  The nature of the optical--radio correlations for powerful radio galaxies is
investigated using spectroscopic observations of a complete sample of southern
2Jy radio sources. In line with previous work, we find that significant
correlations exist between the luminosities of the [OIII]5007, [OII]3727 and
Hbeta emission lines and the radio luminosity. However, our observations are
not easily reconciled with the idea that these correlations are caused by the
increase in the power of the photoionizing quasar as the jet power increases,
with average ISM properties not changing appreciably with redshift or radio
power: not only do we find that the scatter in the L_[OIII] vs. L_radio
correlation is significantly larger than in L_[OII]} vs. L_radio and L_Hbeta
vs. L_radio correlations, but the ionization state deduced from the emission
lines does not increase with radio power as predicted by the simple, constant
ISM, photionization model. We conclude that: (a) there exists a considerable
range in the quasar ionizing luminosity at a given redshift; and (b) that the
mean density of the emission line clouds is larger in the high redshift/high
power radio sources. The latter density enhancement may either be a consequence
of the increased importance of jet-cloud interactions or, alternatively, due to
a higher pressure in the confining hot ISM, in the high redshift objects. Deep
spectra show that many of the sources in our sample are broad line radio
galaxies (BLRG). The fact that the BLRG are observed out the redshift limit of
the survey, overlapping in redshift with the quasars, argues against the idea
that BLRG are simply the low radio power counterparts of high power, high
redshift quasars.
