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Young Astronomers YoungYoung AstronomersAstronomers,, IAUIAU ElementsElements ofof ObservationalObservational AstronomyAstronomy Elena Terlevich, INAOE TonantzintlaTonantzintla,, JulyJuly 20052005 LectureLecture 11 Thanks to Dave Carter, Liverpool ContentsContents TechniquesTechniques andand TechnologyTechnology ofof ObservationalObservational Astronomy.Astronomy. PhysicalPhysical ProcessesProcesses atat allall wavelengths.wavelengths. ElectromagneticElectromagnetic radiationradiation fromfrom RadioRadio toto GammaGamma--RayRay wavelengths.wavelengths. TTextext BooksBooks AstrophysicalAstrophysical techniquestechniques –– C.R.C.R. Kitchen:Kitchen: IOPIOP Publishing.Publishing. ISBNISBN 00 75037503 04980498 7.7. ToolsTools ofof RadioRadio AstronomyAstronomy –– K.K. RohlfsRohlfs && T.L.T.L. Wilson:Wilson: SpringerSpringer--VerlagVerlag.. ISBNISBN 09410941 7834.7834. ModernModern AstrophysicsAstrophysics –– B.W.B.W. CarrollCarroll && D.A.D.A. OstlieOstlie:: AddisonAddison--Wesley.Wesley. ISBNISBN 00 201201 5473054730 9.9. RadiativeRadiative ProcessesProcesses inin AstrophysicsAstrophysics –– G.B.G.B. RybickiRybicki && A.P.A.P. LightmanLightman:: J.J. Wiley.Wiley. ISBNISBN 00--471471--8275982759--2.2. .. AstronomyAstronomy methodsmethods –– HaleHale BradtBradt CUPCUP –– ISBNISBN 00 521521 535514535514 ToTo taketake homehome We learn from the sky through electromagnetic radiation, or neutrinos, cosmic rays or (in the future) gravitational waves. Electromagnetic radiation.. travels at c and can behave as a flux of photons, each one of E=h ν or as waves. One can convert between λ, ν, and E using algebra. Bands from radio waves (lowest frequency) to gamma rays (highest). If thermal radiation, average photon frequency (energy) is an indicator of source temperature. Absorption of photons in the atmosphere is frequency dpendent, some bands from high altitude balloons or space. Absorption by dust and atoms in the ISM --> cosmos more or less transparent, depending on frequency band. IRAS - Infrared Astronomical Atacama Large Millimeter Satelite (IR) Array - ALMA (Microwave) Paranal Observatory – VLT (Visble + etc.) 100-meter Green Bank Telescope (Radio) International Ultraviolet Explorer – IUE (UV) International Gamma-Ray Astrophysics Laboratory - INTEGRAL (Gamma) Chandra Spacecraft (X-ray) The VisibleSpectrum Fig. 1.1a The electromagnetic spectrum (the visible Spectrum). Wavelengths shorter than 325 nm can only be observed from rocket, or satellite altitude while atmospheric water vapor makes infrared and microwave observations possible only from high, dry sites or from balloon or satellite altitudes. The atmosphere is very transparent for radio observations at wavelengths shorter than 10 m (Fig. 1.1b). Fig 1.1b Atmospheric absorption. Wavelength regions where the atmosphere is essentially transparent, such as the visible spectrum, are called atmospheric windows. Wavelengths and atmosphere not drawn to scale. ExampleExample ofof atmosphericatmospheric transmissiontransmission aroundaround 1.21.2 µµmm AtmosphericAtmospheric transmissiontransmission aroundaround 1.71.7µµmm AtmosphericAtmospheric transmissiontransmission aroundaround 2.22.2 