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Introduction to Radio Astronomy
What is radio astronomy?
Radio astronomy is the study of radio waves originating outside the Earth. The radio range of
frequencies · or wavelengths Õ is loosely defined by three factors: atmospheric transparency,
current technology, and fundamental limitations imposed by quantum noise. Together they
yield a boundary between radio and far-infared astronomy at frequency · Ø 1 THz (1 THz Ñ 1012 Hz) or wavelength Õ = c=· Ø 0:3 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 and visible wavebands, suitable for ground-based astronomy. The visible window is relatively narrow in terms of logarithmic frequency or wavelength; it spans the wavelengths of peak thermal emission from T Ø 3000 K to T Ø 10000 K blackbodies. Since we can see visible light without 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 nebula in Orion's "sword" is visible as a fuzzy blob to the unaided eye on a dark night), and objects shining by reflected starlight (e.g., planets and moons). Knowing the theoretical spectrum of blackbody radiation, astronomers a century ago correctly deduced that stars having nearly blackbody spectra would be undetectably faint as radio sources, and incorrectly assumed that there would be no other astronomical radio sources. Consequently astronomers failed to pursue radio astronomy until cosmic radio emission was discovered accidentally in 1932 and followed up by radio engineers.
Ground-based astronomy is confined to the visible and radio atmospheric windows of the electromagnetic spectrum. The radio window is much wider than the visible window when plotted on logarithmic wavelength or frequency scales. One Angstrom Ñ 10À10 m = 10À7 1 of 11 09/02/2008 02:03 PM Introduction to Radio Astronomy http://www.cv.nrao.edu/course/astr534/Introradastr...
mm. Image credit Vibrational transitions of atmospheric molecules such as CO 2, O2, and H2O have energies E = h· comparable with those of mid-infrared photons and absorb most extraterrestrial mid-infrared radiation before it reaches the ground. Lower-energy rotational transitions of atmospheric molecules help define the boundary between the far-infrared band and shortwavelength limit of the radio window.
Ground-based radio astronomy is increasingly degraded at wavelengths Õ 1 m (· 300 MHz, where 1 MHz Ñ 106 Hz) by variable ionospheric refraction, which is proportional to Õ 2.
Cosmic radio waves having wavelengths Õ 30 m (· 10 MHz) are usually reflected back into 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 additio n, Rayleigh scattering of sunlight by atmospheric dust at visible and ultraviolet wavelengths brightens the sky enough to prevent nearly all daytime optical observations. Radio wavelengths are much longer than the size of these dust grains and the Sun is not an overwhelmingly bright radio source, so the radio sky is always dark and most radio observations can be made day or night.
The atmosphere is not perfectly transparent at any radio frequency. The figure below shows how the zenith (the direction directly overhead) opacity Üz varies with frequency for a typical summer night in Green Bank, WV, with a water-vapor column density of 1 cm, 55% cloud cover, and surface air temperature T = 288 K = 15 C. The total opacity is the sum of
several components (Leibe, H. J. 1985, Radio Science, 20, 1069):
(1) The continuum opacity of dry air results from viscous damping of the free rotatio ns of nonpolar molecules. It is relatively small ( Üz Ù 0:01) and nearly independent of frequency.
(2) Molecular oxygen (O2 ) has no permanent electric dipole moment, but it has rotational transitions that can absorb radio waves because it has a permanent magnetic dipole mo ment.
The pressure-broadened complex of oxygen lines near 60 GHz is quite opaque ( Üz µ 1 ) and prevents ground-based observations between about 52 GHz and 68 GHz.
(3) Hydrosols are water droplets small enough (radius Ô 0:1 mm) to remain suspended in clouds. Since they are much smaller than the wavelength even at 120 GHz ( Õ Ù 2:5 mm), they follow the Rayleigh approximation and their opacity is proportional to Õ À2 or · 2.
(4) The strong water-vapor line at · Ù 22:235 GHz is pressure broadened to Á· Ù 4 GHz width. The so-called "continuum" opacity of water vapor at radio wavelengths is actually the sum of line-wing optical depths from much stronger water lines at infrared wavelengths. In the plotted frequency range, this continuum opacity is also proportional to · 2. Both the line and continuum 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 as a length (e.g., 1 cm) rather than a true column density (e.g., 1 gm cm À2 ), but the two are equivalent because the density of water is one in cgs units.
The zenith atmospheric opacity for a typical summer night at Green Bank. An opacity Ü attenuates the power received from an astronomical source by the factor exp(ÀÜ ). The oxygen and dry-air opacities are nearly constant, while the water-vapor and hydrosol (water droplets in clouds) contributions vary significantly with weather.
A partially absorbing T Ø 300 K atmosphere doesn't just attenuate the incoming radio radiation; it also emits radio noise that degrades the sensitivity of ground-based radio observations. For example, emission by water vapor in the warm and humid atmosphere above Green Bank, WV precludes sensitive observations near the water-vapor line at · Ø 22 GHz (1 GHz Ñ 109 Hz) during the summer. Green Bank can be quite cold and dry in the winter, allowing observations at frequencies up to about 115 GHz.
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 at frequencies up to about 1 THz. Image credit The very best sites for observing at higher frequencies are exceptionally high and dry, with typical pwv 0.1 cm.
