Theses and Dissertations - Department of Physics & Astronomy

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Coupling semi-analytic models and N-body simulations: a new way of making galaxies and stellar halos
(University of Alabama Libraries, 2017) McCord, Krista; Bailin, Jeremy; University of Alabama Tuscaloosa
Stellar halos give insight into the initial conditions that existed when a host galaxy first formed and provide details on disrupted satellites via their different stellar populations. An algorithm that is computationally inexpensive compared to hydrodynamic simulations is necessary in order to theoretically study the structure and formation of galactic stellar halos in sufficient detail to probe substructure. CoSANG (Coupling Semi-Analytic/N-body Galaxies) is a new computational method that we are developing which couples pure dark matter N-body simulations with a semi-analytic galaxy formation model. At each timestep, results from the N-body simulation feed into the semi-analytic code, whose results feed back into the N-body code making the evolution of the dark matter and baryonic matter dependent on one another. CoSANG will enable a variety of galaxy formation science, including analysis of stellar populations, halo merging, satellite accretion, supermassive black holes, and indirect and direct dark matter detection. In this dissertation, I will describe the new simulation code CoSANG. The results from the extensive testing phase on CoSANG will be presented which indicate CoSANG is properly simulating feedback from galaxies within a dark matter halo. I used this validated code to analyze a CoSANG zoom simulation of a $10^{12} \mathrm{M_{\odot}}$ dark matter halo. Results showed a flatter inner halo near the disk and a more spherical outer halo which is expected when a galaxy exists at the center of a dark matter halo. A comparison is made with a simulation run with the same initial conditions, but with the baryonic component simulated using a hydrodynamic algorithm. The semi-analytic model predicted galaxy types better than the hydrodynamic simulation leading to the conclusion that the CoSANG halo is more accurate. I also present a dark matter direct detection analysis on the CoSANG zoom halo to measure the dark matter velocity distributions and modulation amplitudes. The CoSANG results show that the dark matter velocity distribution does not fit well to a Maxwell Boltzmann distribution and the modulation amplitudes derived indicate an anisotropic dark matter velocity distribution. Future work will include tagging dark matter particles with stellar properties to build and evolve a stellar halo.
Dark matter decays from the galactic center using IceCube-86
(University of Alabama Libraries, 2017) Pepper, James Alan; Williams, Dawn R.; University of Alabama Tuscaloosa
Most searches for Dark Matter primarily focus on the WIMP paradigm, which predicts dark matter masses in the GeV - 10 TeV range. However, these relatively low energy searches continue to produce null results, possibly suggesting that dark matter is something other than WIMPs. Gravitinos, on the other hand, can satisfy the cosmological constraints on dark matter, and decay with a lifetime orders of magnitude longer than the age of the universe, producing extremely high energy neutrinos. The IceCube Neutrino Observatory has already had success detecting EHE extragalactic neutrinos, and is well suited to search for dark matter in this high energy regime. This analysis sets limits on the gravitino lifetime from the high energy neutrino events observed at IceCube using three possible astrophysical explanations of the neutrino flux. The most conservative limit on the gravitino lifetime using the softest two-body decay mode was found to be $\tau_{DM} = 10^{27.6}s$. This is the first analysis developed to place a limit on the gravitino lifetime using IceCube software and simulation files, and the results are comparable to theoretical limits based on the same data set.
Dust mapping of spiral galaxy disks from the Hubble starsmog survey
(University of Alabama Libraries, 2016) Bradford, Sarah; Keel, W. C.; University of Alabama Tuscaloosa
Dust in spiral galaxy disks plays an important role in observations through its interaction with starlight. While arguments have been made in favor of completely opaque spiral disks as a result of the dust (see Valentijn, 1990), most subsequent studies agree that spiral galaxies include both optically thick and optically thin regions. Using the occulting-galaxies technique introduced by White & Keel (1992), we created transmission maps of nine partially overlapped galaxy systems taken from the STARSMOG (STarlight Absorption Reduction through a Survey of Multiple Occulting Galaxies) catalog to probe the optical depth in regions of backlit dust. The measurements of all nine targets show a wide range of opacities, and therefore none of them can be definitively classified as either "optically thick" or "optically thin". In the analysis of the transmission maps of predominantly highly inclined systems, we noticed a distinct edge to the dust structure beyond which no dust was present in the foreground galaxy disk. This radius varied amongst systems but was distinct enough to be measured for three of the systems. The transmission maps also reveal a web-like pattern to the dust structure in some galaxy disks. Four of the targets showed dust structure which not only followed the pitch angle of the spiral arms, but also crossed at other various angles. This web-like pattern was only observed in systems where the foreground galaxy was of Hubble type SBc/Sc/Sbc. While there appears to be no real trend in the statistics of transmission through backlit dust with respect to stellar mass of the galaxies, there does appear to be a correlation with respect to star formation rates. In this nine-system sample, the targets with the least star formation appear more opaque. By continuing this study for the rest of the STARSMOG targets, we will be able to determine how representative this sample is of the entire target set.
