In the early seventies Fanaroff and Riley (1974) made a classification scheme based entirely on the morphology of extra-galactic radio sources. Their sample included high and low luminosity sources. They suggested that there is a definite relationship between the relative positions of the low brightness and high brightness regions of the radio sources and their luminosity.
The authors used a sub-sample of the 3CR complete sample (199 sources, Mackay, 1971), consisting only those sources which were clearly resolved into two or more components. The sources were classified using the ratio of the distance between the regions of highest brightness on opposite sides of the central galaxy or quasar, to the total extent of the source measured from the lowest contour. Any compact component situated on the central galaxy was not taken into account. If the ratio was less than 0.5 it became a class I object, if the ratio was greater than 0.5 the source was of class II. This is exactly the same as saying that class I objects are edge darkened and class II sources are edge brightened.
There is a sharp division in luminosity between the two classes. In their
sample those with luminosities at 178 MHz below
) were nearly all of class I and those above this limit nearly all class II.
This value that devides the two luminosity regions is very close to that which
divides sources showing strong cosmological evolution (Longair, 1971) from
those with little or no evolution.
Class I sources typically show very complex structure. In most cases the structure close to the associated galaxy indicates that material has been ejected in opposite directions. This is presumed also to occur in most of the powerful double sources in class II. There is a great variety of structure in the extended regions away from the core in class I objects. The components often show changes of curvature along their lenghts. This type of distortion is usually not found in class II sources, however, low brightness tails are often inclined to the source axis (Fanaroff, Riley, 1974).
Although the sources are divided in two distinct classes the low brightness
regions and the high brightness regions show in both classes similar
properties. The energy densities of the low brightness regions are in the order
of and are such that these regions could
be confined by the thermal pressure of a hot intergalactic gas. The spectral
indices of these regions are often high
, where
and it is usually assumed that these steep spectra are produced by synchrotron
(or inverse Compton) losses. The equipartition magnetic fields (see chapter
3) in these regions are low (
G). As a result of this, the periods
required to produce such steepening of the spectra are
yr, so the
relativistic particles in these regions could be very old. The energy densities
and equipartition field strengths derived from the high brightness regions are
often high. This leads to the assumption that the relativistic particles must
be replaced over a relatively short timescale (
yr). And this means
that these particles have recently been deposited or accellerated in these high
brightness regions.