1,721,088 research outputs found
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Simulations of Runaway Processes in Astrophysics
Get PDFNumerical and N-body simulations are used across astrophysics in problems ranging from the small-- modeling stellar winds, or planetary atmospheres-- to the large-- in simulations of large patches of cosmos or individual galaxies. Applications studied in this dissertation span this entire range of scales of the cosmos and include the exploration for alternate states of habitability on extrasolar planets, and modeling gravothermal collapse in Self-Interacting-Dark-Matter (SIDM) halos. I develop simulation codes to study planetary habitability on the edge of the runaway greenhouse and (runaway) core collapse in DM halos that galaxies reside in. A variety of common code implementations for SIDM N-body simulations is tested and numerical effects are outlined for typical usages of such codes. With these models, the Eccentric Habitable Zone (EHZ) is constrained for wide range of planetary orbits, and I present a set of equations to predict the evolution of SIDM halos under any velocity-dependent self-interaction cross section. Additionally, we explore the potential for a new planetary habitable state we termed ''Terminator Habitability" on water rich planets
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Gravothermal Evolution of Generic Self-Interacting Dark Matter Models
Get PDFIn self-interacting dark matter (SIDM) halos undergoing gravothermal evolution, the cores of halos are driven into the short-mean-free-path (SMFP) regime and undergo core collapse. In this dissertation, I study generic SIDM models with velocity-dependent cross-sections and elastic scattering, in SIDM halos experiencing gravothermal collapse. We study the structure and dynamics of halo cores in the SMFP and discover a new approximate universality deep in the SMFP regime. This new SMFP universality allows for the mapping of velocity- dependent cross sections to constant ones in this regime, offering improved predictions for physics occurring in the SMFP evolution of SIDM halos. We devise a semi-analytic recipe to predict the entire SMFP evolution of halos with our newfound SMFP universality by relating this new phase to the long-mean-free-path (LMFP) regime. This methodology streamlines the procedure of acquiring core properties within the SMFP regime and allows for estimates of the minimum mass of the black hole left behind, using simply the initial halo characteristics and particle physics parameters
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Cosmological Simulations of Self-Interacting Dark Matter: A Study of Gravothermal Collapse in Low-Mass Galaxies
Get PDFDark matter drives structure formation of the universe from the largest to smallest scales, yet we lack a complete understanding of its fundamental nature. Self-interacting dark matter, a compelling alternative to cold dark matter, is well motivated by particle physics and impacts the density profiles of dark matter halos, particularly at the centers of low-mass (∼ 1010 M⊙) halos. I use novel cosmological simulations of low-mass halos in SIDM models with moderate (1 − 5 cm2/g) to large (30 − 140 cm2/g) self-interaction cross-sections to analyze the gravothermal evolution of subhalos and isolated halos. I find that moderate cross-sections are not sufficient to produce subhalos with core densities large enough to host the densest Milky Way dwarf spheroidal galaxies. This motivates larger cross-sections that induce gravothermal collapse in such systems. When simulating isolated halos with large cross-sections in cosmological environments, I find that while some halos undergo gravothermal collapse, others experience a delay in gravothermal collapse, driven by mergers. How massive and radial a merger is, and also how frequent mergers are affect the time it takes a halo to reach gravothermal collapse. These findings indicate that accounting for mergers is essential when modeling gravothermal collapse in low-mass halos, since they influence constraints on the SIDM parameter space. This work will impact future research on self-interacting dark matter models, allowing for more robust predictions of dark sector models
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Investigations of the Inner Dark Matter Density Profiles of Dwarf Galaxies using Multiple Chemodynamical Populations and Rotation Curves.
Get PDFThe Λ cold dark matter (DM) model successfully explains the distribution of large scale structure and the cosmic microwave background but, there are several problems concerning the distribution of DM on sub-galactic scales. Robust measurements of the distribution of DM in low mass dwarf galaxies are key to understand the true nature of DM. In this work, we present two new techniques for characterizing the kinematics of dispersion supported systems. The first method identifies localized kinematic substructure in line-of-sight velocity data while the second separates global stellar populations utilizing metallicty, line-of-sight velocity, and spatial information. We apply the first method to the dwarf spheroidal galaxy Ursa Minor and find two localized kinematic substructures at high significance. We present new Keck/DEIMOS spectroscopic observations of Ursa Minor, motivated by the previous detection, which form the largest spectroscopic data set of Ursa Minor. With the new data, we identify two chemodynamical stellar population at high significance with distinct kinematic, metallicity, and spatial distributions. By utilizing the dynamics of multiple stellar populations we break halo profile degeneracies and find the DM slope is more consistent with a ‘cored’ halo than a ‘cuspy’ halo. We present a complementary study comparing a large sample of literature rotation curves to dark matter halos influenced by baryonic processes. The analysis suggests that baryonic processes are an inconsistent solution to the ‘core-cusp’ problem
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Astrophysical Probes of Particle Dark Matter
Get PDFAlthough many independent astrophysical and cosmological observations imply the existenceof a new dark matter particle, experimental searches have so far been unable to provideany conclusive evidence of such a particle. The study of dark matter and its behaviorin astrophysical systems is a promising directions for current and future searches, as thesheer scale of these systems allows physicists to probe regions of parameter space that areunreachable in laboratory or collider experiments. This thesis will describe several ways inwhich we may test dark matter models using astrophysical observations. Chapters 2, 3, and4 will describe the use of gamma-ray and cosmic-ray data in testing and constraining WIMPdark matter models. Chapter 5 will discuss dark hydrogen as a possible self-interacting darkmatter candidate, and provide testable predictions for future observations