Finding fossil galaxy system progenitors using strong gravitational lensing
(University of Alabama Libraries, 2018) Johnson, Lucas Edward; Irwin, Jimmy A.; University of Alabama Tuscaloosa
Fossil galaxy systems have been described as some of the oldest systems in the universe, where the central galaxy has cannibalized most nearby member galaxies over cosmic time. While the progenitors to fossil systems have been predicted to exist in numerical simulations, little effort has gone into locating them until now. The discovery of fossil progenitors in the CASSOWARY catalog of strong gravitational lensing demonstrates that not all fossils are old, and their formation histories are more complex than originally thought. These progenitors have optical characteristics consistent with them being the transition phase between non-fossils and fossils, as we are observing the central galaxies in mid-assembly. We also identify a bias where systems acting as strong gravitational lenses are ~5 times more likely to be seen as fossils than non-lensing systems. Chandra X-ray images of eight CASSOWARY fossil progenitors show them being significantly over-luminous and hotter than comparable non-fossils which could be due to the strong lensing bias in our data, or fossils have characteristically deeper potential wells than non-fossils. Two progenitors were luminous enough to see a rise in gas temperature toward their cores which suggests these may be undergoing group mergers akin to the previously studied progenitor CSWA 2 verifying this as a viable fossil formation mechanism. Refinements to our original CASSOWARY data using the Hubble Space Telescope allowed us to disentangle complex merging environments at the centers of these eight progenitors, which further solidified the notion that progenitors are indeed transitioning toward fossil systems.
Investigating the effects of progenitor metallicity on observable features of chandrasekhar mass type ia supernovae and improving detonation models of sub-chandrasekhar mass type ia supernovae
(University of Alabama Libraries, 2018) Miles, Broxton Jake; Townsley, Dean M.; University of Alabama Tuscaloosa
Type Ia Supernovae are generally considered to be the result of the thermonuclear disruption of carbon oxygen white dwarfs. However, the exact mechanism behind the explosion remains uncertain. The pre-explosion progenitor of a white dwarf has never been observed, so all conclusions must be drawn from comparisons between observed events and computational models. Here, work is presented on identifying spectral features indicative of progenitor metallicity. Metallicity affects the production of alpha-chain elements, which leaves imprints in the spectra. Two features are found that may be signals of progenitor metallicity, a Ti feature at 4200 \AA\ and an Fe feature at 5500 \AA . The second portion of this work focuses on the accurate modeling of detonations in sub-Chandrasekhar mass type Ia supernovae. The scales of the burning processes involved, compared to the size of the white dwarf, make fully resolving the detonation computationally impossible in full-star simulations. To mitigate this problem, past studies have used sub-grid scale models that attempt to capture the energetics of the explosion and post-process the results to calculate their models' nucleosynthetic products. If sub-grid models are to be believed, they must have accurate treatments of detonation physics such as curvature and shock strengthening. In low-density regions of the white dwarf, the curvature of the detonation front slows its propagation, affecting the production of intermediate mass elements. We find that the sharp density gradient in the outer radii of the white dwarf counteracts the weakening effect of curvature, resulting in more complete burning than expected in this low density region.