µµmm AtmoshericAtmosheric transmissiontransmission AroundAround 2.72.7 µµmm NearNear IRIR atmosphericatmospheric transmissiontransmission AtmosphericAtmospheric transmissiontransmission atat aroundaround 2222 µµmm RadiationRadiation ProcessesProcesses SpectralSpectral LinesLines ContinuumContinuum Radio Neutral Hydrogen (HI) 21cm Thermal Bremsstrahlung (20m- 1mm) fine structure line – neutral gas (free-free emission) – HII Hydrogen recombination lines – regions ionised gas Synchrotron Radiation – Radio Galaxies, Pulsars, OH, H2O etc. Masers – dense, warm molecular gas Supernovae. Molecular Rotation lines – cold Thermal emission from dust molecular gas – cold, dense gas. Submillimetre Molecular Rotation Lines – Thermal emission – warm and far IR warm, dense gas. dust. (600 microns – Solid State features (silicates) – 5 microns) dust. Hydrogen recombination lines – HII regions. RadiationRadiation ProcessesProcesses SpectralSpectral LinesLines ContinuumContinuum Near IR (5 Hydrogen recombination lines – Thermal emission – Hot microns – ionised gas gas. 800nm) Molecular Vibration-Rotation Stars. lines – shock or UV excited gas Optical (800- Atomic Forbidden Lines – hot, Starlight 300 nm) low density gas. Extinction by dust. Hydrogen recombination lines – HII regions, denser gas. Ultraviolet Atomic Forbidden Lines – hot, Continuum absorption at (300-10 nm) low density gas – Quasars & λ<912 Angstroms by AGN. Hydrogen. Hydrogen recombination lines – HII regions, denser gas 220nm extinction feature – carbon dust. RadiationRadiation ProcessesProcesses SpectralSpectral LinesLines ContinuumContinuum X-Ray (10 – Hydrogen like lines from Thermal emission – Hot gas 0.01 nm) highly ionised gas e.g in supernovae, accretion disks. Thermal Bremsstrahlung – hot gas in clusters of galaxies Synchrotron - Jets Gamma-Ray Electron-Positron annihilation. Thermal emission from (0.01 – 0.0001 . Relativistic shocks –Supernovae nm) and GRBs. MultiMulti--WavelengthWavelength ObservationsObservations DifferentDifferent processesprocesses dominatedominate atat differentdifferent wavelengthswavelengths ToTo establishestablish thethe physicsphysics ofof aa sourcesource youyou oftenoften needneed toto observeobserve itit atat aa varietyvariety ofof wavelengthswavelengths ClustersClusters ofof GalaxiesGalaxies Left X-ray Thermal Bremsstrahlung, right optical starlight RadioRadio GalaxiesGalaxies Red radio synchrotron, blue-green optical starlight RadioRadio GalaxiesGalaxies Red radio synchrotron, blue-green optical starlight JupiterJupiter Left is in radio, synchrotron (high magnetic field); right is optical which reflected sunlight. CCrabrab NeNebulabula X-ray X-ray + Optical Infrared X-ray is synchrotron radiation Optical and Infrared mostly atomic line radiation Radio is both synchrotron and thermal bremsstrahlung radiation Radio (VLA) M87M87 Radio, optical, X-ray (from left). There is a synchrotron jet in the centre at all wavelengths RadioRadio imageimage ofof HIIHII regionregion Thermal bremsstrahlung and thermal dust radiation BasicBasic DefinitionsDefinitions IntensityIntensity IIν defineddefined by:by: dWdW == IIν coscosθθ ddΩΩ ddσσ ddνν WW isis power;power; ddΩΩ isis sourcesource solidsolid angle;angle; ddσσ isis detectordetector area;area; ddνν isis frequencyfrequency bandwidth;bandwidth; θθ isis thethe angleangle betweenbetween thethe normalsnormals toto sourcesource andand detectordetector FluxFlux DensityDensity FFν defineddefined by:by: FFν == ∫∫Ω IIν coscosθθ ddΩΩ NormalNormal unitsunits areare JanskysJanskys:: 1Jansky1Jansky == 1010-26 WattsWatts mm-2 HzHz-1 RadiativeRadiative TransferTransfer dIdIλ == --κκλ ρρ IIλ dsds ++ jjλ ρρ dsds WhereWhere IIλ isis intensity,intensity, ρρ isis density,density, κκλ isis thethe absorptionabsorption coefficient,coefficient, andand jjλ isis thethe emissionemission coefficient.coefficient. DefineDefine thethe SourceSource FunctionFunction:: SSλ ≡≡ jjλ // κκλ dIdIλ/ds/ds == --κκλ ρρ ((IIλ -- SSλ )) RadiativeRadiative TransferTransfer DefineDefine thethe OpticalOptical depth:depth: ddττλ == κκλ ρρ dsds RadiativeRadiative transfertransfer equationequation becomes:becomes: dIdIλ//ddττλ == -- IIλ ++ SSλ IfIf thethe sourcesource functionfunction SSλ isis constantconstant thenthen thethe solutionsolution is:is: -τ -τ IIλ((ττλ)) == IIλ(0)(0) ee λ ++ SSλ (1(1 -- ee λ )) IfIf insteadinstead wewe useuse frequencyfrequency νν asas thethe variable:variable: -τν -τν IIν((ττν)) == IIν (0)(0) ee ++ SSν (1(1 -- ee )) BlackbodyBlackbody RadiationRadiation BlackbodyBlackbody radiationradiation isis radiationradiation whichwhich isis inin thermalthermal equilibrium.equilibrium. DerivationDerivation ofof thethe functionalfunctional formform ofof thethe spectrumspectrum isis aa problemproblem inin statisticalstatistical physics.physics. 2 5 hc/λkT BBλ(T)(T) == (2(2 hh cc // λλ )) (1(1 // ((ee –– 1))1)) WithWith FrequencyFrequency νν ratherrather thanthan wavelengthwavelength λλ asas thethe variable:variable: 3 2 hν/kT BBν(T)(T) == (2(2 hh νν //cc )) (1(1 // ((ee –– 1))1)) BlackbodyBlackbody RadiationRadiation B(T+B(T+∆∆T)T) >> B(T)B(T) AtAt allall νν forfor positivepositive ∆∆TT BlackbodyBlackbody RadiationRadiation InIn thethe LowLow FrequencyFrequency limitlimit hhνν <<<< kTkT:: eehν/kT == (1(1 ++ hhνν//kTkT ++ …………)) 2 2 BBν(T)(T) == (2(2 νν kk TT//cc )) ThisThis isis thethe RayleighRayleigh--JeansJeans approximation,approximation, andand isis usedused atat lowlow frequency,frequency, particularlyparticularly inin RadioRadio Astronomy.Astronomy. BrightnessBrightness TemperatureTemperature BrightnessBrightness temperaturetemperature TTb isis defineddefined by:by: IIν == BBν(T(Tb)) InIn thethe RayleighRayleigh--JeansJeans approximationapproximation 2 2 IIν == 22 νν kk TTb // cc 2 2 TTb == IIν cc // (2(2 νν k)k) TTb isis thethe temperaturetemperature thatthat aa blackbodyblackbody wouldwould havehave inin orderorder toto radiateradiate atat thethe measuredmeasured intensityintensity IIν atat aa givengiven frequencyfrequency νν.. ItIt isis aa lowerlower limitlimit toto thethe truetrue thermodynamicthermodynamic temperature.temperature. RadiativeRadiative TransferTransfer inin thethe RR--JJ limitlimit -τν -τν IIν((ττν)) == IIν (0)(0) ee ++ SSν (1(1 -- ee )) InIn ThermodynamicThermodynamic EquilibriumEquilibrium SSν == BBν(T)(T) 2 2 2 2 UsingUsing IIν == 22 νν kk TTb // cc ;; BBν(T)(T) == 22 νν kk T/cT/c -τν -τν
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