A submillimetre difference between radio galaxies and radio quasars: evidence for quasar-heated dust and synchronized submillimetre and radio source activity

We present submillimetre photometry of 11 3CR and 6CE radio quasars of similar 151-MHz radio luminosity and redshifts; to the radio galaxies studied at 1.3 < z < 2 by Archibald et al. We detect all bar one quasar at 850 mum, and five quasars are confirmed as dusty hyperluminous objects by detections at shorter wavelengths. Our observations reveal a clear difference between the submillimetre luminosity distributions of the radio quasars and a matched sample of radio galaxies: the quasars are similar to5 times brighter than the radio galaxies and greater than or equal to2 times brighter accounting for possible synchrotron contamination. This difference is in quantitative agreement with a receding-torus unified scheme for radio sources in which the torus opening angle depends on quasar optical luminosity, provided that there is a close relationship between the optical and submillimetre luminosities. The implication is that quasar-heated dust dominates the submillimetre emission for powerful radio quasars at z similar to 1.5. We find a significant anti-correlation between submillimetre/far-infrared luminosity L-FIR and radio source age in the sense that hyperluminous quasars tend to be associated with young (<10(7) yr) sources. This suggests that the processes controlling L-FIR are synchronized with the jet-triggering event and that LFIR declines on a ∼10(7) yr time-scale. There is evidence for a small amount of obscuration in the hyperluminous quasars from reddening and associated or broad absorption lines. We speculate that shortly after jet triggering, dust is more widely distributed than at later times and is heated by the quasar nucleus and/or a short-lived synchronized starburst. Any such starburst would form only a few per cent of the total stellar mass in agreement with the evidence for dominant old stellar populations in z ∼ 1.5 radio galaxies. In contrast, high-redshift (z > 3) radio galaxies with similar submillimetre luminosities could have longer star-formation time-scales owing to the greater availability of gas and could be forming the bulk of their stellar populations.