Mass dependent galaxy transformation mechanisms in the complex environment of SuperGroup Abell 1882
(University of Alabama Libraries, 2013) Sengupta, Aparajita; Buta, Ronald James; University of Alabama Tuscaloosa
We present our data and results from panchromatic photometry and optical spectrometry of the nearest (extremely rich) filamentary large scale structure, SuperGroup Abell 1882. It is a precursor of a cluster and is an inevitable part of the narrative in the study of galaxy transformations. There has been strong empirical evidence over the past three decades that galaxy environment affects galaxy properties. Blue disky galaxies transform into red bulge-like galaxies as they traverse into the deeper recesses of a cluster. However, we have little insight into the story of galaxy evolution in the early stages of cluster formation. Besides, in relaxed clusters that have been studied extensively, several evolutionary mechanisms take effect on similar spatial and temporal scales, making it almost impossible to disentangle different local and global mechanisms. A SuperGroup on the other hand, has a shallower dark-matter potential. Here, the accreting galaxies are subjected to evolutionary mechanisms over larger time and spatial scales. This separates processes that are otherwise superimposed in rich cluster-filament interfaces. As has been found from cluster studies, galaxy color and morphology tie very strongly with local galaxy density even in a complex and nascent structure like Abell 1882. Our major results indicate that there is a strong dependence of galaxy transformations on the galaxy masses themselves. Mass- dependent evolutionary mechanisms affect galaxies at different spatial scales. The galaxy color also varies with radial projected distance from the assumed center of the structure for a constant local galaxy density, indicating the underlying large scale structure as a second order evolutionary driver. We have looked for clues to the types of mechanisms that might cause the transformations at various mass regimes. We have found the thoroughly quenched low mass galaxies confined to the groups, whereas there are evidences of intermediate-mass quenched galaxies even in the far outskirts. However, unlike what we observe in this system, ideally would we expect the dwarf galaxies with their shallow potentials to be more vulnerable than more massive galaxies, and hence be quenched earlier. We propose harassment and/or ram-pressure stripping as the mechanism that might lead to the quenched galaxies near or inside the high density, high velocity dispersion region in and near the groups; and mergers as the mechanism for the intermediate mass quenched galaxies at the low density, low velocity dispersion outskirts. We also identify a starburst population preferentially occurring within the filaments, at least a subset of which must be progenitors of the quenched galaxies at the core of Abell 1882. This also indicates a higher degree of preprocessing within the filaments as compared to that of the field.
A Monte Carlo approach to 7Be solar neutrino analysis with KamLAND
(University of Alabama Libraries, 2012) Grant, Christopher; Piepke, Andreas; University of Alabama Tuscaloosa
Terrestrial measurements of neutrinos produced by the Sun have been of great interest for over half a century because of their ability to test the accuracy of solar models. The first solar neutrinos detected with KamLAND provided a measurement of the ^8B solar neutrino interaction rate above an analysis threshold of 5.5 MeV. This work describes efforts to extend KamLAND's detection sensitivity to solar neutrinos below 1 MeV, more specifically, those produced with an energy of 0.862 MeV from the ^7Be electron-capture decay. Many of the difficulties in measuring solar neutrinos below 1 MeV arise from backgrounds caused abundantly by both naturally occuring, and man-made, radioactive nuclides. The primary nuclides of concern were ^210Bi, ^85Kr, and ^39Ar. Since May of 2007, the KamLAND experiment has undergone two separate purification campaigns. During both campaigns a total of 5.4 ktons (about 6440 m^3) of scintillator was circulated through a purification system, which utilized fractional distillation and nitrogen purging. After the purification campaign, reduction factors of 1.5 x 10^3 for ^210Bi and 6.5 x 10^4 for ^85Kr were observed. The reduction of the backgrounds provided a unique opportunity to observe the ^7Be solar neutrino rate in KamLAND. An observation required detailed knowledge of the detector response at low energies, and to accomplish this, a full detector Monte Carlo simulation, called KLG4sim, was utilized. The optical model of the simulation was tuned to match the detector response observed in data after purification, and the software was optimized for the simulation of internal backgrounds used in the ^7Be solar neutrino analysis. The results of this tuning and estimates from simulations of the internal backgrounds and external backgrounds caused by radioactivity on the detector components are presented. The first KamLAND analysis based on Monte Carlo simulations in the energy region below 2 MeV is shown here. The comparison of the Δχ^2 between the null hypothesis and the existence of the ^7Be solar neutrino signal in the data shows a change of 27.9 units, providing evidence that the signal is statistically favored. This analysis reports a measured interaction rate from ^7Be solar neutrinos of R = 343.3 ± 65.0(stat) ± 99.2(syst) events/(kton·day), which corresponds to a total flux of ɸ = (3.41 ±; 1.18) x 10^9 cm^-2 s^-1. The ^7Be solar neutrino flux reported in this work is only the second measurement made of this quantity worldwide. It provides an important cross-check of the Borexino experiment. The flux measurement reported here agrees within 1σ with the standard solar model predictions thus validating our basic understanding of solar fusion reaction processes.
