The ultraviolet excess in nearby powerful radio galaxies: evidence for a young stellar component

Abstract

We present William Herschel Telescope spectropolarimetry observations of a complete RA-limited sample of nine low-redshift (0.05 < z < 0.2) 3CR radio sources in order to investigate the nature of the ultraviolet (UV) excess in nearby powerful radio galaxies. Of the nine galaxies studied in detail from this sample, we find that four show a measurable UV excess following nebular continuum subtraction, but none of the sources shows significant polarization in the UV. One of the radio galaxies with a UV excess - 3C 184.1 - shows evidence for broad permitted lines and hence direct active galactic nucleus (AGN) light. In the remaining three galaxies we argue that the most likely contributor to the UV excess is a young stellar component. For these three galaxies we find that the best-fitting model for the optical/UV continuum consists of a combination of an old stellar population (10-15 Gyr old elliptical galaxy) plus a reddened young stellar population (0.05-2 Gyr). The reddened young stellar component typically accounts for half of the total flux at 4780 A, following nebular continuum subtraction, and E(B - V) values of between 0.2 and 0.7 mag are required. However, for the majority of sources in our sample (six out of nine), continuum modelling provides no evidence for a significant young stellar component in the nuclear regions of the host galaxies. Our results are discussed in the context of far-infrared evidence for star formation activity.