introduction to radio astronomy · 2008. 9. 2. · thermal emission from k to k blackbodies. since...

Introduction to Radio Astronomy

What is radio astronomy?

Radio astronomy is the study of radio waves originating outside the Earth. The radio range offrequencies · or wavelengths Õ is loosely defined by three factors: atmospheric transparency,current technology, and fundamental limitations imposed by quantum noise. Together theyyield a boundary between radio and far-infared astronomy at frequency THz (1 THz

Hz) or wavelength mm.

Atmospheric Windows

The Earth's atmosphere absorbs electromagnetic radiation at most infrared, ultraviolet, X-ray,and gamma-ray wavelengths, so there are only two atmospheric windows, in the radio andvisible wavebands, suitable for ground-based astronomy. The visible window is relativelynarrow in terms of logarithmic frequency or wavelength; it spans the wavelengths of peakthermal emission from K to K blackbodies. Since we can see visible lightwithout the aid of instruments, early observational astronomy was limited to visible objects—primarily stars, clusters and galaxies of stars, hot gas ionized by stars (e.g., the Orion nebulain Orion's "sword" is visible as a fuzzy blob to the unaided eye on a dark night), and objectsshining by reflected starlight (e.g., planets and moons). Knowing the theoretical spectrum ofblackbody radiation, astronomers a century ago correctly deduced that stars having nearlyblackbody spectra would be undetectably faint as radio sources, and incorrectly assumed thatthere would be no other astronomical radio sources. Consequently astronomers failed topursue radio astronomy until cosmic radio emission was discovered accidentally in 1932 andfollowed up by radio engineers.

Ground-based astronomy is confined to the visible and radio atmospheric windows of theelectromagnetic spectrum. The radio window is much wider than the visible window whenplotted on logarithmic wavelength or frequency scales. One Angstrom m

· Ø 1Ñ 0 1 12 Õ =· :3 = c Ø 0

T 000 Ø 3 T 0000 Ø 1

Ñ 0 1 À10 = 0 1 À7

Introduction to Radio Astronomy http://www.cv.nrao.edu/course/astr534/Introradastr...

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mm. Image credit

Vibrational transitions of atmospheric molecules such as CO2, O2, and H2O have energies

E · comparable with those of mid-infrared photons and absorb most extraterrestrialmid-infrared radiation before it reaches the ground. Lower-energy rotational transitions ofatmospheric molecules help define the boundary between the far-infrared band and short-wavelength limit of the radio window.

Ground-based radio astronomy is increasingly degraded at wavelengths Õ m (· 00MHz, where 1 MHz Hz) by variable ionospheric refraction, which is proportional to Õ . Cosmic radio waves having wavelengths Õ 0 m (· 0 MHz) are usually reflected backinto space by the Earth's ionosphere.

Ultraviolet photons have energies close to the binding energies of the outer electrons in atoms,so electronic transitions in atoms account for the high ultraviolet opacity of the atmosphere.Higher-energy nuclear transitions produce X-ray and gamma-ray absorption. In addition,Rayleigh scattering of sunlight by atmospheric dust at visible and ultraviolet wavelengthsbrightens the sky enough to prevent nearly all daytime optical observations. Radiowavelengths are much longer than the size of these dust grains and the Sun is not anoverwhelmingly bright radio source, so the radio sky is always dark and most radioobservations can be made day or night.

The atmosphere is not perfectly transparent at any radio frequency. The figure below showshow the zenith (the direction directly overhead) opacity Ü varies with frequency for a typicalsummer night in Green Bank, WV, with a water-vapor column density of 1 cm, 55% cloudcover, and surface air temperature T 88 K 5 C. The total opacity is the sum ofseveral components (Leibe, H. J. 1985, Radio Science, 20, 1069):(1) The continuum opacity of dry air results from viscous damping of the free rotations ofnonpolar molecules. It is relatively small ( ) and nearly independent of frequency.(2) Molecular oxygen (O ) has no permanent electric dipole moment, but it has rotationaltransitions that can absorb radio waves because it has a permanent magnetic dipole moment. The pressure-broadened complex of oxygen lines near 60 GHz is quite opaque ( ) andprevents ground-based observations between about 52 GHz and 68 GHz. (3) Hydrosols are water droplets small enough (radius mm) to remain suspended inclouds. Since they are much smaller than the wavelength even at 120 GHz ( mm),they follow the Rayleigh approximation and their opacity is proportional to or · .(4) The strong water-vapor line at GHz is pressure broadened to GHzwidth. The so-called "continuum" opacity of water vapor at radio wavelengths is actually thesum of line-wing optical depths from much stronger water lines at infrared wavelengths. In theplotted frequency range, this continuum opacity is also proportional to · . Both the line andcontinuum opacities are directly proportional to the column density of precipitable water vapor(pwv) along the line-of-sight through the atmosphere. The pwv is conventionally expressed asa length (e.g., 1 cm) rather than a true column density (e.g., 1 gm cm ), but the two areequivalent because the density of water is one in cgs units.

