Galactic Radio Astronomy

240 Pages·2017·8.923 MB·English

by  Yoshiaki Sofue (auth.)

Lecture Notes in Physics 935
Yoshiaki Sofue
Galactic
Radio
Astronomy
Lecture Notes in Physics
Volume 935
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J.Ehlers
K.Hepp
H.Weidenmöller
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M.Bartelmann,Heidelberg,Germany
B.-G.Englert,Singapore,Singapore
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R.A.L.Jones,Sheffield,UK
M.Lewenstein,Barcelona,Spain
H.vonLöhneysen,Karlsruhe,Germany
J.-M.Raimond,Paris,France
A.Rubio,Hamburg,Germany
M.Salmhofer,Heidelberg,Germany
W.Schleich,Ulm,Germany
S.Theisen,Potsdam,Germany
D.Vollhardt,Augsburg,Germany
J.D.Wells,AnnArbor,USA
G.P.Zank,Huntsville,USA
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Yoshiaki Sofue
Galactic Radio Astronomy
123
YoshiakiSofue
InstituteofAstronomy
TheUniversityofTokyo
Mitaka,Tokyo,Japan
ISSN0075-8450 ISSN1616-6361 (electronic)
LectureNotesinPhysics
ISBN978-981-10-3444-2 ISBN978-981-10-3445-9 (eBook)
DOI10.1007/978-981-10-3445-9
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Preface
TheSolarsystem—theSun,theEarth,andplanets—isatinyfamilyintheGalaxyas
oneofthenumeroussimilarfamiliesofstars.TheGalaxyconsistsofstars,gas,and
darkmattertotalingsome1012solarmasses,androtatesaroundtheGalacticCenter
in 200 million years. The Sun was born 4.6 billion years ago in a local gas cloud
25,000 light years away from the Galactic Center, when the Galaxy was 9 billion
yearsold.Starsarebeingborneventodayininterstellarclouds,makingthegalactic
disk and spiral arms active as star-forming sites. The Galactic Center is the most
active region with bursting star formation, exhibiting explosive phenomena, and
nests a super-massive black hole of four million solar masses. The Galaxy is one
of the numerous galaxies in the universe, a typical spiral similar to the beautiful
neighbor,theAndromedagalaxy.
These dazzling galactic objects, sometimes revealing apparently mysterious
phenomena, are understood and described by simple physics. All the objects are
observedbytheelectromagneticwavesradiatedbymovingelectrons.Theradiation
mechanismsaretheestablishedfieldsofphysics,clearlyunderstoodanddescribed
byelectromagnetictheory.Thismeansthatanycelestialobjects,emittingradiation,
are not mysterious zones at all, but are describable by fundamental physics. The
emissions from the objects transport fundamental information of the sources. For
example,theformationofstarsisunderstoodbythegravitationalforceactingonan
interstellar cloud, whose density, size, mass, temperature, and so on are measured
by observations. The rotation of the Galaxy, as observed using the Doppler effect,
isusefultomeasurethegalacticmassbasedonmechanics.
This book aims at describing the galactic and interstellar phenomena in terms
ofsimplephysicallaws,andatunderstandingtheunderlyingphysicsofthedriving
mechanismsoftheuniverse.
