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Galactic Radio Astronomy

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Galactic Radio Astronomy GALACTIC RADIO ASTRONOMY Yoshiaki SOFUE Lecture Note Graduate and Undergraduate Courses at University of Tokyo, Kagoshima University, Meisei University and others Contents 1 INTRODUCTION 1 1.1 Radio Astronomy . 1 1.1.1 Radio window through the Galactic Disk . 1 1.1.2 A History of Radio Astronomy . 2 1.2 The Radio Emission . 3 1.2.1 Plane waves . 3 1.2.2 Acceleration of a charged particle and radiation . 5 1.2.3 Radiation from an electron in a magnetic field . 7 1.3 Thermal Emission . 9 1.3.1 Thermal Bremsstrahlung . 9 1.3.2 Thermal emission and absorption coefficient . 11 1.4 Synchrotron Radiation - Non-thermal Radiation . 12 1.4.1 Emissivity and spectrum . 12 1.4.2 Energy equipartition . 14 1.5 Recombination Lines . 15 1.5.1 Frequency . 15 1.5.2 Line width . 17 1.5.3 Line intensity . 19 1.5.4 Line to continuum intensity ratio and tempera- ture determination . 21 1.6 Molecular Lines . 22 1.6.1 Frequency . 23 1.6.2 Intensity . 24 i 1.6.3 htwo mass estimated from the CO intensity . 26 1.6.4 Other molecules . 27 1.7 HI Line . 28 1.7.1 Frequency . 28 1.7.2 Intensity . 29 1.8 Radiations from various species . 31 1.8.1 Molecular Lines . 31 1.8.2 HI Line Emission . 31 1.8.3 Recombination Lines . 32 1.8.4 Free-Free Emission . 32 1.8.5 Synchrotron Radiation . 32 1.8.6 Black-Body Radiation . 33 1.9 Radiative Transfer . 33 2 INTERSTELLAR MATTER 39 2.1 ISM and Radio Emission . 39 2.1.1 ISM at various temperatures . 39 2.1.2 Population in the Galaxy . 39 2.2 Energy Balance in ISM . 41 2.2.1 Energy-density and Pressure balance . 41 2.2.2 `Activity' in ISM . 44 2.3 Molecular Clouds . 45 2.3.1 Mass, size and intensity . 45 2.3.2 The Distribution of MC and GMC . 47 2.3.3 Giant Molecular Clouds (GMC) . 48 2.3.4 GMC and Star-forming sites . 49 2.4 HI Gas and Clouds . 50 2.4.1 Mass, size, and intensity of HI clouds . 50 2.4.2 The Distribution of HI gas . 52 2.5 HI vs H2 in the ISM . 52 2.5.1 The HI to H2 Transition . 52 2.5.2 The Molecular Fraction . 55 ii 2.5.3 Molecular Fraction and the ISM Evolution . 55 2.6 HII Gas . 55 3 STAR FORMATION AND DEATH 57 3.1 Mechanisms of Star Formation . 57 3.1.1 Sites of Star Formation . 57 3.1.2 Schmidt's Law . 58 3.1.3 The Virial Theorem . 59 3.1.4 Gravitational Contraction { Jeans Instability { . 61 3.1.5 Gravitational Instability . 63 3.1.6 The Birth of Stars . 67 3.1.7 Other Instabilities . 68 3.1.8 . Thermal Instability . 68 3.1.9 . Rayleigh-Taylor Instability . 68 3.1.10 . Kelvin-Helmholtz Instability . 68 3.1.11 . Parker Instability (Magnetic Inflation) . 68 3.2 The Environment of Star Formation . 68 3.2.1 Triggering of Cloud Compression . 68 3.2.2 Shock Wave . 69 3.2.3 Formation of Molecular Clouds . 72 3.2.4 Why spiral arms are bright . 73 3.3 HII Regions . 77 3.3.1 Ionization sphere { Str ¨mgrensphere { . 77 3.3.2 Expanding Ionization Front . 79 3.3.3 Shock Compression of Ambient gas . 82 3.4 Sequential Star Formation . 85 3.4.1 Propagation of Shock Compression by an HII region . 85 3.4.2 The Orion region . 88 3.4.3 M16 - M17 Region . 88 3.4.4 Sgr B2 . 89 3.5 Supernova Remnants . 89 iii 3.5.1 Supernovae (SN) and Supernova remnants (SNR) 89 3.5.2 Classification of SNR . 91 3.5.3 The Σ − D Relation and the Distribution of SNR 94 3.5.4 Evolution of a SNR . 96 3.5.5 Interaction with the ISM . 102 3.5.6 Implications of SNRs for the Galaxy Evolution . 103 4 GALACTIC STRUCTURE 109 4.1 The Distance to the Galactic Center . 109 4.1.1 Using Globular Clusters . 109 4.1.2 Using Maser Sources . 110 4.1.3 Using X-ray Bursters . 111 4.2 The Galactic Rotation . 112 4.2.1 Oort's Constants . 112 4.2.2 The Solar Rotation Velocity . 115 4.2.3 Rotation Curve within the Solar Circle . 116 4.2.4 Rotation Curve beyond the Solar Circle . 117 4.2.5 The Rotation Curve . 119 4.2.6 The Mass of the Galaxy . 