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 $\nu$ or wavelengths $\lambda$ is loosely defined by three factors: atmospheric transparency, technology, and quantum noise.  Together they yield a boundary between radio and far-infared astronomy at $\nu \sim 1$ THz  (1 THz $\equiv 10^{12}$ Hz) or $\lambda = c / \nu \sim 0.3$ 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, at radio and visible wavelengths, 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 $T \sim 3000$ K to $T \sim 10000$ K blackbodies. Since we can see visible light without the aid of instruments, early astronomy concentrated on 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 in the early twentieth century correctly deduced that stars having nearly blackbody spectra would be undetectably faint as radio sources and incorrectly decided that there would be no other astronomical radio sources. Consequently astronomers failed to exploit radio astronomy until cosmic radio emission was discovered accidentally and followed up by radio engineers.

atmospheric windows
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 $\equiv 10^{-10}$ m $= 10^{-7}$ mm. Image credit

Vibrational transitions of atmospheric molecules, O2 and H2O in particular, have energies $E = h \nu$ comparable with those of far-infrared photons and  absorb most extraterrestrial far-infrared radiation before it reaches the ground.  Lower-energy rotational transitions of atmospheric molecules help define the short-wavelength limit of the radio window.  Ground-based radio astronomy is increasing degraded at wavelengths $\lambda > 1$ m ($\nu < 300$ MHz, where 1 MHz $\equiv 10^6$ Hz) by variable ionospheric refraction, which is proportional to $\lambda^2$.  Cosmic radio waves having wavelengths $\lambda > 30$ m ($\nu <10$ 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. Likewise, higher-energy nuclear transitions produce X-ray and gamma-ray absorption.  In addition, Rayleigh scattering of sunlight by atmospheric dust at visible and ultraviolet wavelengths brightens the sky enough to prevent daytime optical astronomy.  Radio wavelengths are much larger than 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 completely transparent at any radio frequency.  The figure below shows how the zenith (the direction directly overhead) opacity $\tau_{\rm z}$ varies with frequency for a typical summer day in Green Bank, WV, with a water-vapor column density of 1 cm, 55% cloud cover, and surface air temperature $T = 288{\rm ~K} = 15{\rm ~C}$.  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 rotations of nonpolar molecules.  It is relatively small ($\tau_{\rm z} \approx 0.01$) and nearly independent of frequency.
(2) Molecular oxygen (O$_2$) has no permanent electric dipole moment, but it can absorb radio waves because it does have a permanent magnetic dipole moment.  The pressure-broadened complex of oxygen lines near 60 GHz is quite opaque ($\tau_{\rm z} \gg 1$) and prevent ground-based observations  between about 52 GHz and 68 GHz. 
(3) Hydrosols are water droplets small enough (radius $\leq 0.1$ mm) to remain suspended in clouds.  Since they are much smaller than the wavelength even at 120 GHz ($\lambda \approx 2.5$ mm), they follow the Rayleigh approximation and their opacity is proportional to $\lambda^{-2}$ or $\nu^2$.
(4) The strong water-vapor line at $\nu \approx 22.235$ GHz is pressure broadened to $\Delta \nu \approx 4$ 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 $\nu^2$.  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$^{-2}$), but the two are equivalent because the density of water is one in cgs units.

Zenith opacity over Green Bank in typical summer weather
The zenith atmospheric opacity for a typical summer day at Green Bank. An opacity $\tau$ attenuates the power received from an astronomical source by the factor $\exp(-\tau)$.


A partially absorbing $T \sim 300$ 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 $\nu \sim 22$ GHz (1 GHz $ \equiv 10^9$ Hz) during the summer.

Moon over desert ALMA site

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 good atmospheric transparency at frequencies up to about 1 THz. Image credit


Astronomy in the Radio Window

The radio window in uniquely broad, spaning five decades of frequency and wavelength.  This has both scientific and practical consequences:

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:

Some features of this parallel universe are:

With the advent of astronomy from space, the entire electromagnetic spectrum has become 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.

cosmic em spectrum
The big picture: the electromagnetic spectrum of the universe (Dwek, E., & Barker, M. K. 2002, ApJ, 575, 7). The brightness $\nu I_\nu$ 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. 

The dominant component of electromagnetic radiation in the universe is the cosmic microwave background radiation produced by the hot big bang. It has a nearly perfect 2.73 K blackbody spectrum peaking at $\lambda \approx 1$ mm $= 10^3\,\mu$m.  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 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 its being at the long-wavelength end of the electromagnetic spectrum. Macroscopic wavelengths ($\lambda \sim 0.3$ mm to $\sim 30$ m) enable groups of charged particles to produce 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. 

Telescopes with very large diameters $D$ are required for good angular resolution $\theta \approx \lambda / D$ radians. On the other hand, huge interferometers spanning $D \sim 10^4$ km are practical and precision telescopes (e.g., having reflectors with rms surface errors $\sigma < \lambda/16$) can be built. 

GBT aerial photo

The $D = 100$ m Green Bank Telescope (GBT) in West Virginia is the largest moving structure on land and weighs 16 million pounds ($\approx 7 \times 10^6$ kg), yet the rms deviation of its surface from a perfect paraboloid can be kept below $\sigma \approx 0.3$~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; compare this with the ALMA and VLA site photos. Image credit

VLA photo

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 and synthesize apertures having those diameters and yielding angular resolutions ranging from $\theta \approx 45$ arcsec at $\nu = 1.4$ GHz in the smallest configuration to $\theta \approx 0.04$ arcsec at $\nu = 43$ GHz in the largest.  Coherent (phase preserving) amplifiers allow the signals from each telescope to be combined with signals from the other 26 telescopes without loss of sensitivity, a requirement for making accurate images of faint extended sources. Image credit
VLBA drawing
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 $\theta = 0.00017$ arcsec. Image credit

Low frequencies imply low photon energies $E = h\nu$.  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 number $h \nu / (k T)$ 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 are common.   Radio synchrotron sources live long after their relativistic electrons were produced and provide long-lasting historical records of energetic phenomena.  Plasma effects (scattering, dispersion, Faraday rotation, etc.) are strong.   On the negative side, radio astronomers must deal with large and fluctuating natural backgrounds of emission from the ground and from the atmosphere.

Coherent (phase preserving) amplifiers are required for accurate interferometric imaging of faint extended sources because they allow the signal from each telescope in a multielement interferometer to be amplified before combination with the signals from the other telescopes, rather than being divided among the other telescopes before combination.   The minimum possible noise temperature of a coherent receiver is $T = h\nu /k$ owing to quantum noise, which is proportional to frequency, so coherent amplifiers at visible-light frequencies must have noise temperatures $T > 10^4$ K.  Aperture-synthesis interferometry at radio wavelengths provides unparalleled sensitivity, fidelity, resolution, and position accuracy.  This is a huge practical advantage of radio astronomy.