In this thesis we study aspects of plane wave spacetimes in the hope of shedding light of the nature of holography for plane waves. In particular, we would like to understand better the space of asymptotically plane wave solutions. We first review the necessary background on plane waves, variational principles for gravity and black holes in higher dimensions. We then propose a definition of asymptotically plane wave spacetimes in vacuum gravity in terms of the asymptotic fall-off of the metric and discuss the relation to previously constructed exact solutions. We construct a well-behaved action principle for such spacetimes, using the formalism developed by Mann and Marolf. We show that the action is finite on-shell and that the variational principle is well-defined for solutions of vacuum gravity satisfying our asymptotically plane wave fall-off conditions. Next we investigate the construction of black holes and black strings in vacuum plane wave spacetimes using the method of matched asymptotic expansions. We find solutions of the linearised equations of motion in the asymptotic region for a general source on a plane wave background. We observe that these solutions have some unusual propeties and do not satisfy our previously defined conditions for being asymptotically plane wave. Hence, the space of asymptotically plane solutions is restricted. We consider the solution in the near horizon region, treating the plane wave as a perturbation of a black object, and find that there is a regular black string solution. We find that no regular black hole solution exists, which is a counter-example to the conjecture of Emparan et. al. We end with a discussion of our results and suggest possible directions for future work.

In this thesis we investigate numerically how ionizing photons emitted by stars in galaxies cause the reionization of the Universe, the transition during which most of the gas in the Universe from a mostly neutral, to a highly ionised state it is in today. To this end, we discuss and improve two techniques for the transport of ionising radiation across cosmological volumes, analyse the sources of ionising photons at high redshifts predicted by a semi-analytical galaxy formation model (GALFORM), and combine these to make consistent model of how reionization proceeds. Our improvements to the hybrid characteristics (HC) radiative transport scheme are significant, making the code faster and more accurate, as demonstrated by our contribution to a code comparison paper (Iliev et al., 2009). Our improvements to the SimpleX radiative transport scheme allow for accurate and significantly better numerically converged calculations of the speeds of ionization fronts of cosmological HII regions. This is accomplished by a much more thorough analysis of how to properly model the density field on the unstructured density field in SimpleX. The dependence of the ionizing emissivity of GALFORM galaxies on various parameters of the model is examined. We show that massive stars formed in abundance because of the assumed top-heavy stellar initial mass function during starbursts in the Baugh et al. (2005) model, triggered by galaxy mergers, are the dominant source of ionizing photons. We show that the luminosity functions predicted by this model are in good agreement with the most recent Hubble Space Telescope results at z \gtrsim 8. The model also demonstrates that most photons are produced in faint galaxies which are not yet seen in the current data. We then combine the sources predicted by GALFORM with the SimpleX RT scheme to model inhomogeneous reionization including the effects of source suppression. We investigate how the morphology of reionization depends on the model for the sources, which may be crucial for future observations of this cosmic epoch.

This thesis presents a study of some current questions regarding the evolution of galaxies and AGN. We first argue that the bright submillimetre number counts may be dominated by obscured AGN rather than ultraluminous starbursts. Through statistical analyses of the ECDFS and WHDF, we show that N_H > 10^22 cm^−2 AGN are typically bright at 870um, with less absorbed AGN being much weaker sub-mm emitters. Overall, X-ray-detected AGN contribute 1.5+/-0.1 Jy deg^−2 to the sub-mm background, ~3% of the total, in agreement with the prediction of an AGN model which also fits the bright sub-mm counts. When Compton-thick, X-ray-undetected sources are included, this non-unified model predicts a total AGN contribution to the sub-mm background of 25-40%. The measured dependences of sub-mm flux on X-ray flux, luminosity and column density all agree well with model predictions. We therefore suggest that sub-mm galaxies may contain the absorbed AGN population which has long been sought to explain the X-ray background. Moving to shorter wavelengths, we find that AGN may continue to dominate the source counts down to ~200um. Below this, dusty star-forming galaxies are shown to fit the observed number counts and colours reasonably well. Below ~5um starlight takes over from dust as the main source of flux. These conclusions are supported by the good fit to the observations of phenomenological models assuming pure luminosity evolution. Finally, using a new survey of z~3 Lyman-break galaxies (LBGs), we find evidence for the supernova-driven feedback which is thought to be an essential ingredient of galaxy evolution. By cross-correlating LBG positions with Lya absorption in quasar sightlines, we find (a) that the intergalactic medium shows excess neutral hydrogen within 4 h^−1 Mpc of LBG positions, consistent with their lying in overdense regions, but also (b) that a Lya transmission spike may exist extending out to ~1.5 h^−1 Mpc around LBGs, indicating that galactic winds from LBGs may have ionised the IGM within this radius.

