The emission line – radio correlation for radio sources using the 7C Redshift Survey

We have used narrow emission line data from the new 7C Redshift Survey to investigate correlations between the narrow-line luminosities and the radio properties of radio galaxies and steep-spectrum quasars. The 7C Redshift Survey is a lowfrequency (151 MHz) selected sample with a flux-density limit about 25-times fainter than the 3CRR sample. By combining these samples, we can for the first time distinguish whether the correlations present are controlled by 151 MHz radio luminosity L151 or redshift z. We find unequivocal evidence that the dominant effect is a strong positive correlation between narrow line luminosity LNLR and L151, of the form LNLR ∝ L 0.79±0.04 151 . Correlations of LNLR with redshift or radio properties, such as linear size or 151 MHz (rest-frame) spectral index, are either much weaker or absent. We use simple assumptions to estimate the total bulk kinetic power Q of the jets in FRII radio sources, and confirm the underlying proportionality between jet power and narrow line luminosity first discussed by Rawlings & Saunders (1991). We make the assumption that the main energy input to the narrow line region is photoionisation by the quasar accretion disc, and relate Q to the disc luminosity, Qphot. We find that 0.05< ∼ Q/Qphot ∼ 1 so that the jet power is within about an order of magnitude of the accretion disc luminosity. Values of Q/Qphot ∼ 1 require the volume filling factor η of the synchrotron-emitting material to be of order unity, and in addition require one or more of the following: (i) an important contribution to the energy budget from protons; (ii) a large reservoir of mildly-relativistic electrons; and (iii) a substantial departure from the minimum energy condition in the lobe material. The most powerful radio sources are accreting at rates close to the Eddington limit of supermassive black holes (MBH ∼ 10 M⊙), whilst lower power sources are accreting at sub-Eddington rates.