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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Empirical Measurements of Massive Galaxy and Active Galaxy Evolution

Cool, Richard Jacob January 2008 (has links)
Using new wide-area galaxy redshift surveys, we explore the evolutionof the most massive galaxies and the most luminous quasars in the universe over much of cosmic history. Quasars and massive red galaxies both areextremes; the most luminous high redshift quasars likely play a key role in shaping their nearby environment and the universe as a whole. The mostmassive galaxies represent the end points of galaxy evolution and containa fossil record of the galaxy evolution process.Using the AGES redshift survey completed with the MMT and the Hectospecmulti-object spectrograph as well as new $z$-band observations of the NOAO Deep Wide-Field Survey Bootes field, we report the discovery of threenew quasars at z>5. We explore new mid-infrared selection in light of thesethree new quasars and place constraints on the slope of the high-redshiftquasar luminosity function.At lower redshift (0.1<z<0.4) we measure the scatter in red galaxy colorsaround the optical red-sequence using imaging and spectroscopy from theSloan Digital Sky Survey. With our sample of nearly 20,000 massiveearly-type galaxies (L>2.2L*), we find that the scatter around the color-magnitude relation is quite small in colors studied.Each of three model star formation histories can reproduce the scatter we measure, none of the models producecolor distributions matching those observed.We measure the evolution of the LRG luminosity function in the redshift range 0.1<z<0.9. We find that theLRG population has evolved little beyond the passive fading of its stellar populations since z~0.9. The most massive (L>3L*)red galaxies have grown by less than 50% (at 99% confidence) since z=0.9 in stark contrast to the factor of 2 to 4 growth observed in the L* red galaxy population over the same epoch.Finally, we introduce the PRIsm MUlti-object Survey (PRIMUS), a new redshiftsurvey aimed at collecting ~300,000 galaxy spectra over 10 sq. deg toz~1. We summarize the current status of PRIMUS observations and datareductions and present several survey statistics. PRIMUS is the largestexisting redshift survey at intermediate redshift and holds the largestsample of redshifts for Spitzer and X-ray detected objects.

Analysis of Titan's Neutral Upper Atmosphere from Cassini Ion Neutral Mass Spectrometer Measurements in the Closed Source Neutral Mode

Cui, Jun January 2008 (has links)
In this thesis I present an in-depth study of the distribution of various neutral species in Titan’s upper atmosphere, at altitudes between 950 and 1,500 km for abundant species (N₂, CH₄ as well as their isotopes) and between 950 and 1,200 km for most minor species. However, the study of the H2 distribution on Titan is extended to an altitude as high as 6,000 km in the exosphere. The analysis is based on a large sample of Cassini/INMS (Ion Neutral Mass Spectrometer) measurements in the CSN (Closed Source Neutral) mode, obtained during 15 close flybys of Titan. The densities of abundant species including N₂, CH₄ and H₂ are determined directly from their main channels. However, to untangle the overlapping cracking patterns of minor species, the technique of Singular Value Decomposition (SVD) is used to determine simultaneously the densities of various hydrocarbons, nitriles and oxygen compounds. All minor species except for ⁴⁰Ar present density enhancements measured during the outbound legs. This can be interpreted as a result of wall effects, which could be either adsorption/desorption or heterogeneous surface chemistry on the chamber walls. In the thesis, I use a simple model to describe the observed time behavior of minor species. Results on their atmospheric abundances are provided both in terms of direct inbound measurements assuming ram pressure enhancement and values corrected for wall adsorption/desorption. Among all minor species of photochemical interest, the INMS data provide direct observational evidences for C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₄H₂, C₆H₆, HC₃N and C₂N₂ in Titan’s upper atmosphere. Upper limits are put for other minor species. The globally averaged distribution of N₂, CH₄ and H₂ are each modeled with the diffusion approximation. The N₂ profile suggests an average thermospheric temperature of 154 K. The CH₄ and H₂ distribution constrains their fluxes to be 3.0×10⁹ cm⁻² s⁻¹ and 1.3×10¹⁰ cm⁻² s⁻¹, referred to Titan’s surface. The H₂ escape flux is about a factor of ∼3 higher than the Jeans value, which is interpreted as enhanced thermal escape driven primarily by an upward conductive heat flux. Such a conclusion is based on kinetic model calculations in the 13-moment approximation that require energy continuity at the upper boundary. On the other hand, a proper interpretation of the observed CH4 escape has to rely on the detailed nonthermal processes, which are still unknown at the present time. The INMS observations of the nitrogen isotope ratio implies ¹⁴N/¹⁵N= 131.6 near Titan’s surface. The profile of carbon isotope ratio combining INMS and GCMS results implies that both CH₄ and its isotope escape from Titan’s exobase with roughly the same drift velocity, in contrast to the Jeans case which requires that CH₄ escapes with a much larger velocity due to its smaller mass. The INMS data also suggest horizontal/diurnal variations of temperature and neutral gas distribution in Titan’s thermosphere. The equatorial regions, the ramside, as well as the nightside hemisphere of Titan appear to be warmer and present some evidences for the depletion of light species such as CH₄. Meridional variations of most heavy species are also observed, with a trend of depletion toward the north pole. Though some of the above variations might be interpreted by either the solardriven models or plasma-driven models, a physical scenario that reconciles all the observed horizontal/diurnal variations in a consistent way is still missing, With a careful evaluation of the effect of restricted sampling, some of the features shown in the INMS data are more likely to be observational biases.

