Radio galaxies in the 2SLAQ Luminous Red Galaxy survey - II. the stellar populations of radio-loud and radio-quiet LRGs

We present an analysis of the optical spectra of a volume-limited sample of 375 radio galaxies at redshift 0.4 < z < 0.7 from the 2dF-SDSS (Sloan Digital Sky Survey) Luminous Red Galaxy (LRG) and QSO (quasi-stellar object) (2SLAQ) redshift survey. We investigate the evolution of the stellar populations and emission-line properties of these galaxies. By constructing composite spectra and comparing with a matched sample of radio-quiet sources from the same survey, we also investigate the effect on the galaxy of the presence of an active nucleus. The composite spectra, binned by redshift and radio luminosity, all require two components to describe them, which we interpret as an old and a younger population. We found no evolution with redshift of the age of the younger population in radio galaxies, nor were they different from the radio-quiet comparison sample. Similarly, there is no correlation with radio power, with the exception that the most powerful radio sources (P_1.4 > 10²⁶W Hz^-1) have younger stars and stronger emission lines than the less powerful sources. This suggests that we have located the threshold in radio power where strong emission lines 'switch on', at radio powers of around 10²⁶ W Hz^-1. Except for the very powerful radio galaxies, the presence of a currently active radio active galactic nucleus (AGN) does not appear to be correlated with any change in the observed stellar population of a luminous red galaxy at z ∼ 0.5.