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<p> Through HI observations, galactic gas discs can be observed to extend past the edge of the star forming disk. Observations of HI in these extended galactic disks consistently show significant velocity dispersion, which suggests a non-thermal origin. This suggests that turbulence in the gas is contributing significantly to the observed velocity dispersion. To address this, a new parallel adaptive mesh three dimensional shearing-box implementation with adaptions for evening numerical diffusion effects, self-gravity in the shearing boundary conditions and appropriate vertical boundary conditions has been built, based on the FLASH code. This code is used to perform local simulations, incorporating differential rotation, self-gravity, stratification, hydrodynamics and cooling. These simulations explore possible mechanisms for driving turbulent motions through thermal and self-gravitational instabilities coupling to differential rotation. In isothermal simulations a marginally stable disk is found to be stable against forming a gravitoturbulent quasi-steady state. In simulations including cooling, where the disk conditions do not trigger the formation of a two-phase medium, it is found that perturbations to the flow damp without leading to a sustained mechanism for driving turbulence. In cases where a two-phase medium develops, gravitational angular momentum transporting stresses are much greater, creating a possible mechanism for transferring energy from galactic rotation to turbulence, though a gravitoturbulent quasi-steady state is not found. The differing angular momentum transport properties of the single phase and two phase regimes of the disk suggests a significant dynamical division can be drawn between the two, which may occur far outside the star formation cutoff in a galactic disk. </p> / Thesis / Master of Science (MSc)

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-10-09T19:22:16Z No. of bitstreams: 1 CAMILLA DOS SANTOS RODRIGUES LEITE – DISSERTAÇÃO (PPGFísica) 2012.pdf: 1293712 bytes, checksum: 38858a36d1603d9c94c3a7861fe88368 (MD5) / Made available in DSpace on 2018-10-09T19:22:16Z (GMT). No. of bitstreams: 1 CAMILLA DOS SANTOS RODRIGUES LEITE – DISSERTAÇÃO (PPGFísica) 2012.pdf: 1293712 bytes, checksum: 38858a36d1603d9c94c3a7861fe88368 (MD5) Previous issue date: 2012-05 / Capes / Uma caracterização bem elaborada das propriedades físicas das galáxias é de fundamental importância para entendermos o comportamento do Universo. Por outro lado, um dos grandes desafi os da Teoria da Relatividade Geral é encontrar soluções exatas com clara interpretação física. Nosso trabalho visa obter soluções exatas das equações de Einstein que possam representar modelos de discos galácticos, seguindo um método indireto para evitar a árdua tarefa de resolver as equações de Einstein diretamente. Para tanto, consideramos a ideia de uma hipersuperfície imersa em um espaço de dimensão superior, e utilizamos o formalismo da imersão associado ao Método "deslocar, cortar e refletir" (que pode ser considerado como uma adaptação do conhecido método das imagens, estudado em eletrostática), com o qual "cortando" e "colando" soluções conhecidas de vácuo, geramos soluções com fonte do tipo disco. Este procedimento é aqui denominado método da imersão, e constitui-se como uma ferramenta e ciente na modelagem de discos, visto que permite uma maior liberdade quanto à escolha da hipersuperfície de corte, e a consequente determinação das propriedades físicas (densidade, pressão, etc) do disco de matéria gerado. Este método, portanto, torna-se mais abrangente, uma vez que o método convencional se limita à analise de hipersuperfícies "planas", nas coordenadas consideradas. Através da aplicação desse método, verifi camos que o conteúdo material de um disco galáctico, idealizado como um disco de matéria infinitamente fino, é descrito por um tensor energia-momento super ficial cujas componentes podem ser escritas explicitamente em termos das funções de imersão. Estudando alguns casos particulares, reproduzimos os resultados encontrados na literatura. / The study of the physical properties of galaxies is very important to understand the behavior of the Universe. On the other hand, one of the great challenges of the General Theory of Relativity is to nd exact solutions which have a clear physical interpretation. Our work aims to obtain exact solutions of Einstein's equations that can represent models of galactic disks, by following an indirect method to avoid the di cult task of solving Einstein's equations directly. To this end, we consider the idea of a hypersurface embedded in a space of higher dimension, and we use the embedding formalism associated with the method of "displace", cut and reflect" (which can be considered as an adaptation of the known method of images, studied in electrostatics) on known vacuum solutions in order to generate solutions with disklike sources. This procedure, called the Embedding Method, is an e cient tool for modeling disks, as it allows great freedom in the choice of cutting hipersurfaces, and the consequent determination of physical properties (density, pressure, etc.) of the matter in the disk which is generated. Therefore, this method becomes more general than the conventional formulation of the method of "displace, cut and reflect", which works only to "plane" hypersurfaces (as viewed in the employed coordinate system). By applying the embedding formalism, we found that the material content of galactic disks, idealized as a in nitely thin disk of matter, is described by an energy-momentum tensor whose components can be written explicitly in terms of the embeeding functions. By studying individual cases, we reproduce some disk models found in the literature.

