Emission lines and optical continuum in low-luminosity radio galaxies

Abstract

We present spectroscopic observations of a complete subsample of 13 low-luminosity radio galaxies selected from the 2-Jy sample of Tadhunter et al. The underlying continuum in these sources was carefully modelled in order to make a much-needed comparison between the emission-line and continuum properties of Fanaroff-Riley type Is (FRIs) and those of other classes of radio sources. We find that five galaxies in the sample show a measurable ultraviolet (UV) excess: two of these sources are BL Lacs, but in the remaining three galaxies we argue that the most likely contributor to the UV excess is a young stellar component. Therefore, excluding the BL Lacs, we find that ∼30 per cent of the sample show evidence for young stars, which is similar to the results obtained for higher luminosity samples. We compare our results with far-infrared measurements in order to investigate the far-infrared-starburst link. The nature of the optical-radio correlations is investigated in light of these new available data and, in contrast to previous studies, we find that the FRI sources follow the correlations with similar slopes to those found for the Fanaroff-Riley type IIs. Finally, we compare the luminosities of the emission lines in the FRI and BL Lac sources and find a significant difference between the [O III] line luminosities of the two groups. Our results are discussed in the context of the unified schemes for low-powered radio sources.