Thesis (Ph. D.)--Michigan State University. Dept. of Physics and Astronomy, 2008. / Title from PDF t.p. (viewed on July 22, 2009) Includes bibliographical references (leaves 170-174). Also issued in print.

<p>When studying the structure formation and fragmentation of complex atomic and nuclear systems it is preferable to start with simple systems where all details can be explored. Some of the knowledge gained from studies of atomic dimers can be generalised to more complex systems. Adding a third atom to an atomic dimer gives a first chance to study how the binding between two atoms is affected by a third. Few-body physics is an intermediate area which helps us to understand some but not all phenomena in many-body physics.</p><p>Very weakly bound, spatially very extended quantum systems with a wave function reaching far beyond the classical forbidden region and with low angular momentum are characterized as halo systems. These unusual quantum systems, first discovered in nuclear physics may also exist in systems of neutral atoms.</p><p>Since the first clear theoretical prediction in 1977, of a halo system possessing an Efimov state, manifested in the excited state of the bosonic van der Waals helium trimer ⁴₂He₃, small helium and different spin-polarised halo hydrogen clusters and their corresponding isotopologues have been intensively studied the last three decades.</p><p>In the work presented here, the existence of the spin-polarized tritium trimer ground state, ³₁H₃, is demonstrated, verifying earlier predictions, and the system's properties elucidated. Detailed analysis has found no found evidence for other bound states and shape resonances in this system. The properties of the halo helium trimers, ⁴₂He₃ and ⁴₂He₂-³₂He have been investigated. Earlier predictions concerning the ground state energies and structural properties of these systems are validated using our three-dimensional finite element method. In the last part of this work we present results on the bound states and structural properties of the van der Waals bosonic atomic trimers Ne₃ and Ar₃. We believe to be the first to find evidence of a possible shape resonance just above the three-body dissociation limit of the neon trimer.</p>

We present Hubble Space Telescope far-UV spectra of four QSOs whose sightlines pass through the halo of NGC 1097 at impact parameters of rho = 48-165 kpc. NGC 1097 is a nearby spiral galaxy that has undergone at least two minor merger events, but no apparent major mergers, and is relatively isolated with respect to other nearby bright galaxies. This makes NGC 1097 a good case study for exploring baryons in a paradigmatic bright-galaxy halo. Ly alpha absorption is detected along all sightlines and Si III lambda 1206 is found along the three sightlines with the smallest.; metal lines of C II, Si II, and Si IV are only found with certainty toward the innermost sightline. The kinematics of the absorption lines are best replicated by a model with a disk-like distribution of gas approximately planar to the observed 21 cm H I disk, which is rotating more slowly than the inner disk, and into which gas is infalling from the intergalactic medium. Some part of the absorption toward the innermost sightline may arise either from a small-scale outflow or from tidal debris associated with the minor merger that gives rise to the well known "dog-leg" stellar stream that projects from NGC 1097. When compared to other studies, NGC 1097 appears to be a "typical" absorber, although the large dispersion in absorption line column density and equivalent width in a single halo goes perhaps some way toward explaining the wide range of these values seen in higher-z studies.

We develop two new methods to measure the structure of matter around the Milky Way using stellar tidal streams from disrupting dwarf galaxies and globular clusters. The dark matter halo of the Milky Way is expected to be triaxial and filled with substructure, but measurements of the shape and profile of dark matter around the Galaxy are highly uncertain and often contradictory. We demonstrate that kinematic data from near-future surveys for stellar streams or shells produced by tidal disruption of stellar systems around the Milky Way will provide precise measures of the gravitational potential to test these predictions. We develop a probabilistic method for inferring the Galactic potential with tidal streams based on the idea that the stream stars were once close in phase space and test this method on synthetic datasets generated from N-body simulations of satellite disruption with observational uncertainties chosen to mimic current and near-future surveys of various stars. We find that with just four well-measured stream stars, we can infer properties of a triaxial potential with precisions of order 5--7 percent. We then demonstrate that, if the Milky Way's dark matter halo is triaxial and is not fully integrable (as is expected), an appreciable fraction of orbits will be chaotic. We examine the influence of chaos on the phase-space morphology of cold tidal streams and show that streams even in weakly chaotic regions look very different from those in regular regions. We discuss the implications of this fact given that we see several long, thin streams in the Galactic halo; our results suggest that long, cold streams around our Galaxy must exist only on regular (or very nearly regular) orbits and potentially provide a map of the regular regions of the Milky Way potential. We then apply this understanding of stream formation along chaotic orbits to the interpretation of a newly-discovered, puzzling stellar stream near the Galactic bulge. We conclude that the morphology of this stream is consistent with forming along chaotic orbits due to the presence of the time-dependent Galactic bar. These results are encouraging for the eventual goal of using flexible, time-dependent potential models combined with larger data sets to unravel the detailed shape of the dark matter distribution around the Milky Way.