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Looking into the Cores of Galaxy Clusters and Dwarf Galaxies
Get PDFHere are three studies of dark matter ("DM") dominated objects, with the aim of constraining the shapes of the inner regions of their DM halos. The first chapter is an introduction to the dissertation. Using galaxy cluster Abell 611 as a prototype, in Chapters 2 and 3 we model the strong lensing of eight galaxy clusters to infer the mass distribution in the range of 10 kpc to 150 kpc from the centers of the clusters, then subsequently derive constraints on self-interaction cross-section over mass of DM particles. We infer the mass profiles of the central DM halos, bright central galaxies, key member galaxies, and DM subhalos for the member galaxies for all 8 clusters using the QLens code. The inferred DM halo surface densities are fit to a self-interacting dark matter ("SIDM") model, which allows us to constrain the self-interaction cross section over mass . When our full method is applied to mock data generated from two clusters in the Illustris-TNG simulation, we find results consistent with no DM self-interactions as expected. For the eight observed clusters with average relative velocities of km/s, we infer and \sigma/m < 0.13~ \rm cm^2/g at the 95\% confidence level. In Chapter 4 we examine the bright dwarf spheroidal galaxies of the Milky Way, using a novel approach not yet completed in literature on these 9 objects: the use of phase space distribution functions. We use as data the observed surface density, velocity dispersion and fourth-order velocity moment. We use mock data from the Gaia Challenge project to show that the model can infer important characteristics such as , , the half-light radius, the density at 150 pc and the core radius. We find that the accuracy of the predictions is highest for data sets in which the stellar component is not too deeply embedded within the DM halo. For the observed sample, we infer these same parameters with accuracy comparable to those using a Jeans analysis approach. We confirm the wide diversity of inner densities in these objects, in particular that the Draco, Leo I, Leo II and Ursa Minor dwarf spheroidal galaxies are approximately four times more dense than Carina, Sextans and Fornax
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Cosmology and Astro-particle Physics: What is Dark Matter and What is Dark Energy?
Get PDFWe model the expansion history of the Universe as a Gaussian process and find constraints on the dark energy density and its low-redshift evolution using distances inferred from the Luminous Red Galaxy (LRG) and Lyman-alpha (Ly) datasets of the Baryon Oscillation Spectroscopic Survey, supernova data from the Joint Light-curve Analysis (JLA) sample, Cosmic Microwave Background (CMB) data from the Planck satellite, and local measurement of the Hubble parameter from the Hubble Space Telescope (). Our analysis shows that the CMB, LRG, Ly, and JLA data are consistent with each other and with a CDM cosmology, but the data is inconsistent at moderate significance. Including the presence of dark radiation does not alleviate the tension in our analysis. While some of these results have been noted previously, the strength here lies in that we do not assume a particular cosmological model. We calculate the growth of the gravitational potential in General Relativity corresponding to these general expansion histories and show that they are well-approximated by given the current precision. We assess the prospects for upcoming surveys to measure deviations from CDM using this model-independent approach.We incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma Ray Space Telescope. The range of particle annihilation rate and masses expand when including these unknowns. However, empirical determinations of the Milky Way halo's local density and density profile leave the signal region to be in considerable tension with dark matter annihilation searches from combined dwarf galaxy analyses. The GCE and dwarf tension can be alleviated if: one, the halo is extremely concentrated or strongly contracted; two, the dark matter annihilation signal differentiates between dwarfs and the Galactic Center; or, three, local stellar density measures are found to be significantly lower, like that from recent stellar counts, pushing up the local dark matter density.The Milky Way's Galactic Center harbors a gamma-ray excess that is a candidate signal of annihilating dark matter. Dwarf galaxies remain predominantly dark in their expected commensurate emission. We quantify the degree of consistency between these two observations through a joint likelihood analysis. In doing so I incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the {\em Fermi Gamma-Ray Space Telescope}. The preferred range of annihilation rates and masses expands when including these unknowns. Even so, using two recent determinations of the Milky Way halo's local density leave the GCE preferred region of single-channel dark matter annihilation models to be in strong tension with annihilation searches in combined dwarf galaxy analyses. A third, higher Milky Way density determination, alleviates this tension. This joint likelihood analysis allows us to quantify this inconsistency. As an example, we test a representative inverse Compton sourced self-interacting dark matter model, which is consistent with both the GCE and dwarfs.Self-interacting dark matter (SIDM) models have been proposed to solve the small-scale issues with the collisionless cold dark matter (CDM) paradigm. We derive equilibrium solutions in these SIDM models for the dark matter halo density profile including the gravitational potential of both baryons and dark matter. Self-interactions drive dark matter to be isothermal and this ties the core sizes and shapes of dark matter halos to the spatial distribution of the stars, a radical departure from previous expectations and from CDM predictions. Compared to predictions of SIDM-only simulations, the core sizes are smaller and the core densities are higher, with the largest effects in baryon-dominated galaxies. As an example, we find a core size around 0.3 kpc for dark matter in the Milky Way, more than an order of magnitude smaller than the core size from SIDM-only simulations, which has important implications for indirect searches of SIDM candidates
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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