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1 Introduction


1.1.1 What Is Radio Astronomy?

Radio astronomy is the study of natural radio emission from celestial sources. The

range of radio frequencies or wavelengths is loosely defined by atmospheric opacity and by quantum noise in coherent amplifiers. Together they place the boundary between radio and far-infrared astronomy at frequency ν ∼ 1 THz (1 THz ≡ 1012 Hz) or wavelength λ = c/ν ∼ 0.3 mm, where c ≈ 3 × 1010 cm s−1 is the vacuum speed of light. The Earth’s ionosphere sets a low-frequency limit to ground-based radio astronomy by reflecting extraterrestrial radio waves with frequencies below ν ∼ 10 MHz (λ ∼ 30 m), and the ionized interstellar medium of our own Galaxy absorbs extragalactic radio signals below ν ∼ 2 MHz.

The radio band is very broad logarithmically: it spans the five decades between 10 MHz and 1 THz at the low-frequency end of the electromagnetic spectrum.

Nearly everything emits radio waves at some level, via a wide variety of emission mechanisms. Few astronomical radio sources are obscured because radio waves can penetrate interstellar dust clouds and Compton-thick layers of neutral gas. Because only optical and radio observations can be made from the ground, pioneering radio astronomers had the first opportunity to explore a “parallel universe” containing unexpected new objects such as radio galaxies, quasars, and pulsars, plus very cold sources such as interstellar molecular clouds and the cosmic microwave background radiation from the big bang itself.

Telescopes observing from above the atmosphere have since opened the entire electromagnetic spectrum to astronomers, but radio astronomy retains a unique observational advantage. Coherent amplifiers, which preserve phase information, allow the construction of sensitive multielement aperture-synthesis interferometers that can image complex sources with angular resolution and absolute astrometric accuracies approaching 10−4 arcsec. Quantum noise forever restricts sensitive coherent amplification to the low photon energies E = hν (where h = Planck’s constant ≈ 6.626 × 10−27 erg s) of the radio band. Also, coherent signals can be shifted to lower frequencies and digitized, permitting the construction of radio spectrometers with extremely high spectral resolution and frequency accuracy.

For general queries, contact webmaster@press.princeton.edu © Copyright, P

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1.1.2 Atmospheric Windows The Earth’s atmosphere absorbs electromagnetic radiation at most infrared (IR), ultraviolet, X-ray, and gamma-ray wavelengths, so only optical/near-IR and radio observations can be made from the ground (Figure 1.1). The visible-light window is relatively narrow and spans the wavelengths of peak thermal emission from T ∼ 3000 K to T ∼ 10,000 K blackbodies. Early observational astronomy was limited to visible objects—hot thermal sources such as stars, clusters and galaxies of stars, and 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 to cooler objects shining by reflected starlight (e.g., planets and moons). Knowing the 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 celestial radio sources. Consequently they failed to develop radio astronomy until strong radio emission from our Galaxy was discovered accidentally in 1932 and followed up by radio engineers.

What physical processes limit the atmospheric windows? At the high-frequency end of the radio window, vibrational transitions of atmospheric molecules such as CO2, O2, and H2 O have energies E = hν comparable with those of mid-infrared photons, so vibrating molecules absorb most extraterrestrial mid-infrared radiation.

Lower-energy rotational transitions of atmospheric molecules define the fairly broad transition between the far-infrared band and the high-frequency limit of the radio window at ν ∼ 1 THz. Ground-based radio astronomy is increasingly degraded at frequencies ν 300 MHz (wavelengths λ 1 m) by variable ionospheric refraction,

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ν (GHz) Figure 1.2. The atmospheric zenith opacity τz at Green Bank during a typical summer night.

An opacity τ attenuates the power received from an astronomical source by the factor exp(−τ ). The oxygen and dry-air opacities are nearly constant, while the water vapor and hydrosol contributions vary significantly with weather.

and celestial radio waves having frequencies ν 10 MHz (wavelengths λ 30 m) are usually reflected back into space by the Earth’s ionosphere. Total internal reflection in the ionosphere makes the Earth look like a mirror from space, like the glass face of an underwater wristwatch viewed obliquely.