Tokyo,Japan YoshiakiSofue
August2016
v
Contents
1 Introduction .................................................................. 1
1.1 RadioAstronomy...................................................... 1
1.1.1 AHistoryofRadioAstronomy.............................. 1
1.2 RadioEmission........................................................ 2
1.2.1 PlaneWaves.................................................. 2
1.2.2 AccelerationofaChargedParticleandRadiation.......... 3
1.2.3 RadiationfromanElectroninaMagneticField............ 5
1.3 ThermalEmission ..................................................... 7
1.3.1 ThermalBremsstrahlung .................................... 7
1.3.2 ThermalEmissionandAbsorptionCoefficient............. 8
1.4 SynchrotronRadiation:NonthermalRadiation ...................... 9
1.4.1 EmissivityandSpectrum .................................... 9
1.4.2 EnergyEquipartition......................................... 11
1.5 RecombinationLines.................................................. 12
1.5.1 Frequency .................................................... 12
1.5.2 LineWidth ................................................... 13
1.5.3 LineIntensity................................................. 14
1.5.4 LinetoContinuumIntensityRatioandTemperature
Determination................................................ 16
1.6 MolecularLines........................................................ 17
1.6.1 Frequency .................................................... 17
1.6.2 Intensity ...................................................... 19
1.6.3 H MassEstimatedfromtheCOIntensity.................. 20
2
1.6.4 OtherMolecules.............................................. 21
1.7 HILine................................................................. 21
1.7.1 Frequency .................................................... 22
1.7.2 HIIntensityandColumnDensity........................... 23
1.7.3 HIVolumeDensityfromBrightnessTemperature ......... 25
1.8 RadiationsfromVariousSpecies...................................... 26
1.8.1 MolecularLines.............................................. 26
1.8.2 HILineEmission ............................................ 26
vii
viii Contents
1.8.3 RecombinationLines ........................................ 26
1.8.4 Free-FreeEmission .......................................... 27
1.8.5 SynchrotronRadiation....................................... 27
1.8.6 Black-BodyRadiation ....................................... 27
1.9 RadiativeTransfer ..................................................... 28
1.10 RadioAstronomicalObservables..................................... 31
1.10.1 ObservablesattheAntenna.................................. 31
1.10.2 PhysicalObservablesAfterConversion..................... 32
References..................................................................... 32
2 InterstellarMatter........................................................... 33
2.1 EnergyBalanceinISM................................................ 33
2.1.1 Energy-DensityandPressureBalance...................... 33
2.1.2 “Activity”inISM ............................................ 37
2.2 MolecularClouds...................................................... 38
2.2.1 Mass,Size,andIntensity .................................... 38
2.2.2 DistributionofMolecularClouds........................... 39
2.2.3 GiantMolecularClouds ..................................... 40
2.2.4 GMCandStar-FormingSites ............................... 40
2.3 TheCO-to-H Conversion ............................................ 41
2
2.3.1 MetallicityDependenceoftheConversionFactor ......... 41
2.3.2 RadialVariationoftheConversionFactor.................. 43
2.4 HIGasandClouds..................................................... 43
2.4.1 Mass,Size,andIntensityofHIClouds..................... 43
2.4.2 DistributionofHIGas....................................... 45
2.5 HIVersusH intheISM .............................................. 45
2
2.5.1 TheHItoH Transition ..................................... 45
2
2.5.2 MolecularFraction........................................... 46
2.6 GalacticScaleHIandCO............................................. 47
2.6.1 GalacticHIGasDistributions............................... 48
2.6.2 GalacticCO(H )GasDistribution.......................... 48
2
2.6.3 CentralConcentrationofCO................................ 49
2.6.4 COVersusHIinthePosition-Velocity(PV)Diagram ..... 51
2.7 RadialDistributionsofHIandH Densities ......................... 51
2
2.8 PhaseTransitionBetweenHIandH :MolecularFraction
2
andtheMolecularFront............................................... 54
2.8.1 MolecularFront.............................................. 54
2.8.2 PhaseTransitionBetweenHIandH ....................... 54
2
References..................................................................... 56
3 StarFormationandDeath.................................................. 57
3.1 MechanismofStarFormation ........................................ 57
3.1.1 SitesofStarFormation ...................................... 57
3.1.2 Schmidt’sLaw ............................................... 58
3.1.3 BirthofStars................................................. 60
3.1.4 InitialMassFunction(IMF)................................. 61
Contents ix
3.1.5 TheVirialTheorem.......................................... 62
3.1.6 GravitationalContractionofClouds........................ 63
3.1.7 JeansInstability.............................................. 66
3.1.8 ThermalInstability........................................... 68
