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.

Thesis (Ph.D.)--Indiana University, Dept. of Physics, 2005. / Source: Dissertation Abstracts International, Volume: 66-08, Section: B, page: 4269. Adviser: Charles J. Horowitz. Title from dissertation home page (viewed Oct. 10, 2006).

Despite significant advances in the numerical modeling of galaxy formation and evolution, it is clear that a satisfactory theoretical picture of how galaxies acquire their baryons across cosmic time remains elusive. In this thesis we present a computational study which seeks to address the question of how galaxies get their gas. We make use of new, more robust simulation techniques and describe the first investigations of cosmological gas accretion using a moving-mesh approach for solving the equations of continuum hydrodynamics. We focus first on a re-examination of past theoretical conclusions as to the relative importance of different accretion modes for galaxy growth. We study the rates and nature of gas accretion at z=2, comparing our new simulations run with the Arepo code to otherwise identical realizations run with the smoothed particle hydrodynamics code Gadget. We find significant physical differences in the thermodynamic history of accreted gas, explained in terms of numerical inaccuracies in SPH. In contrast to previous results, we conclude that hot mode accretion generally dominates galaxy growth, while cold gas filaments experience increased heating and disruption. Next, we consider the impact of feedback on our results, including models for galactic-scale outflows driven by stars as well as the energy released from supermassive black holes. We find that feedback strongly suppresses the inflow of "smooth" mode gas at all redshifts, regardless of its temperature history. Although the geometry of accretion at the virial radius is largely unmodified, strong galactic-fountain recycling motions dominate the inner halo. We measure a shift in the characteristic timescale of accretion, and discuss implications for semi-analytical models of hot halo gas cooling. To overcome the resolution limitations of cosmological volumes, we simulate a suite of eight individual 10^12 solar mass halos down to z=2. We quantify the thermal and dynamical structure of the gas in and around these halos. A radial sightline analysis allows us to measure the angular variability of halo gas properties, and demonstrate its increasing complexity at higher numerical resolution. We study the presence and characteristics of a strong virial shock, and make the link to recent observations of the circumgalactic medium surrounding galaxies. We conclude with a technically oriented presentation of the full public data release of the Illustris simulation. Our goal is to facilitate a new era of robust comparisons, between state of the art theoretical models of galaxy formation and the many rich observational surveys of galaxy populations across cosmic time. We describe the data itself, as well as the comprehensive interface and set of tools we have developed for its analysis. We discuss scientific issues relevant when interpreting the simulations, technical details of the release effort, and future goals. / Astronomy

Understanding the interior structures and chemistry of Earth-like exoplanets is crucial for us to characterize exoplanets, and to find potentially habitable planets. First, I provide a model grid of the mass-radius relations for solid planets in between 0.1 and 100 Earth masses. Planets are modeled as consisting of three layers: Fe, MgSiO3 and H2O. This model is made into an interactive tool available online: http://www.astrozeng.com/ Second, I explore the effects of thermal evolution and phase transitions on the interior structures of H2O-rich planets. It is shown that the bulk H2O in such planets may exist in the plasma, superionic, ionic, Ice VII, or Ice X states depending on sizes, ages, and cooling rates. The results suggest that super-Earth sized planets which are not significantly irradiated by parent stars and which are older than approximately 3 billion years, are mostly solid. Third, I describe a new, semi-empirical mass-radius relation for solid exoplanets. It is based on the recent mass and radius measurements of 5 exoplanets within 1 to 10 Earth masses and an extrapolation of the seismically derived pressure-density relation of the Earth's interior (PREM). The implication of common core mass fractions of 0.2~0.3 among these solid exoplanets is also discussed. Fourth, I model the elemental abundance patterns of solid exoplanets based on that of their host stars. This model is constructed from the following steps of planet formation: volatile depletion, core formation, and late delivery. This model could provide constraints on the chemical compositions of solid exoplanets in addition to the constraints derived from their masses and radii. In terms of future directions of this research, I hope to link my chemical model of solid exoplanets with the chemical evolution model of our galaxy, such as the one being developed by the Lars Hernquist group, which may indicate a different mineralogy of solid exoplanets formed at different ages of our galaxy, as well as the implications for the habitability of these planets. I also hope to understand the origins of the volatile contents on the surfaces of solid planets, which are important prerequisites for possible origins of life on them. / Astronomy

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The determination of the timing of impact events and the ages of cratered planetary surfaces is a complex and challenging undertaking. A powerful approach to this endeavor is a multidisciplinary study; understanding and using data from both rock samples and remote sensing. Shocked material (especially melt material) found in meteorites like Orvinio provides critical material dating impact craters. Orvinio, in spite of a complex degassing history shows evidence for multiple impacts at 4.2 Ga, 7.5 Ma, and possibly 330 Ma. Correlating impact histories for inner solar system bodies and the asteroid belt will constrain the genesis of impactor populations. Determining the recent cratering history of the Earth, however, is complicated by surface processes which erode and destroy impact craters. The Gardnos impact structure, for example, while possessing samples suitable for dating, has suffered substantial post-impact degassing due to metamorphism in the Caledonian orogeny ∼385 Ma. We must therefore look to the Moon to unravel the recent cratering history of the Earth-Moon system The Clementine mission data set provides an excellent resource for research into the bright rayed craters on the lunar surface. Studies of large rayed craters using the OMAT (optical maturity parameter) technique of Lucey and colleagues has revealed much information on the maturation of the crater ejecta. Profiles of OMAT values for the ejecta of large craters as a group show no evidence for an increase in the cratering rate during the Copernican era as advocated by Shoemaker. Future studies of both remote sensing and sample data will allow a better understanding of meteorite parent body impact histories and their implications for widespread epochs of increased impactor flux; the age-size correlation in lunar craters; the calibration of a large crater relative age scheme based on optical maturity with implications for the nature of the impactor flux in recent history; the nature of the recent small impactors on the lunar surface; implications for impact hazards on Earth today; and the best target sites for future landings and sample acquisition on the lunar surface.

When applied to radio astronomy instrumentation, practical limitations on circuitry restrict digital correlators to extremely coarse quantization. This paper examines non-linear effects of modified two-bit quantization on astronomical correlators. Equations are presented to correct quantization bias in estimates. The degradation due to quantization is given and plotted. The necessary tolerance in threshold levels is established to avoid systematic errors in power spectrum measurement. An alternative method of measuring power is derived that reduces parameter sensitivity.

A systematic search (Dow and Sykes 1988) for cometary dust trails in the Infrared Astronomical Satellite (IRAS) Sky Brightness Images (1988) resulted in the discovery of 466 sources (Dow et al. 1990) that are not in the IRAS Point Source Catalog (Version 2 1988) or in the IRAS Small Scale Structure Catalog (1988). Nearly all of the sources that were found are best explained as artificial satellites or pieces of Earth orbiting debris. This study addresses two questions. To what degrees have the Sky Brightness Images been contaminated by orbital debris? Second, can valuable information concerning the thermal characteristics of these sources be obtained by suitably analyzing IRAS data? Fifty-four sources, covering a range of positions and observed fluxes, were selected from the main sample to determine their angular motion, flux density and color temperature distributions. Four of these objects were correlated with known artificial satellites. (Abstract shortened with permission of author.)

<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>