Dependence of Fanaroff--Riley break of radio galaxies on luminosity and
  redshift
  We investigate the dependence of the Fanaroff-Riley (FR) 1/2 dichotomy of
radio galaxies on their luminosities and redshifts. Because of a very strong
redshift-luminosity correlation (Malmquist bias) in a flux-limited sample, any
redshift-dependent effect could appear as a luminosity related effect and vice
versa. A question could then arise - do all the morphological differences seen
in the two classes (FR 1 and 2 types) of sources, usually attributed to the
differences in their luminosities, could as well be primarily a
redshift-dependent effect? A sharp break in luminosity, seen among the two
classes, could after all reflect a sharp redshift-dependence due to a rather
critical ambient density value at some cosmic epoch. A doubt on these lines
does not seem to have been raised in past and things have never been examined
with this particular aspect in mind. We want to ascertain the customary
prevalent view in the literature that the systematic differences in the two
broad morphology types of FR 1 and 2 radio galaxies are indeed due to the
differences in their luminosities, and not due to a change in redshift. Here we
investigate the dependence of FR 1/2 dichotomy of radio galaxies on luminosity
and redshift by using the 3CR sample, where the FR 1/2 dichotomy was first
seen, supplemented by data from an additional sample (MRC), that goes about a
factor of 5 or more deeper in flux-density than the original 3CR sample. This
lets us compare sources with similar luminosities but at different redshifts as
well as examine sources at similar redshifts but with different luminosities,
thereby allowing us a successful separation of the otherwise two intricately
entangled effects. We find that the morphology type is not directly related to
redshift and the break between the two types of morphologies seems to depend
only upon the radio luminosity.