3.1.9 Rayleigh-TaylorInstability.................................. 71
3.1.10 Kelvin-HelmholtzInstability................................ 72
3.1.11 ParkerInstability(MagneticInflation)...................... 73
3.2 EnvironmentofStarFormation....................................... 74
3.2.1 TriggeringofCloudCompression........................... 74
3.2.2 ShockWave .................................................. 75
3.2.3 FormationofMolecularClouds............................. 78
3.2.4 WhySpiralArmsAreBright................................ 78
3.3 HIIRegions............................................................ 80
3.3.1 IonizationSphere:Strm¨grenSphere ........................ 80
3.3.2 ExpandingIonizationFront ................................. 82
3.3.3 ShockCompressionofAmbientGas ....................... 84
3.4 SequentialStarFormation............................................. 84
3.4.1 PropagationofShockCompressionbyanHIIRegion..... 84
3.4.2 VariousSFRegions.......................................... 86
3.5 SupernovaRemnant ................................................... 88
3.5.1 Supernovae(SN)andSupernovaRemnants(SNR) ........ 88
3.5.2 ClassificationofSNR........................................ 89
3.5.3 ˙ (cid:2)DRelationandDistributionofSNR .................. 91
3.5.4 EvolutionofaSNR.......................................... 92
3.5.5 InteractionwiththeISM..................................... 94
3.5.6 ImplicationsofSNRsforGalaxyEvolution................ 96
References..................................................................... 99
4 GalacticStructure ........................................................... 101
4.1 TheMilkyWayGalaxy................................................ 101
4.1.1 Edge-OnViewoftheGalaxy................................ 101
4.1.2 DistancetotheGalacticCenter,R ......................... 101
0
4.1.3 RotationoftheSunAroundtheGalacticCenter ........... 106
4.1.4 GalacticRotationCurve..................................... 109
4.2 RotationCurveoftheMilkyWay..................................... 110
4.3 DistributionofISM.................................................... 113
4.3.1 (l;v )and(b;v )Diagrams .................................. 113
r r
4.3.2 Velocity-to-Space Transformation Using a
Radial-VelocityDiagram .................................... 114
4.3.3 ReliabilityofKinematicDistances.......................... 117
4.3.4 Face-OnViewoftheGalaxy ................................ 117
4.3.5 dv=dlMethodtoMeasureDistancestoArmsinthe
GCDirection................................................. 119
4.3.6 SpiralArmsintheMilkyWay............................... 121
x Contents
4.4 DensityWaves ......................................................... 122
4.4.1 GravitationalInstabilityofaDisk........................... 122
4.4.2 PatternSpeed................................................. 123
4.4.3 Resonances................................................... 124
4.5 GalacticShockWaves................................................. 125
4.5.1 ShockedStreamLine ........................................ 125
4.5.2 DensityandVelocityJumpsandInflowofGas ............ 127
4.5.3 DeterminationofPatternSpeed............................. 128
4.5.4 NuclearFuelingbyaBar.................................... 129
4.6 MagneticandRadioArms ............................................ 130
4.7 RotationofGalaxies................................................... 130
4.7.1 RotationCurves.............................................. 130
4.7.2 MeasurementofRotationVelocity.......................... 132
4.7.3 RotationCurvesandGalaxyTypes ......................... 133
4.7.4 RadialMassDistributionforFlatRotation................. 133
4.7.5 MassDistributionPerpendiculartotheDisk ............... 135
4.8 MassDistributioninGalaxies......................................... 136
4.8.1 ApproximateMassoftheGalaxy........................... 136
4.8.2 AxisymmetricMassModel.................................. 138
4.8.3 DecompositionofRotationCurve .......................... 139
4.8.4 DecompositionbyMoreEmpiricalLaws................... 142
4.8.5 DarkMatterHalo ............................................ 143
4.8.6 MassModelsforRotationCurveDecompositions......... 143
4.8.7 Direct Calculation of Mass Distribution from
RotationCurves.............................................. 148
4.8.8 VelocityFieldandGalaxyWarp ............................ 150
4.9 EvidenceforDarkMatterinGalaxies................................ 151
4.9.1 FlatRotationCurves......................................... 151
4.9.2 BinaryGalaxies.............................................. 152
4.9.3 VirialMassofaClusterofGalaxies ........................ 152
4.9.4 DarkMatter .................................................. 152
4.10 Tully-FisherRelation.................................................. 153
4.10.1 HITully-FisherRelation..................................... 153
4.10.2 COVersusHILineProfiles.................................. 154
4.10.3 TheCOTully-FisherRelation:mm-WaveCosmology .... 155
References..................................................................... 156
5 TheGalacticCenterandActivity.......................................... 159
5.1 RadioContinuumFeatures............................................ 159
5.1.1 RadioMaps................................................... 159
5.1.2 RadioSpectrum.............................................. 160
5.1.3 LinearPolarizationandMagneticField..................... 162
5.2 CentralDiskandStarFormation...................................... 163
5.2.1 SgrB:AMolecularComplexwithStar-FormingRegion.. 165