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 electronic and nuclear transitions produce X-ray and gamma-ray absorption. In addition, Rayleigh scattering of sunlight by atmospheric gas molecules and dust particles at visible and ultraviolet wavelengths brightens the sky enough to preclude daytime optical observations of faint objects.

Radio wavelengths are much longer than atmospheric dust grains and the Sun is not an overwhelmingly bright radio source, so the radio sky is always dark and many radio observations can be made day or night.

The atmosphere is not perfectly transparent at any radio frequency. Figure 1.2 shows how its zenith (the zenith is the point directly overhead) opacity τz in Green Bank, WV varies with frequency during a typical summer night with a water-vapor column density of 1 cm, 55% cloud cover, and surface air temperature T = 288 K = 15◦ C. The total zenith opacity (solid curve) is the sum of several

components [65]:

1. The broadband or continuum opacity of dry air (long dashes) results from viscous damping of the free rotations of nonpolar molecules. It is relatively small (τz ≈ 0.01) and nearly independent of frequency.

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2. Molecular oxygen (O2 ) has no permanent electric dipole moment, but it does have rotational transitions that can absorb radio waves because it has a permanent magnetic dipole moment. The atmospheric-pressure-broadened complex of oxygen

1) and precludes ground-based spectral lines (short dashes) is quite opaque (τz observations in the frequency range 52 GHz ν 68 GHz (1 GHz ≡ 109 Hz).

3. Hydrosols are liquid water droplets small enough (radius ≤ 0.1 mm) to remain suspended in clouds. They are much smaller than the wavelength even at 120 GHz (λ ≈ 2.5 mm), so their emission and absorption obey the Rayleigh scattering approximation and their opacity (dot-dash curve) is proportional to λ−2 or ν 2.

4. The strong water-vapor spectral line at ν ≈ 22.235 GHz is pressure broadened to ν ≈ 4 GHz width. The so-called “continuum” opacity of water vapor at radio wavelengths is actually the sum of line-wing opacities from much stronger water lines at infrared wavelengths [106]. In the plotted frequency range, this continuum opacity is also proportional to ν 2. Both the line and continuum zenith opacities (dotted curves) are directly proportional to the column density of precipitable water vapor (pwv) along the vertical line of sight through the atmosphere.

Conventionally pwv is expressed as a length (e.g., 1 cm) rather than as a true column density (e.g., 1 gm cm−2 ), but the two forms are numerically equivalent because the mass density of water is unity in CGS units.

The partially absorbing atmosphere doesn’t just attenuate incoming radio radiation; it also emits radio noise that can seriously degrade the sensitivity of

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ground-based radio observations. If the atmospheric opacity is τ, the atmospheric transparency is exp(−τ ) and emission from the atmosphere at kinetic temperature T ∼ 300 K adds Ts = T [1 − exp(−τ )] to the system noise temperature Ts. Radio astronomers use Ts ≡ Pν /k, where k = Boltzmann’s constant ≈ 1.38 × 10−16 erg K−1, as a convenient measure of the noise power per unit bandwidth Pν.

The system noise temperature is normally much smaller than the atmospheric kinetic temperature, so the added noise from atmospheric emission degrades sensitivity even more than pure absorption alone. 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 ν ∼ 22 GHz during the summer. Green Bank can be quite cold and dry in the winter, allowing observations at frequencies up to ≈ 115 GHz.

The very best sites for observing at higher frequencies are exceptionally high and dry.

For example, the Atacama Large Millimeter Array (ALMA) shown in Color Plate 5 is located at 5000 m elevation on a desert plain near Cerro Chajnator in Chile, where the typical pwv is 1 mm. Figure 1.3 shows the zenith atmospheric opacity at the ALMA site when pwv = 0.5 mm, for frequencies up to the ν ∼ 1 THz atmospheric limit.