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> 1 < 3Ñ 0 1 6 2

> 3 < 1

z

= 2 = 1

Ü :01 z Ù 0

2

Ü z µ 1

Ô :1 0Õ :5 Ù 2

Õ À2 2

· 2:235 Ù 2 Á· Ù 4

2

À2


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The zenith atmospheric opacity for a typical summer night at Green Bank. An opacity Üattenuates the power received from an astronomical source by the factor . Theoxygen and dry-air opacities are nearly constant, while the water-vapor and hydrosol (waterdroplets in clouds) contributions vary significantly with weather.

A partially absorbing K atmosphere doesn't just attenuate the incoming radioradiation; it also emits radio noise that degrades the sensitivity of ground-based radioobservations. For example, emission by water vapor in the warm and humid atmosphereabove Green Bank, WV precludes sensitive observations near the water-vapor line at GHz (1 GHz Hz) during the summer. Green Bank can be quite cold and dry in thewinter, allowing observations at frequencies up to about 115 GHz.

exp(ÀÜ )

T 00 Ø 3

· 2 Ø 2Ñ 0 1 9


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The Atacama Large Millimeter Array (ALMA) is being built on this extremely high (5000 m)and dry desert site near Cerro Chajnator in Chile with excellent atmospheric transparency atfrequencies up to about 1 THz. Image credit

The very best sites for observing at higher frequencies are exceptionally high and dry, withtypical pwv < 0.1 cm.


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The Atacama Large Millimeter Array (ALMA) will ultimately cover the ten frequency bandsindicated by numbered horizontal bars. The gaps between bands 8 and 9 and between bands9 and 10 match frequency ranges with very low atmospheric transmission. Image credit

Astronomy in the Radio Window

The radio window in uniquely broad, spaning roughly five decades of frequency (10 MHz to 1THz) and wavelength. This has both scientific and practical consequences:

A wide variety of astronomical sources, thermal and nonthermal radiation mechanisms,and propagation phenomena can be studied at radio wavelengths. A wide variety of radio telescopes and observing techniques are needed to cover the radiospectrum effectively.

The radio window was opened before observations in other wavebands could be made fromabove the atmosphere, so early radio astronomy was a science of discovery and serendipity. It revealed a "parallel universe" of unexpected sources never seen, or at least not recognized,by optical astronomers. Major discoveries of radio astronomy include:

Nonthermal radiation from our Galaxy and other astronomical sourcesThe violent universe of radio galaxies and quasars powered by supermassive black holesCosmological evolution of galaxies and quasarsEmission from cold interstellar gasCosmic microwave background radiation from the big bangNeutron starsGravitational radiationExtrasolar planetsGravitational lensing

Some features of this parallel universe are:

It is often violent, emphasizing radio galaxies, quasars, supernovae, pulsars, etc. ratherthan long-lived stars.It filled with sources ultimately powered by gravity instead of nuclear fusion, the principalenergy source of visible stars.It is cosmologically distant. Most continuum radio sources are extragalactic, and theyhave evolved so strongly over cosmic time that most are at cosmological lookback times.It can be very cold. The cosmic microwave background dominates the electromagneticenergy of the universe, but its 2.7 K blackbody spectrum is confined to radio andfar-infrared wavelengths. Atoms and molecules of cold interstellar gas emit spectral linesat radio wavelengths.

With the advent of astronomy from space, the entire electromagnetic spectrum has becomeaccessible. Many sources discovered by radio astronomers can be now studied in otherwavebands, and new objects discovered in other wavebands (e.g., gamma-ray bursters) canbe studied by radio astronomers. Radio astronomy is no longer a separate field; it is one facetof multiwavelength astronomy.


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The big picture: the electromagnetic spectrum of the universe (Dwek, E., & Barker, M. K.2002, ApJ, 575, 7). The brightness ·I per logarithmic frequency (or wavelength) interval isshown as a function of the logarithm of the wavelength, so the highest peaks correspond tothe strongest spectral components.

Most of the electromagnetic energy of the universe is in the cosmic microwave backgroundradiation produced by the hot big bang. It has a nearly perfect 2.73 K blackbody spectrumpeaking at mm = 0 m. The strong UV/optical peak is primarily thermal emissionfrom stars supplemented by a smaller contribution of thermal and nonthermal emission fromthe active galactic nuclei (AGN) in Seyfert galaxies and quasars. Most of the comparablystrong cosmic infrared background is thermal re-emission from interstellar dust heated byabsorbing that UV/optical radiation. The cosmic X-ray and gamma-ray backgrounds aremixtures of nonthermal emission (e.g., synchrotron radiation or inverse-Compton scattering)from high-energy particles accelerated by AGN and thermal emission from very hot gas (e.g.,intracluster gas). By comparison, the cosmic radio-source background is extremely weak. Nevertheless, radio sources trace most phenomena that are detectable in other portions ofthe electromagnetic spectrum, and modern radio telescopes are sensitive enough to detectextremely faint radio emission.

Long Wavelengths and Low Frequencies

Many unique scientific and technical features of radio astronomy result from radio wavesoccupying the long-wavelength end of the electromagnetic spectrum. Macroscopicwavelengths ( mm to m) enable groups of charged particles moving together involumes < to produce strong coherent emission, accounting for the astounding radiobrightness of pulsars. Dust scattering is negligible because dust grains are much smaller thanradio wavelengths, so the dusty interstellar medium (ISM) is transparent. Radio astronomers

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Õ :3 Ø 0 Ø 0 3Õ3


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were the first to see through the dusty disk of our Galaxy and discover the compact radiosource Sgr A* powered by a supermassive black hole at the Galaxy center.

The nucleus of the Milky Way Galaxy observed with the VLA at 1.3 cm at an angular resolutionof 0.1 arcsec (Zhao, J.-H., & Goss, W. M. 1998, ApJ, 499, L163). Sgr A*, the brightunresolved radio source in the middle of this image, is powered by the supermassive( solar masses) black hole in the Galactic center.

Low frequencies imply low photon energies E ·. Thus radio spectral lines trace extremelylow-energy transitions produced by atomic hyperfine splitting (e.g., the ubiquitous 21 cm lineof neutral hydrogen), quantized rotation of polar molecules (e.g., carbon monoxide) ininterstellar space, and high-level recombination lines from interstellar atoms. The low values ofthe dimensionless quantity at radio frequencies ensure that nearly everythingemits radio photons at some low level. Cold astronomical sources may emit most strongly atradio wavelengths (e.g., the 2.73 K cosmic microwave background, cold interstellar gas). Stimulated emission is important, and natural masers ("maser" is an acronym for microwaveamplification by stimulated emssion of radiation) are common. Radio synchrotron sourceslive long after their electrons were accelerated to relativistic energies, so they providelong-lasting archaeological records of past energetic phenomena. Plasma effects (scattering,

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= h

h·=(kT ) Ü 1


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dispersion, Faraday rotation, etc.) are strong enough to trace the interstellar electron densityand magnetic field strength. On the negative side, radio astronomers must deal with large andfluctuating natural backgrounds of emission from the ground and from the atmosphere.

Telescopes having very large diameters D are required for good angular resolution

radians at radio wavelengths. On the other hand, huge interferometers spanning kmare practical and precision telescopes (e.g., having reflectors with rms surface errorsÛ =16) can be built. Paradoxically, the finest angular resolution is obtainable at thelong-wavelength (radio) end of the electromagnetic spectrum.

The D 00 m Green Bank Telescope (GBT) in West Virginia is the largest moving structureon land and weighs 16 million pounds ( kg), yet the rms deviation of its surfacefrom a perfect paraboloid can be kept below mm, the thickness of three sheets ofpaper. The two semitrailers at the lower right are each 53 feet (16 m) long. The green grassand trees are not good signs for high-frequency observing; compare this with the ALMA andVLA site photos. Image credit

Ò =D Ù Õ

D 0 Ø 1 4

< Õ

= 1Ù 0 7Â 1 6

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The 1 km configuration of the Very Large Array (VLA) of 27 25-m telescopes located on thesemi-desert plains of San Augustin in New Mexico at an elevation of 7,000 feet (about 2100m). The individual dishes can be moved to span D 1, 3.4, 11, or 36 km to synthesizeapertures having those diameters and yield angular resolutions ranging from arcsec at· :4 GHz in the smallest configuration to arcsec at · 3 GHz in the largest. Coherent (phase preserving) amplifiers allow the signals from each telescope to be combinedwith signals from the other 26 telescopes without loss of sensitivity, a requirement for makingaccurate images of faint extended sources. Image credit

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= 1 Ò :04 Ù 0 = 4


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The Very Long Baseline Array (VLBA) of 10 25-m telescopes spanning the continent from St.Croix, VI to Mauna Kea, HI yields angular resolution as fine as Ò :00017 arcsec, surpassingthe resolution of the Hubble Space Telescope by two orders of magnitude. Image credit

Coherent (phase preserving) amplifiers are required for accurate interferometric imaging offaint extended sources because they allow the signal from each telescope in a multielementinterferometer to be amplified before combination with the signals from the other telescopes,rather than being divided among the other telescopes before combination. The minimumpossible noise temperature of a coherent receiver is T ·=k owing to quantum noise, whichis proportional to frequency, so coherent amplifiers at visible-light frequencies must have noisetemperatures T 0 K. Aperture-synthesis interferometry at radio wavelengths providesunparalleled sensitivity, fidelity, resolution, and position accuracy. This is a huge practicaladvantage of radio astronomy.

= 0

= h

> 1 4


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Multi-epoch VLBA position measurements of T Tau Sb, a companion of the well-known youngstellar object T Tauri, allowed Loinard et al. (2007, ApJ, 671, 546) to determine its parallaxdistance with unprecedented accuracy: pc, a significant improvement overthe Hipparchos distance pc, and even detect accelerated proper motion.

d 46:7 :6 = 1 Æ 0d 77 = 1 +68

À39


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introduction to radio astronomy · 2008. 9. 2. · thermal emission from k to k blackbodies. since...

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