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Galactic Astronomy Astrophysics III: Galactic Astronomy Lecture, D-PHYS, ETH Zurich, Spring Semester 2016 Tuesday: 12.45{13.30, HIT F13, and Wednesday: 8.45{10.30, HIT J51, H¨onggerberg Exercises: Wednesday: 10:45{12:30, HIT J51 Dates: Feb. 23 to June 1, 2016 (except for Easter break, March 27 { April 3) Website: www.astro.ethz.ch/education/courses/Astrophysics 3 Lecturer: Prof. Dr. H.M. Schmid, Office, HIT J22.2, Tel: 044-63 27386; e-mail: [email protected] Teaching Assistants and Co-Lecturers: Natalia Engler, HIT J 41.2, [email protected] Bruderer Claudio, HIT J 41.2, [email protected] ETH Zurich, Institut f¨urAstronomie, Wolfgang Pauli Str. 27 ETH-H¨onggerberg, 8093 Zurich Contents 1 Introduction 1 1.1 The Milky Way and the Universe . 1 1.2 Short history of the research in galactic astronomy . 3 1.3 Lecture contents and literature . 4 2 Components of the Milky Way Galaxy 7 2.1 Geometric components . 7 2.2 Stars . 9 2.2.1 Properties of main-sequence stars . 10 2.2.2 Observational Hertzsprung-Russell diagrams . 12 2.2.3 Stellar clusters and associations . 15 2.2.4 Globular clusters. 17 2.2.5 Age and metallicity of stars . 19 2.2.6 Cepheids and RR Lyr variables as distance indicators . 21 2.2.7 Star count statistics . 23 2.3 Stellar Dynamics . 29 2.3.1 Velocity parameters relative to the sun . 29 2.3.2 Solar motion relative to the local standard of rest . 30 2.3.3 Velocity dispersion in the solar neighborhood . 32 2.3.4 Moving groups . 33 2.3.5 High velocity stars . 33 2.3.6 Radial velocity dispersion in clusters . 34 2.3.7 Kinematics of the galactic rotation . 35 2.3.8 The GAIA revolution . 41 2.4 Interstellar matter (ISM) in the Milky Way . 43 2.4.1 The ISM in the solar neighborhood . 43 2.4.2 Global distribution of the ISM in the Galaxy . 45 2.4.3 Galactic rotation curve from line observations . 45 2.4.4 H i and CO observations in other galaxies . 47 3 Galactic dynamics 49 3.1 Potential theory . 49 3.1.1 Basic equations for the potential theory . 49 3.1.2 Newton's theorems . 50 3.1.3 Equations for spherical systems . 52 3.1.4 Simple spherical cases and characteristic parameters . 53 3.1.5 Spherical power law density models . 55 iii 3.1.6 Potentials for flattened systems . 56 3.1.7 The potential of the Milky Way . 57 3.2 The motion of stars in spherical potentials . 59 3.2.1 Orbits in a static spherical potential . 59 3.2.2 Radial and azimuthal velocity component. 62 3.2.3 Motion in a Kepler potential . 63 3.2.4 Motion in the potential of a homogeneous sphere . 64 3.3 Motion in axisymmetric potentials . 65 3.3.1 Motion in the meridional plane . 65 3.3.2 Nearly circular orbits: epicycle approximation . 66 3.3.3 Density waves and resonances in disks . 69 3.4 Two-body interactions and system relaxation . 71 3.4.1 Two-body interaction . 71 3.4.2 Relaxation time . 74 3.4.3 The dynamical evolution of stellar clusters . 74 4 Physics of the interstellar medium 81 4.1 Gas . 81 4.1.1 Description of a gas in thermodynamic equilibrium . 81 4.1.2 Description of the diffuse gas . 83 4.1.3 Ionization . 85 4.1.4 H ii-regions . 87 4.2 Dust . 89 4.2.1 Extinction, reddening and interstellar polarization . 89 4.2.2 Particle properties . 91 4.2.3 Temperature and emission of the dust particles . 92 4.2.4 Evolution of the interstellar dust . 93 4.3 Magnetic fields . 95 4.4 Radiation field . 96 4.5 Cosmic rays . 96 4.5.1 Properties of the cosmic rays . 96 4.5.2 Motion in the magnetic field . 97 4.5.3 The origin of the cosmic rays . 98 4.6 Radiation processes . 99 4.6.1 Radiation transport . 99 4.7 Spectral lines: bound-bound radiation processes . 101 4.7.1 Rate equations for the level population . 102 4.7.2 Collisionally excited lines . 103 4.7.3 Collisionally excited molecular lines . 108 4.7.4 Recombination lines: excitation through recombination . 109 4.7.5 Absorption lines . 110 4.8 Free-bound and free-free radiation processes . 115 4.8.1 Recombination continuum . 115 4.8.2 Photoionization or photo-electric absorption . 116 4.9 Free-free radiation processes or bremsstrahlung . 117 4.9.1 Radiation from accelerated charges . 117 4.9.2 Thermal bremsstrahlung . 118 v 4.10 Compton and Thomson scattering . 120 4.11 Temperature equilibrium . 122 4.11.1 Heating function H for neutral and photo-ionized gas . 122 4.11.2 Cooling of the gas . 123 4.11.3 The cooling function Λ(T ) . 