Gas Evolution in Disk Galaxies
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Gas Evolution in Disk Galaxies Disk Stability, Gas Accretion & The Impacts of Spiral Density Waves Hsiang-Hsu Wang Max-Planck-Institut f¨ur Astronomie (MPIA) Zentrum f¨ur Astronomie der Universit¨at Heidelberg, Institut f¨ur Theoretische Astrophysik (ITA) Heidelberg 2010 Dissertation in Astronomy submitted to the Combined Faculties of the Natural Sciences and Mathematics of the Ruperto-Carola-University of Heidelberg, Germany, for the degree of Doctor of Natural Sciences Put forward by Lic. Hsiang-Hsu Wang born in Tainan, Taiwan Oral examination: 19.01.11 Gas Evolution in Disk Galaxies Disk Stability, Gas Accretion & The Impacts of Spiral Density Waves Referees: Prof. Dr. Ralf Klessen Priv. Doz. Dr. Henrik Beuther Abstract This thesis studies many aspects of gas evolution in disk galaxies. A simple, effective method is developed for initializing a three-dimensional gaseous disk which is in detailed equilibrium. With this method, theo- retical predictions for disk stability and swing amplification are numerically studied for three-dimensional disks. The missing link between intergalactic gas accretion and the star formation activity is found for the galaxy M83. We improve the analysis method to search for the signature of gas infall. For the first time, gas accretion with sufficient fresh gas to fuel star forming disk is kinematically confirmed. The impacts of spiral density waves on gas motions are studied numerically. Shock driven turbulence is quantified and is found to match excellently with observations. Furthermore, the evolution of shock itself has pro- found impacts on redistributing gaseous surface density, angular momentum and on the development of substructures. Zusammenfassung Diese Arbeit untersucht viele Aspekte der zeitlichen Entwicklung von Gas in Scheibengalaxien. Eine einfache und effektive Methode zur Initialisierung einer dreidimensionalen Gasscheibe in detailliertem Gleichgewicht wurde entwickelt. Mithilfe dieser Methode untersuchen wir theoretische Vorhersagen ber Stabilitt und ’Swing’ Verstrkung dreidimensionaler Scheiben durch numerische Simulationen. Fr die Galaxie M83 wurde der fehlende Zusammenhang zwischen Akkretion intergalaktischen Gases und der Sternentstehungsrate gefunden. Wir verbesserten die Analysemethode zum Auffinden von Gaseinfall. Zum ersten Mal wurde eine ausreichende Gasakkretion zum speisen einer Sterne formenden Scheibe kinematisch besttigt. Der Einfluss von spiralfrmigen Dichtewellen auf die Gasbewegung wurde numerisch untersucht. Schock-getriebene Turbulenz wurde quantifiziert und stimmt mit Beobachtungen exzellent berein. Des Weiteren hat die Entwicklung des Schocks selbst weitreichende Einflsse auf die Umverteilung von Gasoberflchendichte, Drehimpuls und die Entwicklung von Substrukturen. Contents 1 Introduction 1 1.1 GalaxiesintheUniverse . ... 1 1.1.1 ClassificationofGalaxies . .. 1 1.1.2 GalaxyDistributioninTheUniverse . ..... 3 1.1.3 Environmental EffectsonGalaxies..................... 5 1.2 SpiralGalaxies .................................. 6 1.2.1 SpiralDensityWavesandSubstructures . ...... 6 1.2.2 StarFormation,TurbulenceandGasAccretion . ....... 8 1.2.3 DarkMatterHalo.............................. 10 1.2.4 StellarDisks ................................ 11 1.2.5 GaseousDisks ............................... 12 1.3 LayoutofTheThesis ............................... 13 2 Equilibrium Initialization and Stability of Three-DimensionalGasDisks 15 ix x Contents 2.1 Introduction.................................... 15 2.2 FormulationofEquations. .... 18 2.2.1 AzimuthalRotationVelocity . ... 20 2.2.2 DensityDistribution . 21 2.3 ImplementationandTests. .... 25 2.3.1 SimulationParameters . 25 2.3.2 AStableDisk................................ 27 2.4 AxisymmetricInstability . ..... 29 2.4.1 Theimpactofthicknessondiskstability. ....... 31 2.4.2 Theinclusionofstellarpotentials . ...... 32 2.5 SpontaneouslyInducedSpiralStructure . ......... 33 2.6 Summary ...................................... 35 3 Evidence for Radial Inflow In The Extended HI Disk of M83 (NGC5236) 43 3.1 Introduction.................................... 43 3.2 HImapsofM83................................... 45 3.3 FourierDecomposition . ... 46 3.3.1 Axi-symmetricflow ............................ 46 3.3.2 InclusionofHarmonics. 48 3.4 ApplicationtoM83 ................................ 50 3.4.1 TheMethod................................. 50 3.4.2 Results ................................... 53 3.5 Discussions ..................................... 54 3.5.1 TheHIRingAsAnAngularMomentumBarrier . .. 54 3.5.2 RadialInflowInTheOuterDisk . 55 Contents xi 3.5.3 TheInnerDiskandTheTransitionZone. .... 57 3.6 Summary ...................................... 