The halos of galaxies - consisting of gas, stars, and satellite galaxies - are formed and shaped by the most fundamental processes: hierarchical merging and the flow of gas into and out of galaxies. While these processes are hard to disentangle, metals are tied to the gas that fuels star formation and entrained in the wind that the deaths of these stars generate. As such, they can act as important indicators of the star formation, the chemical enrichment, and the outflow histories of galaxies. Thus, this thesis aims to take advantage of such metal signatures in the stars and gas to place observational constraints on current theories of galaxy evolution as implemented in cosmological simulations. The first two chapters consider the metallicities of stars in the stellar halo of the Milky Way and its surviving satellite dwarf galaxies. Chapter 2 pairs an N-body simulation with a semi-analytic model for supernova-driven winds to examine the early environment of a Milky Way-like galaxy. At z=10, progenitors of surviving z=0 satellite galaxies are found to sit preferentially on the outskirts of progenitor halos of the eventual main halo. The consequence of these positions is that main halo progenitors are found to more effectively cross-pollute each other than satellite progenitors. Thus, inhomogeneous cross-pollution as a result of different high-z spatial locations of different progenitors can help to explain observed differences in abundance patterns measured today. Chapter 3 expands this work into the analysis of a cosmological, hydrodynamical simulation of dwarf galaxies in the early universe. We find that simple assumptions for modeling the extent of supernova-driven winds used in Chapter 2 agree well with the simulation whereas the presence of inhomogeneous mixing in the simulation has a large effect on the stellar metallicities. Furthermore, the star-forming halos show both bursty and continuous SFHs, two scenarios proposed by stellar metallicity data. However, the metallicity distribution functions of the simulated halos are both too metal rich and too peaked when compared to the data. This comparison reveals that a complex SFH and a broad metallicity distribution can develop rapidly in the early Universe. The third chapter moves to the present day with a consideration of the circumgalactic medium (CGM) around nearby Milky Way-like galaxies. We compare a cosmological simulation of a Milky Way-like galaxy to recent absorption line data and find that a reduced extragalactic ultraviolet background brings the column density predictions into better agreement with the data. Similarly, when the observationally derived physical properties of the gas are compared to the simulation, we find that the simulation gas is always at temperatures approximately 0.5 dex higher. Thus, similar column densities can be produced from fundamentally different gas. Metal-line emission is then considered as a complementary approach to studying the CGM. From the simulations, we find that the brightest emission is less sensitive to the extragalactic background and that it closely follows the fundamental filamentary structure of the halo. This becomes increasingly true as the galaxy evolves from z = 1 to z = 0 and the majority of the gas transitions to a hotter, more diffuse phase. Finally, resolution is a limiting factor for the conclusions we can draw from emission observations but with moderate resolution and reasonable detection limits, upcoming instrumentation should place constraints on the physical properties of the CGM. Future work advancing the techniques in this thesis remain promising for putting new observational constraints on our theories of galaxy evolution.