Multiwavelength selection of obscured agn and contributions to the X-ray background
(University of Alabama Libraries, 2011) May, Branyon; Keel, W. C.; University of Alabama Tuscaloosa
Obscuration in AGN is a crucial component to understanding the observed spectrum of the X-ray Background. We tested numerous AGN selection techniques in X-ray, mid-infrared, and optical to test for multiwavelength correlations and to help establish selection criteria for obscured AGN. With AGN sources dominating background X-ray sources, we selected medium-depth archival Chandra observations covering 5.6 deg^2 of sky and generating a large sample of serendipitous X-ray sources (greater than 10,000). The mid-infrared component came from archival Spitzer data, with ~3,500 sources being detected in at least two IRAC bands and 1,485 in all 4 bands. For the optical component, greater than 70% of the Chandra observations also had full coverage within the SDSS Data Release 7, >2,300 optical counterparts and 125 spectra. In analyzing the sample, we have identified the parameter spaces in the X-ray/mid-infrared/optical that are optimized for containing members of the elusive class of obscured AGN, and provide a candidate list. We cross-check our X-ray number counts and source densities with contributions to the X-ray background, and find that we resolve approximately 90% of the X-ray Background in the 0.5-8.0 keV range. Testing populations divided on X-ray hardness and flux-level confirms that the unresolved hard X-ray background will be dominated by large populations of increasingly fainter and harder sources.
A photometric study of pure disk galaxies
(University of Alabama Libraries, 2014) Brockett, Timmy; Buta, Ronald James; University of Alabama Tuscaloosa
Pure disk galaxies are galaxies that form and evolve without a central bulge region. This morphology of galaxy is relatively unexplained and has yet to be successfully simulated using Lambda-Cold Dark Matter (ΛCDM) model parameters. The ΛCDM model is the standard framework from which astronomers and physicists understand and predict the Universe due to confirmed predictions such as the cosmic microwave background and the large scale structure of galaxy clusters. However, ΛCDM has yet to have a benchmark, observationally confirmed prediction on the galactic scale. This thesis is a study of eleven pure disk galaxies. Understanding this type of galaxy is very important in rectifying the incompatibility with the ΛCDM model. The method of analysis includes obtaining, cleaning and sky subtracting images from the Sloan Digital Sky Survey Data Release 7, deprojecting the images for a face on perspective, using g- and i-bands to construct color-index maps, using Fourier decompositions to create mode-dependent intensity ratio plots, surface density maps, mass-to-light ratio maps and surface brightness profiles, from which the radial scale length is derived. The future of this area of study is vital to understand a common feature of our Universe. Future studies can include looking for early supernova remnants or evidence of recent active galactic nuclei activity in young pure disk galaxies. Surveys and photometric analysis of edge-on pure disk galaxies may also reveal vital information to the origin and evolution of this class of galaxy.
Quantitative analysis of 3-arm spiral galaxies
(University of Alabama Libraries, 2019) Hancock, Colin; Keel, William C.; University of Alabama Tuscaloosa
A relatively small fraction of spiral galaxies has three spiral arms. It has been theorized that these "3-arm spirals" are relatively unstable and prone to decaying into even numbered patterns due to tidal interactions. We present a series of quantitative analyses on a large sample of 3-arm spiral galaxies selected by the Galaxy Zoo 2 group. Much of this analysis used the image processing interface known as the Spiral Arc Finder and Reporter (SpArcFiRe). This program traced spiral arms on submitted images of galaxies and provided information that allowed us to replicate the arms in another program like MS Excel. Most of our work involved images taken from the Sloan Digital Sky Survey (SDSS), supplemented with high-resolution Hubble Space Telescope (HST) observations. This allowed us to study the morphological demographics of our sample, for both the internal structure and arm symmetry. The HST images provided special insight into spiral structure and star formation for these galaxies. We analyzed the star formation in particular using the MIRA AL software for photometry. We also attempted in improve the signal-to-noise of our images using data from the Stripe 82 region of SDSS. Unexpectedly, 3-arm patterns coexist with strong central bars roughly as often as other spiral patterns. We also found that 3-arm spirals do not preferentially exist in low-density regions and may be triggered by interactions.