57 4 The Impacts of Spiral Density Waves On Gas Motions 67 4.1 Introduction.................................... 67 4.2 TheModelandParameters . .. 70 4.3 VelocityDispersion .............................. ... 72 4.3.1 TheGenerationofSynthesisMap . .. 73 4.3.2 TheLine-of-SightVelocityDispersion . ....... 74 4.4 AngularMomentumTransportandRadialMotions . ........ 75 4.5 GenerationofVortensity . .... 76 4.6 Substructuresandstreamingmotions . ........ 80 4.7 Discussions ..................................... 82 4.7.1 VelocityDispersion. 82 4.7.2 NonsteadyShocks ............................. 82 4.7.3 AngularMomentumTransportandRadialMotions . ...... 84 4.8 Summary ...................................... 85 5 Summary and Outlook 103 5.1 Summary ......................................103 5.2 Outlook .......................................105 A Appendix 109 A.1 TheDerivationofRotationVelocity . .......109 A.2 Theeffectofthediskthicknessonthemidplanepotential . .110 A.3 TheDerivationofthereductionfactor . ........111 xii Contents A.4 Theverticalforceratio . .112 A.5 Validity check of the reduced Poisson Equation for the gasdisk. .113 A.6 transformationmatrix. .115 A.7 Thepost-shockstreamingflow . .115 Acknowledgments 129 Chapter 1 Introduction Galaxies are complex systems composed of stars, gas, dust and invisible dark matters, embedded in a relatively vast empty space. These galaxies are the basic building blocks of the Universe. Although modern cosmological models suggest that these visible luminous objects occupy only a small fraction of the constituents of the Universe, our current understanding of the cosmos fully relies on the light emitted by the normal matter. Observational, theoretical and numerical studies on the formation and evolution of galaxies in the past century has largely renovated our viewpoint about the Universe. 1.1 Galaxies in the Universe 1.1.1 Classification of Galaxies As the heliocentric model of planetary motions suggested by the Polish-born astronomer Nico- laus Copernicus (1473 1543) shifts our standing point in the solar system, a series of pivotal − works of Edwin Hubble (1889 1953) profoundly revolutionize our understanding of our po- − sition in the Universe. The great Shapley-Curtis debate (1920) over the nature of the nebulae centered on their distances from us and the size of the Milky Way was finally settled conclu- sively by Hubble’s work in 1923. Hubble measured the distances bewteen the Milky Way and several nebulae, including the Andromeda (M31), via the luminosity-period relation of Cepheid variables stars. He concluded that some of the nebulae are too distant to be part of the MilkyWay and they are, in fact, our neighbor galaxies. This realization greatly extends the original Milky Way centered viewpoint to a much larger ecosystem in the Universe, i.e., the Milky Way is just one of the countless galaxies. Galaxies come in different flavors in terms of size and morphology. Hubble’s scheme of classi- fication of galaxies (Hubble 1926, 1936) was the first step to understand the nature of galaxies 1 2 CHAPTER 1 based on morphology. As shown in Fig. 1.1, Hubble arranged galaxies into the tuning-fork di- agram and categorized them into three groups based on their appearance, i.e., ellipticals (E’s), spirals (S’s) and irregulars (Irr’s). Spirals are further divided into two separated sequences, the normal spirals without bars (S’s) and the barred spirals (SB’s). A transition type between ellipti- cals and spirals is designated as lenticulars (S0’s). Galaxies on the left of the diagram are called ‘early’ and on the right ’late’ in type. This is an unfortunate relic of nomenclature derived from the early misunderstanding of the evolution of galaxies. Before, galaxies were believed to be formed from the collapse of proto-galactic nebulae supported by pressure. As gas falls inward, the kinetic energy is converted into thermal energy and dissipates via radiation. Eventually, due to the conservation of angular momentum, a rotational supported gaseous disk results, ensuing the structure development such as spirals and bars. Figure 1.1: Diagram of Hubble’s Tuning Fork classification scheme from Hubblesite. Ellipticals are classified based on their apparent axial ratio, ǫ 1 b/a, where a and b represent ≡ − the apparent major and minor axis, respectively. Ellipticals with an apparent axial ratio, ǫ, are designated with ‘Eα’, with α 10ǫ. For instance, ellipticals with ǫ = 0.3 are classified as E3 ≡ and sphericals with ǫ = 0 as E0. Ellipticals with ǫ > 0.7, however, have never been observed. Spiral galaxies are morphologically more interesting than ellipticals. Spiral galaxies, normal and barred, having the most conspicuous bulge-to-disk luminosity ratios are classified as ‘Sa’ or INTRODUCTION 3 ’SBa’. These galaxies are also the most tightly wound spirals. Those with the least bulge-to- disk luminosity ratios