The abundance of molecular hydrogen (H2), the primary coolant in primordial gas, is critical for the thermodynamic evolution and star–formation histories in early protogalaxies. Suppression of H2–cooling in early protogalaxies can occur via photodissociation of H2 (by ultraviolet Lyman–Werner [LW] photons) or by photodetachment of H−, a precursor in H2 formation (by infrared [IR] photons). It is widely believed that the formation of the first massive black hole “seeds,” with masses 104−6 M⊙, in primordial halos may be enabled if H2–cooling is suppressed. We study the radiative feedback processes that suppress H2–cooling in primordial proto- galaxies. Previous studies have typically adopted idealized spectra, with a blackbody or a power–law shape, in modeling the chemistry of metal–free protogalaxies, and utilized a single parameter, the critical UV flux, or Jcrit, to determine whether H2–cooling is prevented. This can be misleading, as independent of the spectral shape, there is a a critical curve in the (kLW,kH−) plane, where kLW and kH− are the H2–dissociation rates by LW and IR photons, which determines whether a protogalaxy can cool below ∼ 1000 Kelvin. In Chapter 1, we use a one–zone model to follow the chemical and thermal evolution of gravitationally collapsing protogalactic gas, to compute this critical curve, and provide an accurate analytical fit for it. We improve on previous works by considering a variety of more realistic Pop III or Pop II-type spectra from population synthesis models and perform fully frequency–dependent calculations of the H2–photodissociation rates for each spectrum. We compute the ratio kLW/kH− for each spectrum, as well as the minimum stellar mass M∗, for various IMFs and metallicities, required to prevent cooling in a neighboring halo a distance d away. We provide critical M∗/d2 values for suppression of H2–cooling, with analytic fits, which can be used in future studies. Determining the photodissociation rate of H2 by an incident LW flux is crucial, but prohibitively expensive to calculate on the fly in simulations. The rate is sensitive to the H2 rovibrational distribution, which in turn depends on the gas density, temperature, and incident LW radiation field. In Chapter 2, we use the publicly available cloudy package to model primordial gas clouds and compare exact photodissociation rate calculations to commonly–used fitting formulae. We find the fit from Wolcott-Green et al. (2011) is most accurate for moderate densities n ∼ 103cm−3 and temperatures, T ∼ 103K, and we provide a new fit, which captures the increase in the rate at higher densities and temperatures, owing to the increased excited rovibrational populations in this regime. Our new fit has typical errors of a few percent percent up to n ≤ 107 cm−3, T ≤ 8000K, and H2 column density NH2 ≤ 1017 cm−2, and can be easily utilized in simulations. We also show that pumping of the excited rovibrational states of H2 by a strong LW flux further modifies the level populations when the gas density is low, and noticeably decreases self-shielding for J21 > 103 and n < 102cm−3. This may lower the “critical flux” at which primordial gas remains H2–poor in some protogalaxies, enabling massive black hole seed formation. In Chapter 3, we study the thermal evolution of UV–irradiated atomic cooling halos using high–resolution three–dimensional hydrodynamic simulations. We consider the effect of H− photodetachment by Lyα cooling radiation in the optically–thick cores of three such halos, a process which has not been included in previous simulations. H− is a precursor of molecular hydrogen, and therefore, its destruction can diminish the H2 abundance and cooling. We find that the critical UV flux for suppressing H2–cooling is decreased by up to a factor of a few when H− photodetachment by Lyα is included. In a more conservative estimate of the trapped Lyα energy density, we find the critical flux is decreased by ∼ 15 − 50 per cent. Our results suggest that Lyα radiation may have an important effect on the thermal evolution of UV–irradiated halos, and therefore on the potential for massive black hole formation.

When studying the structure formation and fragmentation of complex atomic and nuclear systems it is preferable to start with simple systems where all details can be explored. Some of the knowledge gained from studies of atomic dimers can be generalised to more complex systems. Adding a third atom to an atomic dimer gives a first chance to study how the binding between two atoms is affected by a third. Few-body physics is an intermediate area which helps us to understand some but not all phenomena in many-body physics. Very weakly bound, spatially very extended quantum systems with a wave function reaching far beyond the classical forbidden region and with low angular momentum are characterized as halo systems. These unusual quantum systems, first discovered in nuclear physics may also exist in systems of neutral atoms. Since the first clear theoretical prediction in 1977, of a halo system possessing an Efimov state, manifested in the excited state of the bosonic van der Waals helium trimer 42He3, small helium and different spin-polarised halo hydrogen clusters and their corresponding isotopologues have been intensively studied the last three decades. In the work presented here, the existence of the spin-polarized tritium trimer ground state, 31H3, is demonstrated, verifying earlier predictions, and the system's properties elucidated. Detailed analysis has found no found evidence for other bound states and shape resonances in this system. The properties of the halo helium trimers, 42He3 and 42He2-32He have been investigated. Earlier predictions concerning the ground state energies and structural properties of these systems are validated using our three-dimensional finite element method. In the last part of this work we present results on the bound states and structural properties of the van der Waals bosonic atomic trimers Ne3 and Ar3. We believe to be the first to find evidence of a possible shape resonance just above the three-body dissociation limit of the neon trimer.