Spitzer's Contribution to the AGN Population

Donley, Jennifer Lynn January 2009 (has links)
Using large multiwavelength datasets, we study obscured AGN in the distant universe that have been missed via traditional selection techniques (e.g. UV/ optical/X-ray). To do so, we take particular advantage of the mid-IR, which is minimally affected by obscuration. We first select as AGN candidates those objects whose radio emission is significantly brighter, relative to the mid-IR, than would be predicted by the well known radio/infrared correlation, indicating that the radio emission originates in the central engine. We find that of the 27 such sources identified in the CDF-N, 60% lack solid X-ray detections and 25% lack even 2σ X-ray emission. The absorbing columns of the faint X-ray–detected objects indicate that they are obscured but unlikely to be Compton thick, whereas the radio-excess AGN which are X-ray non-detected are Compton-thick candidates. We similarly use the infrared emission to select IRAC (3.6-8.0 μm) power-law AGN. In these luminous AGN, the hot dust emission from the AGN fills in the gap in a galaxy’s SED between the 1.6 μm stellar bump and the long-wavelength dust emission feature. While sources selected in this way are more luminous than the radio-excess AGN, we find a similar X-ray detection fraction. Of the 62 power-law galaxies in the CDF-N, only 55% are detected in the X-ray, and 15% lack evidence for even weak 2σ X-ray emission. A study of their X-ray properties indicates that ∼ 75% are obscured. Finally, we test IRAC color-color and infrared-excess selection criteria. We find that while these selection techniques identify a number of obscured AGN, they may also select a significant number of star-forming galaxies. By combining only the secure AGN candidates selected via all methods discussed above, we estimate that the addition of Spitzer-selected AGN candidates to the deepest Xray selected AGN samples directly increases the number of known AGN by 54- 77%, and implies a total increase to the number of AGN of 71-94%.

Building a Red Sequence in Cosmological Hydrodynamic Simulations

Gabor, Jared January 2011 (has links)
Despite years of study, the origins of red galaxies are not fully understood in a cosmological context. We develop new models for quenching star-formation and producing red galaxies in cosmological hydrodynamic simulations. We start with phenomenological models applied in post-processing to previously-run simulations. We focus separately on mergers and hot haloes – akin to “quasar mode” and “radio mode” feedback – as the drivers shutting down star-formation. With appropriate parameter choices, each model can produce a reasonably good match observed color-magnitude diagrams and red galaxy luminosity functions at redshift zero. We uncover some difficulties with these models in general, including red galaxy stellar populations that appear too blue by 0.1 magnitudes in g − r due to a metallicity deficit. Building on the post-processing models, we develop quenching models for simulations that run on-the-fly. Again, we test merger quenching and hot halo quenching separately. We model merger feedback as > 1000 km s⁻¹ winds motivated by observations of post-starburst galaxies, and wemodel hot halo feedback by continuously adding thermal energy to circum-galactic gas in haloes dominated by gas above 250, 000 K. Merger quenching temporarily shuts down starformation, butmerger-remnant galaxies typically resume star-formation with 1− 2 Gyrs thanks to accretion of newfuel fromthe IGM.Hot halo quenching successfully produces a realistic red sequence, providing a good match to the observed red galaxy luminosity function. Despite some minor difficulties with hot halo quenching, we examine its effects in more detail. Specifically, we study the evolution of the simulated red sequence over time. We find that galaxies with stellar mass ∼ 10¹¹M⊙ are the first to populate the red sequence at z ≳ 2, with significantly fewer red galaxies around 10¹⁰M⊙ until z ≈ 0. We show that massive galaxies grow substantially after moving onto the red sequence, primarily through minor mergers. We also examine the relationship between quenching and environment.


