THE NATURE OF THE OPTICAL-RADIO CORRELATIONS FOR POWERFUL RADIO GALAXIES

The nature of the optical-radio correlations for powerful radio galaxi es is investigated using spectroscopic observations of a complete samp le of southern 2-Jy radio sources. In line with previous work, we find that significant correlations exist between the luminosities of the [ O III]lambda 5007, [O II]lambda 3727 and H beta 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 t he 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-[O III] versus L -radio correlation is significantly larger than in L-[O II] versus L-r adio and L-H beta versus L-radio correlations, but the ionization stat e deduced from the emission lines does not increase with radio power a s predicted by the simple, constant ISM, photoionization model. We con clude that (a) there exists a considerable range in the quasar ionizin g luminosity at a given redshift, and (b) the mean density of the emis sion-line clouds is larger in the high-redshift/high-power radio sourc es. The latter density enhancement may be either a consequence of the increased importance of jet-cloud interactions or, alternatively, the result of a higher pressure in the confining hot ISM, in the high-reds hift objects. Apart from the general scatter in the correlations, we i dentify a distinct group of objects with [O III]lambda 5007 luminositi es which are more than an order of magnitude lower than in the general population radio galaxies at similar redshift. These weak-line radio galaxies (WLRGs) are likely to be sources in which the central ionizin g quasars are particularly feeble. Deep spectra show that many of the sources in our sample are broad-line radio galaxies (BLRGs). The fact that the BLRGs are observed out to the redshift limit of the survey, o verlapping in redshift with the quasars, argues against the idea that BLRGs are simply the low-radio-power counterparts of high-power, high- redshift quasars. Either there exists a considerable range in the intr insic luminosities of the broad-line AGN for a given redshift or radio power, or the BLRGs represent partially obscured quasars. The degree of scatter present in the L-[O III] versus L-radio correlation support s the former possibility.