Galaxy clustering encodes information about the values of cosmological parameters and also about the physical processes behind galaxy formation and evolution. The GALFORM semi-analytical model is the theoretical approach we used to model galaxy formation. We start by studying the luminosity dependence of galaxy clustering which is measured accurately in the local Universe. We have compared the clustering predictions of three publicly available galaxy formation models with clustering measurements from the 2dFGRS and found that two new processes need to be included in order to understand the observed clustering. We then study the distribution of cold gas in dark matter haloes central to the processes of galaxy formation. We present the cold gas mass function and its evolution with redshift. We have found that the clustering predicted by the semi-analytic models agrees well with the HIPASS measurements of Meyer et al. (2007). We have calculated effective volume for redshift surveys planned with Square Kilometre Array (SKA) and compared with that of the optical Euclid mission. Finally, we study the clustering of faint extragalactic sources which are one of the foregrounds in PLANCK maps. We predict the clustering of faint extragalactic sources using a hybrid GRASIL+GALFORM+N-body model. We have compared the hybrid scheme with analytic clustering estimates. On large scales the two approaches agree, but for multipoles with l>500 the results differ significantly, with the hybrid approach being the more accurate.

In this thesis, we present a study of the identification of large-scale structure in optical astronomical surveys. This encompasses the detection of large connected structures of alaxies in spectroscopic datasets and of galaxy clusters in deep photometric surveys. Beginning with a survey featuring full 3D galaxy data, in chapter 2 we present a method to identify filamentary structure after accounting for the line-of-sight velocity distortions characteristic of the virialised systems we search for. We compare data from a real galaxy survey to a series of realistic mocks. Despite broad similarities between the two, we find models do not reproduce the argest observed structures. To evaluate the exploration of a multi-band survey lacking spectroscopy, we simulate the effects of photometric redshift uncertainties on galaxy redshifts. Our findings provide limits on the accuracy of photometric redshift estimators required to recover the same diverse range of structures detected in the original spectroscopic survey. As an alternative means of exploiting the deep multi-band photometric data common to wide-area observational campaigns, in chapter 3 we present a red sequence-based algorithm to detect galaxy clusters with Voronoi diagrams. This algorithm makes no prior assumptions about cluster properties other than the similarity in colour of their members, and an enhanced projected surface density. Testing the algorithm with mock galaxy survey data reveals a detection performance equalling or exceeding that of alternative detection algorithms. Chapter 4 describes the application of this algorithm to a $270{\rm deg^2}$ survey with deep SDSS photometry. The scientific exploitation of $4,000\ {\rm z}\ \leq\ 0.6$ cluster detections from this survey is ongoing, but work presented here shows: agreement with the red sequence slope evolution derived from semi-analytic galaxy models, evidence stellar age is not responsible for responsible for the sequence slope, and a well-defined colour-colour track of potential use in photometric cluster redshift stimation. We detail improvements made to the cluster algorithm in chapter 5. Through a series of case studies we verify our approach successfully identifies galaxy clusters in a diverse range of surveys, from volumes spanning $2h^{-3}{\rm Gpc}^{3}$ to deep near-IR detections at ${\rm z}\sim 1$. based on our findings, we expect the Pan-STARRS $3\pi$ large-area survey to identify over $10^{5}$ clusters and groups. In chapter 6, we explore the characteristics of randomly-distributed noise in Voronoi diagrams. We verify the model traditionally used to describe the distribution of Voronoi cell areas in Poisson data fails to describe the frequency of high-density random cells. Because high-density cells resemble those expected from a population of galaxy cluster members, using a large dataset generated in this study we propose an alternative model that better estimates the frequency of their areas. This new model may in the future be used to improve Voronoi-based recovery of clustered data in a diverse range of applications, both astronomical and otherwise.

