Optical spectroscopy of radio galaxies in the 7C redshift survey

Abstract

We present optical spectroscopy of all 49 radio galaxies in the 7C-I and 7C-II regions of the 7C Redshift Survey (7CRS). The low-frequency (151 MHz) selected 7CRS sample contains all sources with flux-densities S151 > 0.5 Jy in three regions of the sky; 7C-I and 7C-II were chosen to overlap with the 5C 6 and 5C 7 surveys, respectively, and cover a total sky area of 0.013 sr. The sample has been completely identified and spectroscopy of the quasars and broad-lined radio galaxies has been presented previously by Willott et al. Only seven of the radio galaxies do not have redshift determinations from the spectroscopy, giving a redshift completeness for the sample of >90 per cent. The median redshift of the 7CRS is 1.1. We present a composite 0.2 < z < 0.8 7CRS radio galaxy spectrum and investigate the strengths of the 4000-Å breaks in these radio galaxies. We find an anti-correlation between the 4000-Å break strength and emission line luminosity, indicating that departures from old elliptical galaxy continuum shapes are most likely due to non-stellar emission associated with the active nucleus.