Luminosity dependence of optical activity and alignments into radio galaxies

Abstract

We present the results of deep infrared imaging of matched samples of 3CR and fainter Parkes radio galaxies in a redshift band around z ≃ 1. The difference in radio power of a factor ~ 10 between these galaxies has a marked effect upon their optical/infrared 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 show no statistical tendency for their radio and optical/infrared axes to be aligned. The Parkes galaxies are also redder than most of the 3CR galaxies, consistent with the absence or reduced amplitude of an aligned blue component associated with the radio activity. The amplitude of 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 a underlying correlation with environment. Considering also (i) the universal shape of the UV continuum; (ii) the common detection of significant optical/UV polarization; (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 depolarization and extended optical emission, we conclude that a scattering 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 ∝ v ^{– 0.2}) quasar continuum emitted within a broad cone centred on the radio axis. At least part of the aligned infrared light may have a separate if 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 steeper v^–1.5 blazar continuum, emitted within a narrower cone of opening angle ≲ 10^°. Finally we argue that the small dispersion in colour displayed by the Parkes galaxies indicates that the significant AGN contamination of optical/infrared light 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 elliptical galaxy evolution – and indicate a low degree of star-forming activity at z = 1.