Next: §2 Active galactic nuclei Contents
In the early 1900's, observations of the elliptical galaxy M87 (NGC 4486, 3C 274 or Virgo A) carried out by Heber Curtis (Curtis, 1918) revealed a ``curious straight jet ... apparently connected with the nucleus by a thin line of matter''. These optical observations were not followed up by Curtis and it was not until the development of radio astronomy in the 1960's that jets emanating from the nuclei of certain galaxies became a major theme of research in astrophysics.
The birth of modern radio astronomy began with Karl Jansky who,
in May 1933, announced the discovery of the radio emission of our
Galaxy (Longair, 1995). His observations were made at
with a radio antenna built to identify natural
sources of radio noise which could interfere with radio transmissions.
Grote Reber, a radio engineer and amateur astronomer, built a home radio
antenna operating at a
with which he
confirmed Jansky's discovery and made radio scans along the galactic
plane (Reber, 1940). The results of Jansky and Reber ruled out the
possibility that the emission could be black body. As an alternative,
Reber suggested that it might be bremsstrahlung radiation. However, not
long after that, Henyey and Keenan showed that this was also not possible.
The origin of this radio emission remained a mystery. The culmination
of these early radio astronomical studies was Reber's map of the galactic
radio emission (Reber, 1944).
Alfven & Herlofson (1950) were the first to propose that the galactic radio emission might be synchrotron radiation of high energy electrons gyrating in magnetic fields in the atmospheres of stars. In the early 1950's, Kippenhauer and Ginzburg first applied the synchrotron hypothesis to high energy electrons moving in the interstellar magnetic fields. By the mid-1950s, the power-law form and the degree of polarisation of the galactic radio spectrum provided convincing evidence that the radiation was synchrotron.
Soon after the second World War, scientists who were involved in the development of radar began to analyse the nature of cosmic radio emission. Hey and collaborators at the Army Operational Research Group in the United Kingdom discovered the first discrete radio source in the constellation of Cygnus. This source became known as Cygnus A (see fig.(I.1)) and remains to date the brightest radio galaxy.
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Radio astronomy groups at Cambridge, Manchester and Sydney began
the construction of more powerful telescopes to study the radio sky.
In 1948, Martin Ryle discovered the brightest discrete source in
the northern hemisphere, Cassiopeia A. This object was identified by
Baade & Minkowski (1954) as a supernova remnant. (1954) also identified
the radio source Cygnus A (fig.(I.1)) with a galaxy at redshift
. The faint optical image had a disturbed appearance
comprising two parts. They interpreted the structure of this galaxy as
being the result of the collision of two galaxies. Jennison & Das Gupta (1956)
showed that the radio emission from Cygnus A did not originate from the
galaxy, but rather from two giant patches or radio lobes placed
symmetrically about the galaxy on the sky. These radio structures are
presumed to have a three dimensional structure, like a dumbbell, and are
often called lobes. Later, it was shown that the double optical
structure was only an illusion created by an obscuring lane of dust.
Telescopes of even higher sensitivity and of higher angular
resolution were built in the 60's and 70's, particularly at the
University of Cambridge, the Westerbork Observatory in the Netherlands
and the National Radio Astronomy Observatory (NRAO) in West Virginia.
This work culminated with the construction of the Very Large Array (VLA)
in New Mexico, which consists of 27 linked radio telescopes, each of 25
meters in diameter configured in a Y-shaped array that span 40 kilometres.
With this instrument it became possible to analyse the radio emission of
objects like Cygnus A in detail. The type of objects resembling Cygnus A
became known as radio galaxies. At the outer extremities of
the radio lobes in the bright double radio sources, hot spots,
that is, compact regions of intense radiation, were often observed.
Many of the radio sources also showed bridges or tails,
now called jets, extending from the hot spots towards the centre
of the source, in which there is often a compact region of radio emission
called a core. Optical identifications of these radio sources
revealed that most of the powerful double radio sources are associated
with elliptical galaxies, like Cygnus A, or with a quasar
. In both cases the core is
found to coincide with the galaxy's optical centre.
Even before the beautiful maps made by the VLA were available, theoretical considerations led naturally to a picture in which the hot spots and radio lobes are powered by jets originating in the nucleus of the host galaxy. Rees (1971) suggested that there was a central ``engine'' in the core of the galaxy responsible for all the radio-emitting electrons and magnetic fields. This central engine provided power to the giant lobes to energize their electrons and fields through some sort of ``channel''. Rees thought that the beams, which carry power from the central core to the lobes, were made of ultra-low frequency electromagnetic waves. However, theoretical calculations soon made it clear that electromagnetic beams cannot pass through the galactic interstellar gas.
A few years later, Longair (1973) generalised arguments about the dynamical structure of the double radio sources. Instead of the beams being made up of electromagnetic waves, they proposed that beams were made up of hot, magnetised gas. This idea, of a gas jet was accepted by Rees and gave rise to the standard model for powerful double radio sources, discussed later on in this Chapter.