Skemer, Andrew January 2011 (has links)
I present the results of six papers related the formation and evolution of planets and planetary systems, all of which are based on high-resolution, ground-based, mid-infrared observations.The first three chapters are studies of T Tauri binaries. T Tauri stars are young, low mass stars, whose disks form the building blocks of extrasolar planets. The first chapter is a study of the 0.68"/0.12" triple system, T Tauri. Our spatially resolved N-band photometry reveals silicate absorption towards one component, T Tau Sa, indicating the presence of an edge-on disk, which is in contrast to the other components. The second chapter is an adaptive optics fed N-band spectroscopy study of the 0.88" binary, UY Aur. We find that the dust grains around UY Aur A are ISM-like, while the mineralogy of the dust around UY Aur B is more uncertain, due to self-extinction. The third chapter presents a survey of spatially resolved silicate spectroscopy for nine T Tauri binaries. We find with 90\%-95\% confidence that the silicate features of the binaries are more similar than those of randomly paired single stars. This implies that a shared binary property, such as age or composition, is an important parameter in dust grain evolution.The fourth chapter is a study of the planetary system, 2MASS 1207. We explore the source of 2MASS 1207 b's under-luminosity, which has typically been explained as the result of an edge-on disk of large, grey-extincting dust grains. We find that the edge-on disk theory is incompatible with several lines of evidence, and suggest that 2MASS 1207 b's appearance can be explained by a thick cloudy atmosphere, which might be typical among young, planetary systems.The fifth chapter is a study of the white dwarf, Sirius B, which in the context of this thesis is being studied as a post-planetary system. Our N-band imaging demonstrates that Sirius B does not have an infrared excess, in contrast to previous results.The sixth chapter is a study of mid-infrared atmospheric dispersion, which in the context of this thesis is being studied as an effect that will limit the ability of ELTs to image planetary systems. We measure mid-infrared atmospheric dispersion, for the first time, and use our results to confirm theoretical models.

Infrared Insights on the Nature and Evolution of Star-Forming Galaxies

Rujopakarn, Wiphu January 2012 (has links)
The peak of the star formation rate (SFR) of the Universe is widely accepted to be at 1 < z < 3, after which the SFR declined by more than an order of magnitude to the present level. The mechanisms resulting in the decline and the nature of individual star-forming galaxies at the peak of galaxy evolution, however, are poorly understood. This thesis summarizes an effort to understand both the statistical properties of star-forming galaxies and the physical conditions in individual galaxies at 0 < z < 3. I have studied the star formation (SF) sizes of local and high-z ultraluminous infrared galaxies (ULIRGs) using Pa-alpha, 24 micron and radio continuum observations and discovered that high-z ULIRGs have extended SF regions over 3-10 kpc, similar to local lower LIR SF galaxies, but with a scaled-up star formation rate surface density, ∑(SFR). Local ULIRGs, in contrast, are compact and invariably merger-triggered starbursts. A major implication to galaxy evolution is that there is a route besides major mergers to trigger very high levels of SF activity at z ~ 2, a conclusion further supported by our morphological study. I also find star formation rate surface density to be a good indicator of the IR galaxy spectral energy distribution universally and use this fact to develop a new SFR estimator using single-band 24 micron observations. The resulting indicator predicts IR luminosity and SFR within 0.15 dex of the values measured with far-IR photometry. This affords the deepest unobscured probe of SF at 0 < z < 3. According to my separate study, the spread of extinction values of SF galaxies is larger than previously known from optical observations and also indicates a large variety of dust distribution scenarios, from a uniform mixture that resembles the extinction screen assumption to inhomogeneous mixtures, which could undermine the assumptions commonly used to correct for extinction at high-z and necessitates the use of unobscured SF tracers. Lastly, I present the luminosity functions of galaxies and their evolution measured from IR observations out to z = 1.2.

The Optical Design of a Visible Adaptive Optics System for the Magellan Telescope