This thesis exploits the wealth of information contained in the existing cosmological surveys, and demonstrates how the use of tools such as two-point statistics permit the extraction of such information. In particular, the wide field imaging survey – the Sloan Digital Sky Survey (SDSS) in conjunction with Luminous Red Galaxy (LRG) spectroscopic surveys carried out by the Two-degree-Field (2dF) and AAOmega instruments on the Anglo-Australian telescope (AAT) are utilised here. This also includes the observations of the Cosmic Microwave Background (CMB) radiation from the Wilkinson Microwave Anisotropy Probe (WMAP) experiment. Combining the imaging and spectroscopic surveys, we extract three photometric LRG samples at redshift 0.35, 0.55 and 0.7 which cover 7600 deg2 of the sky, probing a total cosmic volume of 5.5 h^−3 Gpc^3. We find very little clustering evolution in these massive early-type galaxies out to z~0.8 or nearly half the age of the Universe. The shape of the large-scale correlation functions is consistent with a simple ‘high-peaks’ bias and linear theory framework of the standard CDM model. The new z=0.7 LRG sample is then used in the CMB-LSS cross-correlation analysis to look for the the Integrated Sachs-Wolfe (ISW) effect as a dynamical evidence for the accelerated expansion of the Universe. The measured zero CMB-LRG correlation is inconsistent with the CDM model expectation at 2.2sigma significance level. Furthermore, our rotation tests show that the previous detections of the ISW effect may not be as significant as previously claimed. We make independent estimates of the WMAP CMB temperature power spectra and show explicitly how sensitive they are to the instrumental beams. We propose an alternative method for determining the beam profiles by stacking radio point sources and demonstrate its robustness via Monte Carlo simulations plus realistic point source detection algorithm. Using this technique, we find significantly wider W-band beam profiles than the WMAP Jupiter beam analysis. We also find a tentative evidence for a non-linearity in the WMAP radio source fluxes when compared with the ground-based measurements. Finally, we investigate if the recently claimed timing offset in the WMAP time-ordered data can explain the observed wider than expected beam profile.

The early 1970's saw the start of a new era in astronomy with the rst approach to building large telescopes (2 to 5 meters class telescope) and locating them in the right places. The astronomical community started to evaluate these sites using the sky quality "seeing" parameters as one the major constraints to decide where to place the observatory in the world and where is the best location in the observatory. Taking a few words from one of the rst site testing works by Merle Walker (Walker,1971) that considered the Islands like as: Canary Islands, Hawaiian islands or Crete, a possible ideal place for Astronomy. "The best seeing occurs at sites on peaks near sea coasts having cold ocean currents oshore that reduce the height of the [temperature] inversion layer, and where the laminar air-flow set up over the ocean still persists". (....) "Mountain peaks on (small) islands in warm oceans may be good sites, provided that the peaks are suciently high to place the observer above the inversion layer". It is in this new era when many engineers, physicist, meteorologists and astronomers started to search for new places and to develop new instruments to measure the sky quality. Please see the site testing report for La Palma from Mcinees (Mcinnes,1973). Astronomy is now becoming more sophisticated, with new observatories and new adaptive optics instruments that can compensate the atmosphere to the difraction limit of the telescopes. The newer Adaptive Optics require more information about the optical parameters that aect image quality and it is also becoming more important to manage the technical and human resources in the observatory for high resolution observations. To obtain the information needed, a new set of instruments have been developed and used, such as: MASS-DIMM, DIMM, SCIDAR, SLODAR,Lusci, etc to mention only a few. They have been the main contributor to analyzing potential new sites for the future extremely large telescopes. They also provide new data for the analysis of the expected performances of the new generation of AO instruments for the TMT or E-ELT. The purpose of this thesis is to establish the influence of the upper atmosphere, monitored using tropospheric wind speed and temperature, on the local sky conditions, measured by the refractive index parameter C2n and the astroclimate parameters (seeing, isoplanatic angle and coherence time), and their evolution through the years. The meteorological data used come from a launch site located about 150 km away from the observatory and which provides data for meterological forecasting agencies. It has been determined that the best yearly period for the seeing was the year 2005, which corresponds with the year with lower upper wind speed periods. The use of these meteorological data and the good agreement found with the local sky conditions, provide the rst step to constructing a simple parametric optical turbulence model (Trinquet model) of the Observatory Roque de Los Muchachos. This in turn opens the way to performing a nowcast of the astroclimate parameters and the refractive index parameter C2n using the meteorological forecasting data. It can also be used to help design the AO instrumentation capabilities at the ORM (and perhaps by extension elsewhere). Due to the importance of the knowledge of the vertical C2n distribution and how this aacts the Laser Guide Star (LGS) AO observations, an instrument was proposed based on the DIMM technique and using a range-gated CCD detector to measure the backscattered laser light at dierent altitudes. This can then produce a vertical distribution of the Fried parameter r0 and the seeing. A simple prototype was built but due to the lack of time and various faults the rst experiments could not full the proposed requirements. The preliminary prototype design and the initial on-sky results are provided in appendices of this thesis in order to suggest and support future work.

