Thesis (Ph.D.)--Lehigh University, 2008. / Adviser: Slava V. Rotkin.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-11, Section: B, page: 6859. Adviser: Paul M. Ricker. Includes bibliographical references (leaves 141-156) Available on microfilm from Pro Quest Information and Learning.

<p> The stellar halo that surrounds our Galaxy contains clues to understanding galaxy formation, cosmology, stellar evolution, and the nature of dark matter. Gravitationally disrupted dwarf galaxies form tidal streams, which roughly trace orbits through the Galactic halo. The Sagittarius (Sgr) dwarf tidal debris is the most dominant of these streams, and its properties place important constraints on the distribution of mass (including dark matter) in the Galaxy. Stars not associated with substructures form the "smooth" component of the stellar halo, the origin of which is still under investigation. Characterizing halo substructures such as the Sgr stream and the smooth halo provides valuable information on the formation history and evolution of our galaxy, and places constraints on cosmological models. This thesis is primarily concerned with characterizing the 3-dimensional stellar densities of the Sgr tidal debris system and the smooth stellar halo, using data from the Sloan Digital Sky Survey (SDSS). F turnoff stars are used to infer distances, as they are relatively bright, numerous, and distributed about a single intrinsic brightness (magnitude). The inherent spread in brightnesses of these stars is overcome through the use of the recently-developed technique of statistical photometric parallax, in which the bulk properties of a stellar population are used to create a probability distribution for a given star's distance. This was used to build a spatial density model for the smooth stellar halo and tidal streams. The free parameters in this model are then fit to SDSS data with a maximum likelihood technique, and the parameters are optimized by advanced computational methods. Several computing platforms are used in this study, including the RPI SUR Bluegene and the Milkyway@home volunteer computing project. Fits to the Sgr stream in 18 SDSS data stripes were performed, and a continuous density profile is found for the major Sgr stream. The stellar halo is found to be strongly oblate (flattening parameter q=0.53). A catalog of stars consistent with this density profile is produced as a template for matching future disruption models. The results of this analysis favor a description of the Sgr debris system that includes more than one dwarf galaxy progenitor, with the major streams above and below the Galactic disk being separate substructures. Preliminary results for the minor tidal stream characterizations are presented and discussed. Additionally, a more robust characterization of halo turnoff star brightnesses is performed, and it is found that increasing color errors with distance result in a previously unaccounted for incompleteness in star counts as the SDSS magnitude limit is approached. These corrections are currently in the process of being implemented on MilkyWay@home.</p>

<p> Terahertz (THz) heterodyne receiver systems are required by NASA to monitor gas concentrations related to the Earth's ozone depletion. To this end, NASA needs compact, solid state, tunable THz local oscillators. THz LOs have been developed using three means: 1) All-electronic LOs using mixers in combination with Gunn oscillators, 2) Hybrid Photo-electronic LOs using a cw analog of the Auston switch, and 3) All-photonic THz LOs using coherent sources, such as vapor lasers or solid-state Quantum Cascade Lasers, and down converting lasers using nonlinear crystals. In this dissertation, we began with two frequency stabilized Nd:YAG lasers, locked to a common reference cavity, as a starting point to having a stable input into a nonlinear optical frequency conversion system. Following this, we explored the nonlinear crystals useful for THz generation, and the phasematching schemes that could be employed by each. We concluded by settling on highly insulating III-V semiconductor crystals as the proper choice of nonlinear element, and put together a new phasematching method that is most useful for them.</p>