123 4.11.4 Cooling time scale . 126 4.11.5 Equilibrium temperatures. 126 4.12 Dynamics of the interstellar gas . 128 4.12.1 Basic equations for the gas dynamics . 128 4.12.2 Shocks . 130 4.12.3 Example: supernova shells . 133 5 Star formation 135 5.1 Molecular clouds. 135 5.2 Elements of star formation . 136 5.2.1 Time scale for contraction . 140 5.3 Initial mass function . 141 5.4 Proto-stars . 142 6 Milky Way formation and evolution 145 6.1 Virial theorem and galaxy formation . 145 6.2 Timing the Milky Way evolution with high redshift observation . 147 6.3 Gas infall and minor mergers today . 148 6.3.1 Gas inflow . 148 6.3.2 Mergers with dwarf galaxies . 148 6.4 The chemical evolution of the Milky Way . 149 6.4.1 Nucleosynthesis and stellar yields . 149 6.4.2 The role of SN Ia. 151 6.4.3 Modelling the chemical evolution of the Milky Way . 152 vi Chapter 1 Introduction 1.1 The Milky Way and the Universe This lecture concentrates on the physical properties of the Milky Way galaxy and the processes which are important to understand its current structure and properties. Another strong focus is set on observational data which provide the basic empirical information for our models and theories of the Milky Way. The place of our Galaxy in the Universe is roughly illustrated in the block diagram in Fig. 1.1. { The Milky Way is a quite normal spiral galaxy among billions of galaxies in the observable Universe. { The galaxies were born by the assembly of baryonic matter in the growing potential wells of dark matter concentrations in an expanding Universe. This process started about 14 billion years ago with the big bang. The galaxies evolved with time by assembling initially gas rich matter fragments, going through phases of strong star formation, having phases of high activity of the central black hole, and many episodes of minor and perhaps also major interactions with other galaxies. Although the Milky Way belongs to one of the frequent galaxy types, it represents just one possible outcome of the very diverse galaxy evolution processes. { Initially, the big bang produced matter only in the form of hydrogen, helium and dark matter. The heavy elements which we see today were mainly produced in galaxies from H and He by nuclear processes in previous generations of intermediate and high mass stars (see Fig. 1.1). Stars form through the collapse of dense, cool interstellar clouds. Then they evolve due to nuclear reactions until they expel a lot of their mass at the end of their evolution in stellar winds or supernova (SN) explosions. This matter, enriched in heavy elements, goes back to the interstellar gas in the Milky Way and may form again a new generation of stars. The remnants of the stellar evolution, mostly white dwarfs (WD) and neutron stars (NS), contain also a lot of heavy elements which are no more available for the galactic nucleo-synthesis cycle. { Many galaxies, including the Milky Way, have a super-massive black hole (SM-BH) in their center. The black hole grows by episodic gas accretion which may be triggered by galaxy interaction. Supernovae explosions, active phases of the central black hole, 1 2 CHAPTER 1. INTRODUCTION or galaxy interactions are responsible for the loss of interstellar matter of a galaxy to the intergalactic medium. On the other side cold intergalactic matter (IGM), from either primordial origin or gas which was already in a galaxy, can fall onto the Milky Way and enhance the gas content. Big Bang p,e,α,DM (re)-combination H,He,DM ISM IGM SM-BH young stars evolved stars other galaxies low mass stars WD and NS Milky Way Figure 1.1: The Milky Way in relation to the big bang, the intergalactic matter (IGM), the internal interstellar matter (ISM), different types of stars (WD: white dwarfs; NS: neutron stars), the central, super-massive black hole (SM-BH), and other interacting galaxies. 1.2. SHORT HISTORY OF THE RESEARCH IN GALACTIC ASTRONOMY 3 1.2 Short history of the research in galactic astronomy Our knowledge on the Milky Way is constantly improving. The Milky Way research profits also a lot from new results gained in other fields in astronomy, like stellar evolution theory, interstellar matter studies, extra-galactic astronomy, or dark matter research. Most important for the progress is the steady.
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