Finally, the refractive index of water vapor is about 20 times higher at radio than at optical wavelengths because the index of refraction at wavelength λ is proportional to the cumulative strength of the water-vapor absorption lines at shorter wavelengths, the strongest of which lie in the far-infrared range 0.03 λ (mm) 0.6. Water vapor is not well mixed in the atmosphere, so fluctuations in the column density of water vapor along the line of sight blur the image of a point radio source. The scale height of water vapor in the troposphere is ∼ 2 km, so the largest fluctuations have transverse dimensions of several km. Consequently point sources seen by all radio telescopes or radio interferometers smaller than a few km in size are blurred by ∼ 0.5 arcsec, and this blurring is nearly independent of wavelength for all λ

0.6 mm. The angular size of the “seeing” disk for interferometers much larger than a few km is inversely proportional to the size of the interferometer. In contrast, optical seeing at wavelengths λ 0.03 mm is dominated by the much smaller (∼ 10 cm) turbulent density fluctuations of dry air. It is only a coincidence that the optical seeing disk is also ∼ 0.5 arcsec at the best terrestrial sites. For a thorough review of atmospheric and ionospheric propagation effects, see Thompson, Moran, & Swenson [106, Chapter 13].

1.1.3 Astronomy in the Radio Window Because the radio window is so broad, (1) almost all types of astronomical sources, thermal and nonthermal radiation mechanisms, and propagation phenomena can be observed at radio wavelengths; and (2) a wide variety of radio telescopes and observing techniques are needed to cover the radio window effectively.

The radio window was explored before there were telescopes in space, so early radio astronomy was a science of discovery and serendipity. It revealed a “parallel universe” of unexpected sources not previously seen, or at least not recognized as being different from ordinary stars. Major discoveries of radio astronomy include

1. nonthermal radiation from our Galaxy [86] and many other astronomical sources;

2. The “violent universe” of powerful radio galaxies [4] and quasars (quasi-stellar radio sources) [48, 101] powered by supermassive black holes (SMBHs);

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3. cosmological evolution of radio galaxies and quasars [98];

4. thermal spectral-line emission from cold interstellar gas atoms, ions, and molecules;

5. maser (the acronym for microwave amplification by stimulated emission of radiation) emission from interstellar molecules [114];

6. coherent continuum emission from stars and pulsars;

7. cosmic microwave background radiation from the hot big bang [80];

8. pulsars and neutron stars [50];

9. indirect but convincing evidence for gravitational radiation [104];

10. the supermassive black hole at the center of our Galaxy; [8]

11. evidence for dark matter in galaxies, deduced from their HI (neutral hydrogen) rotation curves [92];

12. extrasolar planets [117];

13. strong gravitational lensing [113].

The following items are some of the features of this parallel universe.

1. It is often violent, reflecting high-energy and explosive phenomena in radio galaxies, quasars, supernovae, pulsars, etc., in contrast to the steady light output of most visible stars.

2. Many radio sources are ultimately powered by gravity instead of by nuclear fusion, the principal energy source of visible stars.

3. It is cosmologically distant. Most continuum radio sources are extragalactic, and they have evolved so strongly over cosmic time that most are seen at lookback times comparable with the age of the universe.

4. 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. Cold interstellar gases emit spectral lines at radio wavelengths.

With the advent of telescopes in space, the entire electromagnetic spectrum has become accessible to astronomers. Many sources discovered by radio astronomers can now be studied in other wavebands, and new objects discovered in other wavebands (e.g., gamma-ray bursters) can now be followed up at radio wavelengths.

Radio astronomy is no longer a separate and distinct field; it is one facet of multiwavelength astronomy. Even so, the radio band retains unique astronomical and technical features.

Most of the electromagnetic energy of the universe (Figure 1.4) is in the cosmic microwave background (CMB) radiation left over from the hot big bang.

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