<p> <b>Context:</b> The majority (~85%) of the matter in the universe is composed of dark matter, a mysterious particle that does not interact via the electromagnetic force yet does interact with all other matter via the gravitational force. Many direct detection experiments have been devoted to finding interactions of dark matter with baryonic matter via the weak force. It is still possible that dark matter interacts with itself via a strong scale force and has a self-scattering cross-section of ~0.5 cm²g^-1. In fact such a strong scale scattering force could resolve several outstanding astronomical mysteries: a discrepancy between the cuspy density profiles seen in &Lambda;CDM simulations and the cored density profiles observed in low surface brightness galaxies, dwarf spheroidal galaxies, and galaxy clusters, as well as the discrepancy between the significant number of massive Milky Way dwarf spheroidal halos predicted by &Lambda;CDM and the dearth of observed Milky Way dwarf spheroidal halos. <b>Need:</b> While such observations are in conflict with &Lambda;CDM and suggest that dark matter may self-scatter, each suffers from a baryonic degeneracy, where the observations might be explained by various baryonic processes (e.g., AGN or supernove feedback, stellar winds, etc.) rather than self-interacting dark matter (SIDM). If dark matter lags behind the effectively collisionless galaxies then this is clear evidence that dark matter self-interacts. The expected galaxy-dark matter offset is typically >25 kpc (for cross-sections that would explain the other aforementioned issues with &Lambda;CDM), this is larger than the scales of that are plagued by the baryonic degeneracies. <b>Task:</b> To test whether dark matter self-interacts we have carried out a comprehensive survey of the dissociative merging galaxy cluster DLSCL J0916.2+2951 (also known as the Musket Ball Cluster). This survey includes photometric and spectroscopic observations to quantify the position and velocity of the cluster galaxies, weak gravitational lensing observations to map and weigh the mass (i.e., dark matter which comprises ~85% of the mass) of the cluster, Sunyaev-Zel'dovich effect and X-ray observations to map and quantify the intracluster gas, and finally radio observations to search for associated radio relics, which had they been observed would have helped constrain the properties of the merger. Using this information in conjunction with a Monte Carlo analysis model I quantify the dynamic properties of the merger, necessary to properly interpret constraints on the SIDM cross-section. I compare the locations of the galaxies, dark matter and gas to constrain the SIDM cross-section. This dissertation presents this work. <b>Findings:</b> We find that the Musket Ball is a merger with total mass of 4.8^+3.2_-1.5&times;10¹⁴M_sun. However, the dynamic analysis shows that the Musket Ball is being observed 1.1^+1.3_-0.4 Gyr after first pass through and is much further progressed in its merger process than previously identified dissociative mergers (for example it is 3.4^+3.8_-1.4 times further progressed that the Bullet Cluster). By observing that the dark matter is significantly offset from the gas we are able to place an upper limit on the dark matter cross-section of &sigma;_SIDMm^-1_DM &lt; 8 cm²g^-1. However, we find an that the galaxies appear to be leading the weak lensing (WL) mass distribution by 20.5" (129 kpc at z=0.53) in southern subcluster, which might be expected to occur if dark matter self-interacts. Contrary to this finding though the WL mass centroid appears to be leading the galaxy centroid by 7.4" (47 kpc at z=0.53) in the northern subcluster. <b>Conclusion:</b> The southern offset alone suggests that dark matter self-interacts with ~83% confidence. However, when we account for the observation that the galaxy centroid appears to trail the WL centroid in the north the confidence falls to ~55%. While the SIDM scenario is slightly preferred over the CDM scenario it is not significantly so. <b>Perspectives:</b> The galaxy-dark matter offset measurement is dominated by random errors in each cluster. Thus measuring this offset in other dissociative mergers holds the promise of reducing our uncertainty and enabling us to: 1) state confidently whether dark matter self-interacts via a new dark sector force, or 2) constrain the dark matter cross-section to such a degree that SIDM cannot explain the aforementioned mysteries. To this end we have established the Merging Cluster Collaboration to observe and simulate an ensemble of dissociative merging clusters. We are currently in the process of analyzing six dissociative mergers with existing data, and carrying out multi-wavelength observations of a new sample of 15 radio relic identified dissociative mergers. (Abstract shortened by UMI.)</p>

<p>In this dissertation, we describe observational and theoretical work related to the large-scale clustering of matter in the universe. Such work is crucial in constraining models of the Universe in future surveys and is one of the most powerful probes of the nature of dark energy. In Chapter \ref{magnification}, we present work performed using the Deep Lens Survey (DLS) to measure the growth of structure over cosmic time using weak lensing magnification. This is the first time such a measurement has been performed and represents a significant step forward for this relatively new probe of large-scale structure (LSS) which is known to be complementary to other weak lensing measurements. Later in Chapter \ref{conclusions}, we discuss steps needed for magnification become a competitive, precision probe of cosmology. Chapter \ref{covariance} presents a model for the emulation cosmology dependent error covariances in LSS probes. Estimating these covariances are necessary in order to compare models to the data and require a large amount of computational time to create the simulations required. Tools to reduce the number of simulations required and model the cosmology dependence are needed. We utilize a novel decomposition of LSS error covariances that allows for construction of a emulator that fulfills both of these criteria. In order for future surveys to reach their goals, methods to model measurement error and new probes of LSS complementary to those planned are required. The conclusions of this dissertation in Chapter \ref{conclusions} address the future outlook for this work and research that must be done between now and when the next set of survey data is available. Many systematic errors need to be addressed in magnification before it can be considered a precision cosmology tool. For the error covariances, additional methods to reduce the required number of simulations to estimate the matrices are required. In the Appendix, we present a high level description of an open sourced software package that we developed and implemented over the course of these two projects.

