period minimum. A homogeneously selected sample of CVs, with well defined parameters has been long overdue and the results from SDSS are very promising. With this improved observational evidence, theoretical models can be modified to re-address the discrepancies in the current theories that have long plagued our understanding of CV evolution.

This thesis investigates the role of environment on galaxy formation and evolution, giving particular focus to the transformation of star forming spirals into passive S0s. The data utilised for this study comes from photometric and spectroscopic observations of galaxies at 0 < z < 1 in different environments from the ESO Distant Cluster Survey. We first study the formation history of (172) cluster ellipticals (Es) and S0s, the oldest types of galaxies in the local universe. We examine their colour-magnitude relation (CMR), and find a very small intrinsic colour scatter. Only 7% of the galaxies are significantly bluer than the CMR. The scarcity of blue S0s indicates that, if they are the descendants of spirals, these were already red when they became S0s. We observe no dependence of the CMR scatter with z or cluster velocity dispersion. This implies that by the time cluster E/S0s achieve their morphology, the vast majority have already joined the red sequence. We estimate the galaxy formation redshift z_F for each cluster and find that while it does not depend on the cluster velocity dispersion, it increases weakly with cluster redshift. This suggests that, at any given z, in order to have a population of fully-formed E and S0s they needed to have formed most of their stars ≃2–4Gyr prior to observation. In other words, the galaxies that already have early-type morphologies also have reasonably-old stellar populations. This is partly a manifestation of the "progenitor bias", but also a consequence of the fact that the vast majority of the E/S0s in clusters (in particular the massive ones) were already red by the time they achieved their morphology. Moreover, E and S0 galaxies exhibit very similar colour scatter, implying similar stellar population ages. We also find that fainter E/S0s finished forming their stars later, consistent with the cluster red sequence being built over time and the brightest galaxies reaching the red sequence earlier than fainter ones. Finally, we find that the E/S0s cluster galaxies must have had their star formation truncated over an extended period ∆t ≿1 Gyr. We then move our focus to the evolution of star-forming galaxies. We investigate the effect of the environment on the transformation of star-forming spirals into passive S0s by studying the properties of the gas and the stars in a sample of 422 emission-line galaxies in different environments. We identify galaxies with kinematical disturbances (in their gas disks), and find that they are more frequent in clusters than in the field. The fraction of kinematically-disturbed galaxies increases with cluster velocity dispersion and decreases with distance from the cluster centre, but remains constant with projected galaxy density. We also studied morphological disturbances in the stellar light, finding that the fraction of morphologically disturbed galaxies is independent of environment. Moreover, there is little correlation between the presence of kinematically-disturbed gas and morphological distortions. For the kinematically-undisturbed galaxies, we find that the cluster and field B-band Tully-Fisher relations are remarkably similar. Additionally, we find that the kinematically-disturbed galaxies show a suppressed specific star formation rate. There is also evidence indicating that the gas disks in cluster galaxies have been truncated, and therefore their star formation is more concentrated than in low-density environments. If spirals are the progenitors of cluster S0s, our findings imply that the physical mechanism transforming cluster galaxies efficiently disturbs the star forming gas and reduces their specific star formation rate. This star-forming gas is either removed more efficiently from the outskirts of the galaxies or it is driven towards the centre (or both). In any case, this makes any remaining star formation more centrally concentrated, helping to build the bulges of S0s. All this evidence, together with the fact that the transformation mechanism does not seem to induce strong morphological disturbances on the galaxies, suggests that the physical processes involved are related to the intracluster medium, with galaxy-galaxy interactions playing only a limited role in clusters. Interestingly, in analogy with the "blue" early-type galaxies found in the CMR study in clusters, we have also found several emission-line E/S0 galaxies with extended rotating star-forming gas disks.