Sprites, halos, and elves are members of a family of short-lived, luminous phenomena known as Transient Luminous Events (TLEs), which occur in the middle atmosphere. Sprites are vertical glows occurring at altitudes typically ranging from ~40 to 90 km. In video imagery they exhibit a red color at their top, with blue tendril-like structure at low altitudes. Elves are disk-like glows that occur at the base of the ionosphere, with diameters of ~100-300 km, and have very short lifetimes (~2-3 ms). Halos are diffuse glows that occur at low altitudes, have diameters <100 km, and have a duration that may last up to 10s of ms. A majority of the studies of TLEs have taken place over the Midwestern U.S. where they were first discovered. This area produces large thunderstorms, which in turn generate large lightning discharges that have been associated with the formation of TLEs. Studies have used the low frequency radiation that initiates with these strokes to study characteristics of these events. This low frequency radiation has been used to determine where large numbers of TLEs may occur. Extreme southern Brazil is a region of the globe believed to have many TLEs, but few studies on these phenomena. Two collaborative campaigns involving Utah State University proceeded in 2002-2003, and in 2006. Multiple TLE images were made, proving this is, indeed, a region of the globe where these types of events are prominent. In particular, one storm in February 2003 produced over 440 TLEs imaged by USU video cameras. Of these events, over 100 of them had associated halos. Statistical studies for halos previously had been performed in the U.S., but never abroad. Also, several events from the February storm have been associated with negative cloud to ground lightning, a surprising occurrence, as to date, less than a handful of such events have ever been witnessed or published. In analyzing the TLEs from this campaign, we have shown the halos are similar to those seen in the U.S., even though the storms may be somewhat different. Also, detailed analyses of the negative events show both temporal and spatial morphology heretofore never reported on.

Thèse doctorat : Astronomie : Strasbourg 1 : 2006. / Titre provenant de l'écran-titre. Bibliogr. 7 p.

INTRODUCTION/PURPOSE: Billions of people world wide suffer from refractive errors requiring glasses, contact lenses, or other means of correction to enable them to see better. Many people seeking permanent means to correct their vision consider undergoing corneal refractive laser surgeries (CRLS), photorefractive keratectomy (PRK), laser-assisted subepithelial keratectomy (LASEK), or laser in situ keratomileusis (LASIK). These surgeries have been shown to improve vision, but are not without risks for complications intra-operatively and postoperatively. Few studies have looked at the long-term incidence of postoperative complications such as dry eyes and visual aberrations and the associated preoperative risk factors. It is the aim of this study to examine the long-term incidence of dry eyes and visual aberrations (starbursts, halos, glare) after CRLS, and assess for preoperative risk factors associated with the persistence of these symptoms after surgery. METHODS: This study consisted of 319 patients identified for undergoing PRK, LASEK, or LASIK, at Boston Laser between December 2009 and January 2014. The participants in this study completed a novel online questionnaire consisting of questions to assess dry eye and visual aberration symptoms, and included questions adapted from the Ocular Surface Disease Index (OSDI). Postoperative dry eye symptoms were measured based on the OSDI score and a new dry eye measurement score created for this study's questionnaire. Presence or absence of visual aberration symptoms postoperatively were measured based on a score created for this study and derived from the calculation of the OSDI score. Additionally, a retrospective chart review was conducted of the 319 participants' medical charts to gather and assess for preoperative risk factors related to the long-term incidence of both dry eye and visual aberration symptoms. RESULTS: Our data found a significant association (p < 0.05) that suggests a relationship between development of long-term dry eye symptoms and the following preoperative variables: pupil size, flap thickness, and dry eye risk assessment (including: Zone Quick test values ≤ 9.0mm, contact lens use, and dry eyes with and without contact lenses). No significant association (p > 0.05) was found between the novel dry eye score and the preoperative factors, but it did approach significance with two variables, necessitating further investigation: gender and actual ablation. No significance (p >0.05) was found in the association between the preoperative dry eye risk assessment and severity of postoperative symptoms as gathered using the OSDI score. Our data found a significant association (p < 0.05) that suggests an increased risk for development of long-term visual aberrations symptoms postoperatively with the following preoperative variables: cylindrical manifest refraction, flat K, and greater actual flap thickness. As well as identifying two other possible variables that approached significance requiring further investigation: steep K and preoperative visual aberrations risk (including: spherical manifest refraction &#8805; -6.00, astigmatic manifest refraction ≥ -2.00, and pupil diameter ≥ 7.0). The data showed a significant association (p < 0.05) between postoperative symptom presence and the aforementioned preoperative visual aberrations risk. Our data showed no significance (p > 0.05) when comparing the difference between mean OSDI, dry eye, and visual aberration scores between participants grouped by years since surgery. DISCUSSION/CONCLUSION: Our data found a significant relationship between long-term dry eye risk after CRLS and preoperative pupil size, flap thickness, and dry eye risk assessment. Similarly the data also displayed a significant association between long-term visual aberration risk after CRLS and greater preoperative cylindrical manifest refraction, flat K, and flap thickness. These findings contribute to the risk factors identified in similar short-term follow-up studies, and support the need for increased research into the risk factors and long-term incidence of dry eyes and visual aberrations after CRLS. While the data showed no significance between participants grouped by years since surgery and reported postoperative symptoms, the OSDI mean scores did approach significance (p = 0.088), suggesting that further research with a greater survey population is required.