Luminosity Dependence of Optical Activity in Radio Galaxies

Summary

Abstract

We have previously reported evidence for radio-luminosity dependence of the strength of the near-infrared ‘alignment effect’ in radio galaxies (Dunlop & Peacock 1991). Here we present evidence for an associated radio-luminosity dependence in the level of optical activity found in radio galaxies at z ≃ 1. We find that this correlation, the strength of which is maximised by considering a combination of radio power and spectral index, is very similar and probably closely related to the correlation between radio-jet power and L_NLR found by Rawlings & Saunders (1991); the available evidence suggests that both correlations in fact arise from an underlying correlation with environment.

These correlations along with (i) the universal shape of the supposedly stellar UV-continuum in powerful high-redshift radio galaxies; (ii) the detections of significant optical/UV polarisation in several 3CR radio galaxies; (iii) the inaccuracy of the optical-radio alignments; (iv) the close spatial correspondence between the extended UV continuum and line emission; and (v) the correlation between radio-lobe depolarisation and extended optical emission, indicate that a large fraction of the optical/UV activity and the optical alignment effect in the 3CR sample is the result of Thomson scattering of a ‘flat’ (f_v ∝ v^−0.2) quasar continuum emitted within a broad cone centred on the radio axis.

To investigate the relationship between the optical/UV activity in high-z radio galaxies and their radio properties, we have considered a composite sample of radio galaxies with z ≃ 1 which spans a decade in radio luminosity.