<p> The abstract is not available for copy and paste.</p>

<p> A significant portion of the process of detecting pulsars from radio sky surveys remains a largely manual task. The visual inspection of data in order to detect and validate potential pulsar candidates is by far the most time consuming portion of the overall process. Coupled with the fact that well over a Petabyte of pulsar survey data has been archived, the task of identifying these valuable phenomena is tedious and time consuming.</p><p> Using data from a survey performed with the National Radio Astronomy Observatory&rsquo;s (NRAO&rsquo;s) Green Bank Telescope (GBT) in 2007, this thesis explores the application of machine learning techniques to mitigate the manual efforts involved in pulsar candidate detection. The performance of three different classifiers is explored - Naive Bayes, C4.5 (J48) Decision Tree, and Support Vector Machine. Preprocessing and feature extraction methods are described and a framework for applying the classifiers to the survey data is presented. Multiple features were extracted from the survey data and used to train the classifiers. Cross-validation results of the various feature sets and classifiers are documented. Experiments suggest the potential of the proposed framework in rapidly detecting pulsars from large amounts of survey data.</p>

<p> Optical microscopy is one of the oldest and most important imaging techniques; however, its far-field resolution is diffraction-limited. In this dissertation, we proposed and developed a novel method of optical microscopy with super-resolution by using high-index dielectric microspheres immersed in liquid and placed on the surface of the structures under study. We used barium titanate glass microspheres with diameters of D~2-220 &mu;m and refractive indices <i> n</i>&sim;1.9-2.1 to discern minimal feature sizes &sim;&lambda;/4 (down to &sim;&lambda;/7) of various photonic and plasmonic nanostructures, where &lambda; is the illumination wavelength. We studied the magnification, field of view, and resolving power, in detail, as a function of sphere sizes.</p><p> We studied optical coupling, transport, focusing, and polarization properties of linear arrays of dielectric spheres. We showed that in arrays of spheres with refractive index n=<rad><rcd>3</rcd></rad>, a special type of rays with transverse magnetic (TM) polarization incident on the spheres under the Brewster&#129;'s angle form periodically focused modes with radial polarization and <i> 2D</i> period, where <i> D</i> is the diameter of the spheres. We showed that the formation of periodically focused modes in arrays of dielectric spheres gives a physical explanation for beam focusing and extraordinarily small attenuation of light in such chains. We showed that the light propagation in such arrays is strongly polarization-dependent, indicating that such arrays can be used as filters of beams with radial polarization. The effect of forming progressively smaller focused beams was experimentally observed in chains of sapphire spheres in agreement with the theory.</p><p> We studied optical coupling,transport, focusing, and polarization properties of linear arrays of dielectric spheres. We showed that in arrays of spheres with refractive index n=&#129;&atilde;3, a special type of rays with transverse magnetic (TM) polarization incident on the spheres under the Brewster&#129;'s angle form periodically focused modes with radial polarization and 2D period, where D is the diameter of the spheres. We showed that the formation of periodically focused modes in arrays of dielectric spheres gives a physical explanation for beam focusing and extraordinarily small attenuation of light in such chains. We showed that the light propagation in such arrays is strongly polarization-dependent, indicating that such arrays can be used as filters of beams with radial polarization. The effect of forming progressively smaller focused beams was experimentally observed in chains of sapphire spheres in agreement with the theory.</p>

<p> The ever-present need for reducing greenhouse gas emissions associated with transportation motivates this investigation of a novel ignition technology for internal combustion engine applications. Advanced engines can achieve higher efficiencies and reduced emissions by operating in regimes with diluted fuel-air mixtures and higher compression ratios, but the range of stable engine operation is constrained by combustion initiation and flame propagation when dilution levels are high. An advanced ignition technology that reliably extends the operating range of internal combustion engines will aid practical implementation of the next generation of high-efficiency engines. This dissertation contributes to next-generation ignition technology advancement by experimentally analyzing a prototype technology as well as developing a numerical model for the chemical processes governing microwave-assisted ignition. </p><p> The microwave-assisted spark plug under development by Imagineering, Inc. of Japan has previously been shown to expand the stable operating range of gasoline-fueled engines through plasma-assisted combustion, but the factors limiting its operation were not well characterized. The present experimental study has two main goals. The first goal is to investigate the capability of the microwave-assisted spark plug towards expanding the stable operating range of wet-ethanol-fueled engines. The stability range is investigated by examining the coefficient of variation of indicated mean effective pressure as a metric for instability, and indicated specific ethanol consumption as a metric for efficiency. The second goal is to examine the factors affecting the extent to which microwaves enhance ignition processes. The factors impacting microwave enhancement of ignition processes are individually examined, using flame development behavior as a key metric in determining microwave effectiveness. </p><p> Further development of practical combustion applications implementing microwave-assisted spark technology will benefit from predictive models which include the plasma processes governing the observed combustion enhancement. This dissertation documents the development of a chemical kinetic mechanism for the plasma-assisted combustion processes relevant to microwave-assisted spark ignition. The mechanism includes an existing mechanism for gas-phase methane oxidation, supplemented with electron impact reactions, cation and anion chemical reactions, and reactions involving vibrationally-excited and electronically-excited species. Calculations using the presently-developed numerical model explain experimentally-observed trends, highlighting the relative importance of pressure, temperature, and mixture composition in determining the effectiveness of microwave-assisted ignition enhancement.</p>