The Physics of Extragalactic Radio Sources

  • David S. De Young
University of Chicago Press: 2002. 450 pp. $45

After the Second World War, scientists, especially in England and Australia, turned their wartime radar equipment towards the skies to follow up on the pre-war discovery by Karl Jansky and Grote Reber of radio emission from the Milky Way. Although hampered at first by poor angular resolution and limited sensitivity, they were able to detect powerful sources of non-thermal radio emission from several distant galaxies. During the 1950s and 1960s, powerful interferometric and aperture-synthesis techniques were developed. When combined with theoretical advances, primarily in the Soviet Union, they showed that the observed radio emission was due to synchrotron radiation emitted from extensive clouds of ultrarelativistic electrons moving in weak magnetic fields far removed from the parent galaxy. The energy required in the form of relativistic particles and magnetic fields appeared to be enormous.

But some extragalactic radio sources were found to be very much smaller and surprisingly appeared to vary in intensity on timescales of months or less. Interferometers and lunar occultations were used to locate radio sources accurately in the sky, leading to observations of ever more distant radio galaxies and to the discovery of quasars. By the late 1960s, tape-recording interferometers had stretched radio-interferometer baselines to intercontinental dimensions. With angular resolutions better than 0.001 arcseconds available, radio astronomers were able to show that the source of energy that powers radio galaxies and quasars was as small as a few light years across or less.

Radio star: the Parkes Radio Telescope near Alectown in Australia detects extragalactic signals. Credit: J. SARKASSIAN/CSIRO

Today, radio telescopes are a million times more sensitive than the pioneering post-war instruments, and more than a million extragalactic radio sources have been catalogued. Radio astronomers use wavelengths ranging from less than a millimetre to several metres, which is comparable to the range from the infrared through the optical and ultraviolet to the X-ray parts of the electromagnetic spectrum. Huge radio-telescope arrays show remarkably fine detail in the synchrotron clouds, revealing thin jets that carry the relativistic electron gas from active galactic nuclei (AGN) out to distant radio lobes, as much as a million light years or more away from the central engine. Global radio-interferometer observations using angular resolutions up to 100 times better than that of the Hubble Space Telescope show directly the ejection of relativistic plasma from the central engine with apparent velocities in excess of the speed of light. This is an illusion caused by the finite signal-propagation time from a relativistically moving source of radiation.

However, the new observations may have raised more questions than they answer. The source of energy is still unclear, although it is widely assumed to be associated with accretion of material into a huge black hole up to 109 times as massive as the Sun. How the energy is converted into a highly collimated beam of relativistic particles remains a mystery. Why are some galaxies powerful radio sources, whereas others are not? Why are powerful quasars sometimes radio-loud and sometimes radio-quiet? To what extent is the wide range of observed extragalactic radio-source properties intrinsic, and how much is due to geometrical effects? What is the relationship between quasars, AGN and galaxy formation, especially the burst of star formation often seen in distant galaxies?

In The Physics of Extragalactic Radio Sources De Young discusses all of these issues, starting with a qualitative review of the properties of radio galaxies and quasars. Other chapters discuss the application of synchrotron radiation, plasma and jet physics relevant to extragalactic radio sources. Classical formulae are derived only where the author has a new approach to the derivation that leads to a better understanding of the physics involved. In other cases, the reader is referred to the appropriate literature. In the later chapters De Young applies the theory to both extended and compact radio sources, relativistic jets and AGN, including a discussion of unified radio-source models which attempt to explain the range of observed properties as the result of the orientation of the relativistic jet or an obscuring torus surrounding the central engine. He discusses the nature of the host galaxy or associated quasar and includes a review of their optical and X-ray properties.

There are few original drawings. Most of the illustrations are taken from the literature, but are sometimes out of date in this rapidly moving field and often have inadequate captions or explanation. There are a few errors, some of which seem to have been introduced by the editors. Nevertheless, the book will be a valuable resource for all astrophysics students and to everyone working in the field of extragalactic astronomy. As the author points out in the preface, no other book is devoted to this important field of astrophysics.