Kopon, Derek January 2012 (has links)
The Magellan Adaptive Optics system will achieve first light in November of 2012. This AO system contains several subsystems including the 585-actuator concave adaptive secondary mirror, the Calibration Return Optic (CRO) alignment and calibration system, the CLIO 1-5 μm IR science camera, the movable guider camera and active optics assembly, and the W-Unit, which contains both the Pyramid Wavefront Sensor (PWFS) and the VisAO visible science camera. In this dissertation, we present details of the design, fabrication, assembly, alignment, and laboratory performance of the VisAO camera and its optical components. Many of these components required a custom design, such as the Spectral Differential Imaging Wollaston prisms and filters and the coronagraphic spots. One component, the Atmospheric Dispersion Corrector (ADC), required a unique triplet design that had until now never been fabricated and tested on sky. We present the design, laboratory, and on-sky results for our triplet ADC. We also present details of the CRO test setup and alignment. Because Magellan is a Gregorian telescope, the ASM is a concave ellipsoidal mirror. By simulating a star with a white light point source at the far conjugate, we can create a double-pass test of the whole system without the need for a real on-sky star. This allows us to test the AO system closed loop in the Arcetri test tower at its nominal design focal length and optical conjugates. The CRO test will also allow us to calibrate and verify the system off-sky at the Magellan telescope during commissioning and periodically thereafter. We present a design for a possible future upgrade path for a new visible Integral Field Spectrograph. By integrating a fiber array bundle at the VisAO focal plane, we can send light to a pre-existing facility spectrograph, such as LDSS3, which will allow 20 mas spatial sampling and R~1,800 spectra over the band 0.6-1.05 μm. This would be the highest spatial resolution IFU to date, either from the ground or in space.

Measuring the Structure and Composition of Circumstellar Debris Disks

Ballering, Nicholas, Ballering, Nicholas January 2016 (has links)
In this dissertation, I measure the structure and composition of circumstellar debris disks to probe the underlying planetary systems. In Chapter 1, I provide an introduction to the field of debris disks. I highlight our current observational and theoretical understanding of the field, rather than providing a detailed history. This is intended to give the reader context and motivation for the subsequent chapters. I also describe important developments in debris disk science that are not the focus of this dissertation, but are nevertheless vital for a complete overview. In Chapter 2, I describe my analysis of a large sample of cold (≲130 K) debris disks seen in Spitzer/IRS data. Previous work had suggested a common temperature for these disk components, regardless of spectral type. I find that there is trend with spectral type and argue that the locations of cold disks are not set by snow lines, but more likely by the formation/evolution of planets. This work was published in Ballering et al. (2013). In Chapter 3, I turn my focus to the warm (~190 K) debris components identified in Chapter 2---specifically those exhibiting silicate emission features. I show that these features arise from exozodiacal dust in the habitable zones around these stars. This was published in Ballering et al. (2014). In Chapter 4, I examine the remainder of the warm disks to investigate what mechanism sets their location. I find that for many systems, the locations trace the water snow line in the primordial protoplanetary disk, rather than the current snow line. This favors the interpretation that warm debris components arise from asteroid belts in these systems. This study will be published soon. In Chapter 5, I analyze images of the debris disk around beta Pictoris at five different wavelengths, including in thermal emission and scattered light. I find that matching the disk brightness at all wavelengths constrains the composition of the dust, with a mixture of astronomical silicates and organic refractory material fitting the data well. This was published in Ballering et al. (2016). In Chapter 6, I conclude with a summary of this dissertation and prospects for future progress in these areas.

Effects of Spin on Neutron-Star Observations

Baubock, Michael, Baubock, Michael January 2016 (has links)
Measurements of neutron-star properties provide a natural way to test models of cold dense matter and theories of gravity. In order to correctly interpret these measurements, accurate models taking into account the special and general relativistic effects arising from the strong gravity and fast spin of these sources are necessary. Moreover, for some observables the effects of the rapid spin can dominate the measurement. In this thesis, I develop a ray-tracing algorithm using the Hartle-Thorne metric that allows me to determine the effects of gravity on several observables. I use this algorithm to calculate the bias introduced when observations are interpreted under the common assumption that the source is slowly spinning or not spinning at all. I show that this assumption can lead to errors in mass and radius measurements that are larger than the accuracy needed to distinguish between different models for the equation of state.

An assessment of professional development for astronomy and physics faculty| Expanding our vision of how to support faculty's learning about teaching

Olmstead, Alice Rose 04 October 2016 (has links)
<p> In this thesis, we will explore approaches to faculty instructional change in astronomy and physics. We primarily focus on professional development (PD) workshops, which are a central mechanism used within our community to help faculty improve their teaching. Although workshops serve a critical role for promoting more equitable instruction, we rarely assess them through careful consideration of how they engage faculty. To encourage a shift towards more reflective, research-informed PD, we developed the Real-Time Professional Development Observation Tool (R-PDOT), to document the form and focus of faculty's engagement during workshops. We then analyze video-recordings of faculty's interactions during the Physics and Astronomy New Faculty Workshop, focusing on instances where faculty might engage in pedagogical sense-making. Finally, we consider insights gained from our own local, team-based effort to improve a course sequence for astronomy majors. We conclude with recommendations for PD leaders and researchers.</p>

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