Multiple coherent supernova explosions (SNe) in an OB association can produce a strong shock that moves through the interstellar medium (ISM). These shocks fronts carve out hot and tenuous regions in the ISM known as superbubbles. The density contour plot at three different times (0.5 Myr (left panel), 4 Myr (middle panel), 9.5 Myr (right panel)) showing different stages of superbubble evolution for n0 = 0.5 cm−3, z0 = 300 pc, and for NOB = 104. This density contour plot is produced using ZEUS-MP 2D hydrodynamic simulation with a resolution of 512 × 512 with a logarithmic grid extending from 2 pc to 2.5 kpc. For a detailed description of this figure, see Roy et. al., 2015. The evolution of a superbubble is marked by different phases, as it moves through the ISM. Consider an OB association at the center of a disk galaxy. Initially the distance of the shock front is much smaller than the disk scale height. The superbubble shell sweeps up the ISM material, and once the amount of swept up material becomes comparable to the ejected material during SNe, the superbubble enters a self-similar phase (analogous to the Sedov-Taylor phase of individual SNe). As the superbubble shell sweeps up material, its velocity decreases, and thus the corresponding post-shock temperature drops. At a temperature of ∼ 2 × 105 K (where the cooling function peaks), the superbubble shell becomes radiative and starts losing energy via radiative cooling. This radiative phase is shown in the left panel of Figure 1. The superbubble shell starts fragmenting into clumps and channels due to Rayleigh-Taylor instabilities (RTI) (which is seeded by the thermal instability; for details see Roy et. al., 2013) when the superbubble shell crosses a few times the scale height. This is represented in the middle panel of the same figure. At a much later epoch, RTI has a strong effect on the shell fragmentation and the top of the bubble is completely blown off (the right panel). In the first chapter of the thesis (reported in Sharma et. al., 2014), we show using ZEUS-MP hydrodynamic simulations that an isolated supernova loses almost all its mechanical energy within a Myr whereas superbubbles can retain up to ∼ 40% of the input energy over the lifetime of the starcluster (∼ few tens of Myr), consistent with the analytic estimate of the second chapter. We also compare different recipes (constant luminosity driven model (LD model), kinetic energy driven model (KE model) to implement SNe feedback in numerical simulations. We determine the constraints on the injection radius (within which the SNe input energy is injected) so that the supernova explosion energy realistically couples to the interstellar medium (ISM). We show that all models produce similar results if the SNe energy is injected within a very small volume ( typically 1–2 pc for typical disk parameters). The second chapter concentrates on the conditions for galactic disks to produce superbubbles which can give rise to galactic winds after breaking out of the disk. The Kompaneets formalism provides an analytic expression for the adiabatic evolution of a superbubble. In our calculation, we include radiative cooling, and implement the supernova explosion energy in terms of constant luminosity through out the life-time of the OB stars in an exponentially stratified medium (Roy et. al., 2013). We use hydrodynamic simulations (ZEUS-MP) to determine the evolution of the superbubble shell. The main result of our calculation is a clear demarcation between the energy scales of sources causing two different astrophysical phenomenon: (i) An energy injection rate of ∼ 10−4 erg cm−2 s−1 (corresponding Mach number ∼ 2–3, produced by large OB associations) is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (ii) A larger energy injection scale ∼ 10−3 erg cm−2 s−1, or equivalently a surface density of star formation rate ∼ 0.1 M⊙ yr−1 kpc−2 corresponding to superbubbles with high Mach number (∼ 5–10) produces galactic-scale superwinds (requires superstar clusters to evolve coherently in space and time). The stronger energy injection case also satisfies the requirements to create and maintain a multiphase halo (matches with observations). Roy et. al., 2013 also points out that Rayleigh-Taylor instability (RTI) plays an important role in the fragmentation of superbubble shell when the shell reaches a distance approximately 2–3 times the scale-height; and before the initiation of RTI, thermal instability helps to corrugate the shell and seed the RTI. Another important finding of this chapter is the analytic estimation of the energetics of superbubble shell. The shell retains almost ∼ 30% of the thermal energy after the radiative losses at the end of the lifetime of OB associations. The third chapter considers the escape of hydrogen ionizing (Lyc) photons arising from the central OB-association that depends on the superbubble shell dynamics. The escape fraction of Lyc photons is expected to decrease at an initial stage (when the superbubble is buried in the disk) as the dense shell absorbs most of the ionizing photons, whereas the subsequently formed channels (created by RTI and thermal instabilities) in the shell creates optically thin pathways at a later time (∼ 2–3 dynamical times) which help the ionizing photons to escape. We determine an escape fraction (fesc) of Lyc photons of ∼ 10 ± 5% from typical disk galaxies (within 0 ≤ z (redshift) ≤ 2) with a weak variation with disk masses (reported in Roy et. al., 2015). This is consistent with observations of local galaxies as well as constraints from the epoch of reionization. Our work connects the fesc with the fundamental disk parameters (mid-plane density (n0), scale-height (z0)) via a relation that fescαn20z03 (with a ≈ 2.2) is a constant. In the fourth chapter, we have considered a simple model of molecule formation in the superbubble shells produced in starburst nuclei. We determine the threshold conditions on the disk parameters (gas density and scale height) for the formation of molecules in superbubble shells breaking out of disk galaxies. This threshold condition implies a gas surface density of ≥ 2000 M⊙ pc−2, which translates to a SFR of ≥ 5 M⊙ yr−1 within the nuclear region of radius ∼ 100 pc, consistent with the observed SFR of galaxies hosting molecular outflows. Consideration of molecule formation in these expanding superbubble shells predicts molecular outflows with velocities ∼ 30–40 km s−1 at distances ∼ 100–200 pc with a molecular mass ∼ 106–107 M⊙, which tally with the recent ALMA observations of NGC 253. We also consider different combinations of disk parameters and predict velocities of molecule bearing shells in the range of ∼ 30–100 km s−1 with length scales of ≥ 100 pc, in rough agreement with the observations of molecules in NGC 3628 and M82 (Roy et. al., 2016, submitted to MNRAS).