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_ is proportional to 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 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 steeper
v^-1.5^ blazar continuum, emitted within a narrower cone of opening
angle
Original language | English |
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Pages (from-to) | 936 |
Journal | Monthly Notices of the Royal Astronomical Society |
Volume | 263 |
Publication status | Published - 1 Aug 1993 |