Search for astrophysical tau neutrinos in three years of IceCube data
(University of Alabama Libraries, 2015) Xu, Donglian; Williams, Dawn R.; University of Alabama Tuscaloosa
High-energy cosmic neutrinos are expected to be produced in extremely energetic astrophysical sources such as active galactic nuclei (AGNs) and gamma ray bursts (GRBs). The IceCube Neutrino Observatory has recently detected a diffuse astrophysical neutrino flux at 5.7σ significance. One of the outstanding questions regarding astrophysical neutrinos is their flavor composition. Most standard oscillation scenarios predict tau neutrinos (υ_τ) in the astrophysical flux, which have a negligible background from cosmic ray induced atmospheric neutrinos. This work reports a search for high-energy astrophysical υ_τ with the IceCube neutrino observatory. This analysis has devised a new method to search for astrophysical υ_τ in the IceCube waveforms, and it is the first υ_τ analysis in IceCube that is more sensitive to a υ_τ flux than other neutrino flavors. A total of three years of data was used, yielding a sensitivity of 5.1 x 10^-8 GeV cm^-2 s^-1 sr^-1 near the PeV region assuming a flux limit of E^2 Φ_υ < 1.0 x 10^-8 GeV cm^-2 s^-1 sr^-1 per neutrino flavor. Zero events were found. Differential upper limits for astrophysical υ_τ are derived in the Ο(100) TeV to Ο(10) PeV regime based on the zero findings.
Simulation and identification of non-Poissonian noise triggers in the IceCube neutrino detector
(University of Alabama Libraries, 2013) Larson, Michael James; Williams, Dawn R.; University of Alabama Tuscaloosa
The IceCube neutrino detector, located in the clear glacial ice at the South Pole, completed construction in 2011. The low-energy infill extension, DeepCore, forms a denser sub-detector using higher quantum efficiency photosensors. DeepCore has been taking data since May 2010 and lowers IceCube's energy threshold to about 10 GeV. These low-energy events are dim compared to higher energy events, necessitating the study of low-light backgrounds. While Monte Carlo predictions give an expected rate of approximately 6 Hz due to atmospheric muons, DeepCore records a significantly higher rate of 13.5 Hz with most of the discrepancy due to unsimulated noise events. Much of the rate difference may be resolved by rejecting especially dim events by counting the number of locally coincident hits, retaining 55% of the ν_e and 62% of the ν_μ events sampled with energies of 10-300 GeV while rejecting 96% of noise events. However, differences in the timing distributions of noise hits indicates a need for further study. A new source of correlated noise has been discovered, necessitating an updated noise simulation model. IceCube's new noise generator is able to reproduce the correlated noise in both IceCube and DeepCore sensors. A Metropolis-Hastings algorithm has been used to identify relevant parameters for nearly all of the 5160 sensors that make up the IceCube detector. Initial low quality fits reduce the rate discrepancy between data and simulation from 32% using a Poissonian noise model to 20% using the updated noise model with additional reduction possible. Noise which triggers the DeepCore detector is evaluated and rejected using the NoiseEngine filtering module. Minimal cleaning removes 94% of noise triggers while retaining 85% of ν_e and 87% of ν_μ events with energies of 10-300 GeV. Stringent cleaning removes 99.9% of noise triggers while retaining only 55% of ν_e and 60\% of ν_μ signal events in the same energy range.
