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1.1 The Role of Radio Observations in Astronomy

The data give for the coordinates of the region from which the disturbance comes, a right ascension of 18 hours and declination of -10°.

Karl G. Jansky

Jansky's discovery of radio emission from the Milky Way is now seen as the birth of the new science of radio astronomy. Most astronomers remained unaware of this momentous event for at least the next decade, and its full significance only became apparent with the major discoveries in the 1950s and 1960s of the 21 cm hydrogen line, the quasars, the pulsars and the cosmic microwave background. These are now fully assimilated into astronomy, and radio is now regarded as one among the several tools available to astronomers in their pursuit of the astrophysics of our Galaxy, or of neutron stars, or black holes or cosmology. Nevertheless, radio astronomy has its own distinctive character, not least in its techniques and the particular expertise which they demand. It also has several fields of application in which it is uniquely useful: there is no other way of exploring the cosmic microwave background, it allows spectroscopic investigation of molecular clouds in the Milky Way and it reveals a previously unseen Universe through the synchrotron emission of high-energy particles in stars, galaxies and quasars.

The history of this development is outlined at the end of this book in Appendix C. Our purpose in the main text is to set out those parts of astrophysics and observational techniques which are particularly appropriate in radio astronomy, so that the subject may be fully available to all astronomers and, to physicists with a wide range of backgrounds. There is, throughout all radio astronomy, a close relation with other observational wavebands, and we shall acknowledge, for example, the necessity of using optically measured redshifts for distances of quasars and the components of gravitational lenses, and the need to bring together the X-ray and radio observations to obtain a coherent picture of neutron stars in our Galaxy.

The Milky Way, our Galaxy, which is the origin of the radio noise first observed by Jansky, is a complex assembly of stars of widely varying ages, embedded in an interstellar medium, or ISM, of ionized and neutral gas, itself displaying a great diversity and complexity throughout the electromagnetic spectrum. Optical astronomy primarily addresses the surfaces of the stars, or nearby gas ionized by those stars: the temperatures bring thermal radiation naturally into the visible range. The ISM is partly composed of interstellar dust particles, which absorb radiation in the visible part of the spectrum, severely restricting the optical astronomer's view in the galactic plane (although the absorption effects are small when the observing direction lies far from the plane of the Milky Way). X-ray astronomy deals with much hotter regions, such as the million-degree ionized gas which is found in such diverse places as the solar corona and the centres of clusters of galaxies. Infrared astronomy studies relatively cool regions, where thermal radiation from the dust component of the ISM is a prominent feature: warmer regions are also studied, where the thermal radiation from star-forming regions is also strong. Radio astronomy, using much longer wavelengths, addresses a broad range of phenomena, including the thermal radiation from the 21 cm line of neutral hydrogen, which can be found in a wide variety of thermal regimes, and the thermal radiation from a wide variety of molecular lines, coming from dense, extremely cold gas concentrations that are found within the ISM. The radio noise discovered by Jansky belongs to an entirely different physical regime; it comes from charged particles, usually electrons, with very high energies, moving at relativistic velocities in the magnetic fields of the ISM. This regime of synchrotron radiation also accounts for the intense radio emission from quasars and other interesting objects in the Universe. Synchrotron radiation, although it can generate radiation up to X-ray and beyond, is a particularly prominent long-wavelength phenomenon, giving radio astronomy a unique role in the investigation of some of the most energetic objects in the Universe.

The methods of the radio astronomer often appear to be quite different from those of the optical astronomer, but there is the same aim in all of astronomy, from the radio to the X-ray domain. Nature presents us with a distribution of brightness on the sky, and it is the task of the astronomer to deduce, from this brightness distribution of electromagnetic radiation, what the radiating sources are, and what physical processes are acting, The distinguishing feature of a radio telescope is that the radiation energy gathered by the instrument is not measured immediately, a process known as detection in radio terminology. Instead, the radiation is amplified and manipulated coherently, preserving its wave-like character, before it is finally detected. The instrumental goals of the radio astronomer - obtaining a larger collecting area, greater angular resolution and more sensitive detectors, are otherwise the same as they are for all the astronomical disciplines.

To illustrate the relation between radio and other astronomies, the energy flux of electromagnetic radiation arriving at the earth's surface from the cosmos is plotted in Figure 1.1. The frequency scale runs from the standard radio broadcast band to the X-ray region, and the atmospheric windows are indicated schematically. The optical window is seen to be relatively narrow, blocked at the ultraviolet end by ozone absorption and at still shorter wavelengths by oxygen and nitrogen, while at the infrared end the principal absorbing agents are water vapour and carbon dioxide. The high frequency blockage is so complete that ultraviolet and X-ray work must be carried out above the atmosphere. There are occasional windows in the atmosphere at infrared frequencies, allowing observations to be made from high, dry mountain sites, but for the most part the observations must be taken from airplanes, balloons, or satellites, depending on the particular spectral region. It is easy to see that there is a great stretch of spectrum at the radio end that has relatively little trouble with the atmosphere. It is also obvious why, before Jansky's discovery, there was no reason to expect much of interest in the radio spectrum; if stars were the principal sources of radiation, very little radio emission could be expected. The maximum radiation from even the coolest of the known stars falls at visible or infrared wavelengths, and their contribution to the radio end of the spectrum is almost negligible. The slow response to Jansky's discovery is understandable both in terms of technical difficulty and lack of expectation.

Figure 1.1

Fig. 1.1. The electromagnetic spectrum, showing the wavelength range of the ``windows''. The radio range is limited by the ionosphere at wavelengths greater than a few metres, and by atmospheric absorption at wavelengths shorter than about 2 cm.

The names of the various radio bands are indicated in Figure 1.1: HF (below 30 MHz), VHF (30-300 MHz), UHF (300-1000 MHz), microwaves (1000-30000 MHz), millimetre-wave, and sub-millimetre-wave. Certain microwave bands acquired particular names: L-band (approx 20 cm), S-band (approx 10 cm), X-band (approx 3 cm), Ku-band (sometimes U-band, approx 2 cm), and K-band (approx 1 cm). The names of the microwave bands are rooted in history, like the s, p, d states of atoms; they are commonly met with in practice.

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