The origin of the UV excess in powerful radio galaxies: Spectroscopy and polarimetry of a complete sample of intermediate-redshift radio galaxies

Abstract

We present spectroscopic and polarimetric observations of a complete, optically unbiased sample of 2-Jy radio galaxies at intermediate redshifts (0.15 < z < 0.7). These data - which cover the nuclear regions of the target galaxies - allow us to quantify for the first time the various components that contribute to the UV excess in the population of powerful, intermediate-redshift radio galaxies. We find that, contrary to the results of previous surveys - which have tended to be biased towards the most luminous and spectacular objects in any redshift range - the contribution of scattered quasar light to the UV excess is relatively minor in most of the objects in our sample. Only seven objects (32 per cent of the complete sample) show significant polarization in the rest-frame UV, and none of the objects in our sample is polarized in the near-UVat the P > 10 per cent level. Careful measurement and modelling of our spectra have allowed us to quantify the contributions of other components to the UV excess. We show that nebular continuum (present in all objects at the 3-40 per cent level), direct active galactic nucleus (AGN) light (significant in 40 per cent of objects) and young stellar populations (significant in 15-50 per cent of objects) all make important contributions to the UV continuum in the population of powerful radio galaxies. These results serve to emphasize the multicomponent nature of the UV continuum in radio galaxies. The results also point to an interesting link between the optical/UV and far-IR properties of our sample objects, in the sense that the objects with the clearest evidence for optical/UV starburst activity are also the most luminous at far-IR wavelengths. This supports the idea that the cooler dust components in radio galaxies are heated by starbursts rather than by AGN.