A study of white dwarfs: cataclysmic variables and double-detonation supernovae
(University of Alabama Libraries, 2019) Caldwell, Spencer; Townsley, Dean M.; University of Alabama Tuscaloosa
Novae, be it classical, dwarf, or supernovae, are some of the most powerful and luminous events observed in the Universe. Although they share the same root, they are produced by different physical processes. We research systems capable of experiencing novae with the intention of furthering our understanding of these astrophysical phenomena. A cataclysmic variable is a binary star system that contains a white dwarf with the potential of undergoing classical or dwarf novae. A recent observation of a white dwarf within one of these systems was found to have an unusually high surface temperature for its orbital period. The discovery contradicts current evolutionary models, motivating research to determine a theoretical justification for this outlier. Using MESA (Modules for Experiments in Stellar Astrophysics), we simulated novae for a progenitor designed to represent a white dwarf in an interacting binary. We developed post-novae cooling timescales to constrain the temperature value. We found the rate at which classical novae cool post-outburst (< 1 K yr−1) is in general agreement with the four−year follow-up observation (∼ 2 K). The evolution of white dwarfs during double- detonation type Ia supernovae was also studied. The progenitors capable of producing these events are not fully established, requiring a consistent model to be developed for parametric analysis. Three improvements were made to the simulation model used in (Townsley et al., 2019): the inclusion of a de-refinement condition, a new particle distribution, and a burning limiter. The focus here was to enhance the computational efficiency, offer better representation of particles in the supernova ejecta, and control the nuclear energy release. These developments were employed to test double-detonation scenarios capable of producing spectra analogous to type Ia supernovae, which will offer insight into their prevalence and strengthen their use in measuring cosmological distance.
Testing the Sersic bulge: black hole mass relation in Seyfert galaxies
(University of Alabama Libraries, 2010) May, Alana; Keel, W. C.; University of Alabama Tuscaloosa
Using a sample of Active Galactic Nuclei (AGN), we investigated the relationship between host galaxy and black hole mass using the Sérsic index. We performed two-dimensional (2-D) decompositions of high-resolution Hubble Space Telescope images of (AGN) using GALFIT 3 beta. Taking independent mass estimates for a subsample of the selected galaxies, we test both linear and quadratic regressions in order to find an optimal relation for estimating black hole mass in other galaxies. Our results show that there was little difference between the linear and higher order fits. We examine the effects of these analysis techniques on the black hole mass to luminosity relationship. Application of the data was also looked at concerning properties of pseudo- and classical bulges.
X-ray observations of hot gas in early-type galaxies and groups of galaxies
(University of Alabama Libraries, 2013) Su, Yuanyuan; White, Raymond E.; Irwin, Jimmy A.; University of Alabama Tuscaloosa
Most of the baryonic matter in the Universe is not locked in stars. Instead, groups and clusters of galaxies as well as early-type galaxies contain a large mass of baryons in X-ray emitting hot gas. The study of such hot gas provides us a better understanding not only about the evolution of hierarchical structure formation and metal enrichment processes but also about baryon physics such as radiative cooling, ram pressure stripping, heating from active galactic nucleus (AGNs) and galactic winds. Moreover, such knowledge is invaluable for us to probe cosmology through galaxy clusters. In this dissertation I investigated the hot gas properties of galaxies and galaxy groups in three major scientific projects: (1) The measured metal abundance of the hot gas in early-type galaxies has been known to be lower than theoretical expectations. This may be related to the dilution of hot gas by mixing with cold gas. We studied the hot gas metal abundance with a sample of 32 early-type galaxies observed by Chandra and XMM-Newton. We find that there is virtually no correlation between hot gas Fe abundances and their atomic gas content. In contrast, we demonstrate a negative correlation between the measured hot gas Fe abundance and the ratio of molecular gas mass to hot gas mass. (2) We studied the X-ray brightest fossil group (poor cluster) ESO~3060170 out to its virial radius with Suzaku. The entropy and pressure profiles in the outer regions are flatter than in simulated clusters, similar to what is seen in observations of massive clusters. This may indicate that the gas is clumpy and/or the gas has been redistributed. (3) The nearby group centered on its bright central galaxy NGC~1407 has been thought to be an unusually dark system from previous kinematic studies. It is also known for hosting a bright galaxy NGC~1400 with a huge radial velocity difference (1200 km s$^{-1}$) with respect to the group center. We investigated the NGC~1407/1400 complex with XMM-Newton and Chandra observations. We show that a region of enhanced surface brightness between NGC~1407 and NGC~1400 is likely to be hot gas stripped from NGC~1400's ISM. We inferred that NGC~1407 system has a normal mass-to-light ratio from an X-ray--determined hydrostatic mass estimate.

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