119 4.3 Distributions of Interstellar Gas in the Galaxy . 120 4.3.1 Velocity-to-Space Transformation using a Radial- Velocity Diagram . 120 4.3.2 (l; vr) and (b; vr) Diagrams . 123 4.3.3 `Face-on View' of the Galaxy . 123 4.3.4 `Edge-on View' of the Galaxy . 125 4.3.5 Radial Distributions of HI and CO and the Molec- ular Front . 125 4.4 Mass Distribution in the Galaxy . 133 4.4.1 Radial Mass Distribution for a Flat Rotation Curve . 133 4.4.2 Mass Distribution perpendicular to the Disk . 135 4.4.3 A Mass Distribution Law of the Whole Galaxy . 137 iv 4.4.4 Massive Halo and the Missing Mass Problem . 138 4.4.5 Dynamical Masses of Galaxies and of Cluster of Galaxies . 141 4.5 Spiral Structure . 143 4.5.1 Hubble Types and the Type of the Galaxy . 143 4.5.2 Origin of Spiral Arms . 143 4.5.3 Density Waves . 147 4.5.4 Galactic Shock Waves . 152 5 THE GALACTIC CENTER AND ACTIVITY 159 5.1 The Galactic Center in Radio . 159 5.1.1 Radio Continuum Emission . 159 5.1.2 Flat Radio Spectra . 160 5.1.3 Linear Polarization . 162 5.1.4 Infrared-to-Radio Ratio . 164 5.2 Thermal Emission in the Galactic Center . 165 5.2.1 The Thermal Disk and Star Formation . 165 5.2.2 Molecular Clouds and Star Forming Regions . 167 5.3 Galactic Nucleus and its Surrounding Magnetic Structures175 5.3.1 Sgr A: The Nucleus . 175 5.3.2 Thermal Filaments . 175 5.3.3 Vertical Magnetic Tubes: Radio Arc and Threads 178 5.4 Ejection: Lobes and Jets . 179 5.4.1 The Galactic Center Lobe . 179 5.4.2 The 4-kpc Jet . 182 5.4.3 Giant Shock Wave in the Halo: the North Polar Spur . 183 5.4.4 Extragalactic Radio Bubbles . 183 5.5 Origin of Vertical Structures . 186 5.5.1 oloidal fields . 186 5.5.2 Explosion . 187 5.5.3 The 200-pc Expanding Molecular Cylinder . 188 5.5.4 Infalling-Clouds and \Galactic Sprays" . 189 v 6 NONTHERMAL EMISSION AND MAGNETIC FIELDS191 6.1 Synchrotron Emission and Polarization . 191 6.1.1 Synchrotron intensity and magnetic field strength 191 6.1.2 Linearly Polarized Emission . 194 6.1.3 Faraday Rotation . 195 6.1.4 Determination of Magnetic Field Oeientation . 196 6.2 Magnetic Fields in Disk Galaxies . 197 6.2.1 RM in Disk Magnetic Field . 197 6.2.2 BSS Magnetic Fields and the Primordial Origin Hypothesis . 198 6.2.3 Vertical Fields in spiral Galaxies . 198 6.2.4 Magnetic Fields in the Galactic Halo . 201 6.3 Evolution of Magnetic Fields in Spiral Galaxies . 202 6.3.1 Primordial Origin of Galactic Magnetic Fields . 202 6.3.2 Loss of Angular Momentum by the Vertical Fields204 6.4 Nuclear Activities and the Vertical Fields . 204 6.4.1 Jets from the Nuclei . 204 7 HI and CO IN GALAXIES 205 7.1 HI in Galaxies . 206 7.1.1 HI Gas Distributions . 206 7.1.2 Rotation and Velocity Fields of HI Gas . 206 7.2 CO and Molecular Gas in Galaxies . 209 7.2.1 Distribution of Molecular Gas . 209 7.2.2 Inner CO Kinematics . 213 7.2.3 The CO-to-H2 Conversion Factor in Galaxies . 213 7.3 CO vs HI in Galaxies . 218 7.3.1 CO vs HI in the Position-Velocity Diagram for Edge-on Galaxies . 218 7.4 Radial Variations of HI and H2 Densities . 221 7.4.1 Deriving the Density Distribution from a PV di- agram . 221 vi 7.5 The Molecular Front . 224 7.5.1 Radial Variation of Molecular Fraction . 224 7.5.2 Molecular Front . 225 7.5.3 Phase Transition between HI and H2 ....... 226 7.6 Rotation Curves of Galaxies . 227 7.6.1 Rotation of Galaxies . 227 7.6.2 . Definition of Rotation Curves . 228 7.6.3 Rotation Curves for Nearby Galaxies . 230 7.6.4 Comparison of Rotation Curves . 238 7.6.5 Fitting by Miyamoto-Nagai Potential . 239 7.6.6 The Four-Mass Component Model . 244 7.7 The HI and CO Tully-Fisher Relation and mm-wave Cosmology . 246 7.7.1 HI Tully-Fisher Relation . 246 7.7.2 CO instead of HI . 247 7.7.3 CO vs HI Line Profiles Correlation . 248 7.7.4 The CO Tully-Fisher Relation: mm-wave Cos- mology . 252 8 STARBURST 255 8.1 Starburst Galaxy M82 . ..
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