LUMINOSITY DEPENDENCE OF OPTICAL-ACTIVITY AND ALIGNMENTS IN RADIO GALAXIES

We present the results of deep infrared imaging of matched samples of 3CR and fainter Parkes radio galaxies in a redshift band around z cong ruent-to 1. The difference in radio power of a factor approximately 10 between these galaxies has a marked effect upon their optical/infrare d properties. The 3CR galaxies, despite being more nucleated at K than at optical wavelengths, nevertheless display a clear infrared-radio ' alignment effect'. In contrast, the Parkes galaxies are rounder and sh ow no statistical tendency for their radio and optical/infrared axes t o be aligned. The Parkes galaxies are also redder than most of the 3CR galaxies, consistent with the absence or reduced amplitude of an alig ned blue component associated with the radio activity. The amplitude o f this blue component displays a strong correlation with a combination of radio power and spectral index. We show that this is analogous to the correlation between radio-jet power and L(NLR) found by Rawlings & Saunders, but propose that both correlations arise from an underlying correlation with environment. Considering also (i) the universal shap e of the UV continuum; (ii) the common detection of significant optica l/UV polarization; (iii) the inaccuracy of the optical-radio alignment s; (iv) the close spatial correspondence between the extended UV conti nuum and line emission; and (v) the correlation between radio-lobe dep olarization and extended optical emission, we conclude that a scatteri ng model is preferred. A large fraction of the optical/UV activity and the optical alignment effect in the 3CR sample probably results from Thomson scattering of a 'flat' (f(v) is-proportional-to v-1.2) quasar continuum emitted within a broad cone centred on the radio axis. At le ast part of the aligned infrared tight may have a separate if related origin: when infrared alignments occur, they are tightly aligned with the radio axis. Possible mechanisms include inverse Compton scattering of microwave background photons, or Thomson scattering of a much stee per v-1.5 blazar continuum, emitted within a narrower cone of opening angle less than or similar to 10-degrees. Finally we argue that the sm all dispersion in colour displayed by the Parkes galaxies indicates th at the significant AGN contamination of optical/infrared tight seen in the 3CR galaxies is largely confined to the top decade of radio power . Less extreme radio galaxies may still be useful as probes of ellipti cal galaxy evolution - and indicate a low degree of star-forming activ ity at z = 1.