The Atacama Large Millimeter Array (ALMA) will ultimately cover the ten frequency bands indicated by numbered horizontal bars. The gaps between bands 8 and 9 and between bands 9 and 10 match frequency ranges with very low atmospheric transmission. Image credit
The radio window in uniquely broad, spaning roughly five decades of frequency (10 MHz to 1
THz) 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 radio spectrum effectively.
The radio window was opened before observations in other wavebands could be made from above 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:
It is often violent, emphasizing radio galaxies, quasars, supernovae, pulsars, etc. rather than long-lived stars.
It filled with sources ultimately powered by gravity instead of nuclear fusion, the principal energy source of visible stars.
It is cosmologically distant. Most continuum radio sources are extragalactic, and they have evolved so strongly over cosmic time that most are at cosmological lookback times.
It can be very cold. The cosmic microwave background dominates the electromagnetic energy of the universe, but its 2.7 K blackbody spectrum is confined to radio and far-infrared wavelengths. Atoms and molecules of cold interstellar gas emit spectral lines at radio wavelengths.
With the advent of astronomy from space, the entire electromagnetic spectrum has beco me accessible. Many sources discovered by radio astronomers can be now studied in other wavebands, and new objects discovered in other wavebands (e.g., gamma-ray bursters) can be studied by radio astronomers. Radio astronomy is no longer a separate field; it is one facet of multiwavelength astronomy.
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 is shown as a function of the logarithm of the wavelength, so the highest peaks correspond to the strongest spectral components.
Most of the electromagnetic energy of the universe is in the cosmic microwave backgro und radiation produced by the hot big bang. It has a nearly perfect 2.73 K blackbody spectrum peaking at Õ Ù 1 mm = 103 Öm. The strong UV/optical peak is primarily thermal emission from stars supplemented by a smaller contribution of thermal and nonthermal emission from the active galactic nuclei (AGN) in Seyfert galaxies and quasars. Most of the comparably strong cosmic infrared background is thermal re-emission from interstellar dust heated by absorbing that UV/optical radiation. The cosmic X-ray and gamma-ray backgrounds are mixtures 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 of the electromagnetic spectrum, and modern radio telescopes are sensitive enough to detect extremely faint radio emission.
Long Wavelengths and Low Frequencies
Many unique scientific and technical features of radio astronomy result from radio waves occupying the long-wavelength end of the electromagnetic spectrum. Macroscopic wavelengths (Õ Ø 0:3 mm to Ø 30 m) enable groups of charged particles moving together in volumes Õ 3 to produce strong coherent emission, accounting for the astounding radio brightness of pulsars. Dust scattering is negligible because dust grains are much smaller than radio wavelengths, so the dusty interstellar medium (ISM) is transparent. Radio astro nomers
were the first to see through the dusty disk of our Galaxy and discover the compact radio source 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 resolution of 0.1 arcsec (Zhao, J.-H., & Goss, W. M. 1998, ApJ, 499, L163). Sgr A*, the bright unresolved radio source in the middle of this image, is powered by the supermassive (3:7 Â 106 solar masses) black hole in the Galactic center.
Low frequencies imply low photon energies E = h·. Thus radio spectral lines trace extremely low-energy transitions produced by atomic hyperfine splitting (e.g., the ubiquitous 21 cm line of neutral hydrogen), quantized rotation of polar molecules (e.g., carbon monoxide) in interstellar space, and high-level recombination lines from interstellar atoms. The low values of the dimensionless quantity h·=(kT ) Ü 1 at radio frequencies ensure that nearly everything emits radio photons at some low level. Cold astronomical sources may emit most strongly at radio 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 micro wave amplification by stimulated emssion of radiation) are common. Radio synchrotron sources live long after their electrons were accelerated to relativistic energies, so they provide long-lasting archaeological records of past energetic phenomena. Plasma effects (scattering,
dispersion, Faraday rotation, etc.) are strong enough to trace the interstellar electron density and magnetic field strength. On the negative side, radio astronomers must deal with large and fluctuating natural backgrounds of emission from the ground and from the atmosphere.
Telescopes having very large diameters D are required for good angular resolution Ò Ù Õ =D radians at radio wavelengths. On the other hand, huge interferometers spanning D Ø 104 km are practical and precision telescopes (e.g., having reflectors with rms surface erro rs Û Õ=16) can be built. Paradoxically, the finest angular resolution is obtainable at the long-wavelength (radio) end of the electromagnetic spectrum.
The D = 100 m Green Bank Telescope (GBT) in West Virginia is the largest moving structure on land and weighs 16 million pounds ( Ù 7 Â 106 kg), yet the rms deviation of its surface from a perfect paraboloid can be kept below Û Ù 0:3 mm, the thickness of three sheets of paper. The two semitrailers at the lower right are each 53 feet (16 m) long. The green grass and trees are not good signs for high-frequency observing; compare this with the ALMA and VLA site photos. Image credit
The 1 km configuration of the Very Large Array (VLA) of 27 25-m telescopes located on the semi-desert plains of San Augustin in New Mexico at an elevation of 7,000 feet (about 2100 m). The individual dishes can be moved to span D = 1, 3.4, 11, or 36 km to synthesize apertures having those diameters and yield angular resolutions ranging from Ò Ù 45 arcsec at · = 1:4 GHz in the smallest configuration to Ò Ù 0:04 arcsec at · = 43 GHz in the largest.