We present a survey of 73 galaxy groups and clusters aimed at determining the role played by the cold gas in the feedback process. We use optical Integral Field Spectroscopy from the VIsible Multi Object Spectrograph in conjunction with high spatial resolution X-ray observations from the Chandra X-ray Observatory and extensive multi-wavelength observations to study the interactions of the cold gas with the Intra Cluster Medium and the Brightest Cluster Galaxy. The wealth of information provided by these observations has allowed us to study the kinematic structure of the cold gas in the largest sample of cluster cores to date. We use this information to shed new light three key questions i) Are all line emitting systems highly disturbed? ii) What role does the Brightest Cluster Galaxy play in the cooling of gas from the ICM? iii) What role does the cold gas play in the feedback process and what is its relationship to the fueling of the AGN?. The analysis of the full survey immediately suggests that the line emitting gas in the majority of cluster cores isn’t highly disturbed. While several clusters do show a disturbed Hα morphology (13/73) the majority appeared uniform and quiescent (45/73). Similarly the velocity structure of the ionised gas within most systems also appears to be very ordered and interestingly appears decoupled from the stellar kinematics of the brightest cluster galaxy. Several of the more disturbed systems do show evidence of an interaction with another cluster member, however, this is not common to all disturbed systems. One of the most interesting discoveries to come out of the sample analysis is the identification of a small sub sample of objects which show an offset between the brightest cluster galaxy and the bulk of the ionised gas. Comparison to X-ray observations indicates that the cluster core is spatially coincident with the ionised gas emission and not the galaxy. These objects provided an opportunity to study the role played by the BCG in the cooling of gas from the ICM. The kinematics of these objects suggests that these offsets should be transient and short lived. Despite this we identify a substantial mass of cold molecular gas (∼109 M⊙ ) associated with the offset emission, and not the BCG, for one object. This mass is consistent with the upper limit of the cooling during the lifetime of the offset suggesting that the cooling continues to occur in the core despite being offset from the BCG. This result clearly indicates that while the BCG may typically reside at the centre of most clusters it is not required for cooling gas to condense from the ICM. Finally, we address the ordered velocity structure of the ionised gas in BCG in much greater detail. As a pilot project we study the kinematics of Hydra-A using detailed multi-wavelength observations and identify the presence of a disk of cold gas which has an axis of rotation parallel to the axis along which the AGN is inflating cavities into the ICM. Comparison to the energetics of the cavities suggests that the mass of cold gas in the disk is sufcient to fuel a future outburst of comparable magnitude. Noting the similar velocity structure in many of the clusters in our sample we apply a similar analysis to the full sample. We use kinemetry and determine that ≈62% of the sample have ionised gas kinematics which are consistent with a rotating disk. A linear trend is identified between the peak rotational amplitude of these disks and the linewidth of CO observations which suggests that the large mass of cold molecular gas present in the cores of clusters shares the kinematic structure of the ionised gas. Finally, we compare the axis of rotation of all disks with radio and X-ray observations and identify a clear alignment between the jets/cavities and the disks axis of rotation. The prevalence of these disks and their alignment with the axis along which the AGN is injecting mechanical energy into the ICM clearly suggests that they play an important role in the feedback process and provides a link between the gas cooling on kpc scales and the fueling of the black hole at the centre of the BCG.

No description available.

Two space charge collectors were designed and built incorporating a glass-asbestos medium for filtering ions from the air, the charge being measured using a vibrating reed electrometer. These collectors were rigorously tested and it was demonstrated that they collected over 99.8% of the small ions in the air at certain flow rates through the collector. An analogue-to-digital converter was built in order to digitalise the information displayed on a 16 point recorder. The unit was built incorporating semiconductors; a voltage from the recorder was converted into a frequency which was counted on decatron tubes for a certain period of time, so that the number registered was within 1% of the recorded value. This number was then punched out on paper tape and arrangements were made in order to print 'space' and 'carriage return-line feed' into the output, Four aspirated psychrometers incorporating thermistors were built in order to measure the temperature and humidity gradients on a 21 m steel lattice mast. Wind was recorded at 1 m and 17 m and potential gradient in the surface of the earth and at the top of the mast. Results of space charge concentrations on the mast structure show that during a period of melting snow a charge separation occurs at the ground surface under strong wind conditions. It was also shown that dry blowing snow gives rise to high positive space charge even when the snow is blowing from distant hills. Corona space charge was observed to form at trees after a nearby lightning flash and during misty conditions there was a predominance of negative space charge. Convection cells were apparently detected under cumulus clouds and it was observed that charge was transported by the air circulations within these cells. Perhaps the most important result observed was an electrode effect due to the mast structure towards the top. This effect could completely mask the space charge density expected at that level if the mast were not there.