The evolution of the fundamental plane of radio galaxies from z∼ 0.5 to the present day

We present deep spectroscopic data for a 24-object subsample of our full 41-object z∼ 0.5 radio galaxy sample in order to investigate the evolution of the Fundamental Plane of radio galaxies. We find that the low-luminosity, Fanaroff–Riley type I (FRI), radio galaxies in our sample are consistent with the local Fundamental Plane of radio galaxies defined by Bettoni et al. when corrected for simple passive evolution of their stellar populations. However, we find that the higher luminosity, Fanaroff–Riley type II (FRII), radio galaxies are inconsistent with the local Fundamental Plane if only passive evolution is considered, and find evidence for a rotation in the Fundamental Plane at z∼ 0.5 when compared with the local relation. We show that neither passive evolution, nor a mass-dependent evolution in the mass-to-light ratio, nor an evolution in the size of the host galaxies can, by themselves, plausibly explain the observed tilt. However, we suggest that some combination of all three effects, with size evolution as the dominant factor, may be sufficient to explain the difference between the planes. We also find evidence for a correlation between host galaxy velocity dispersion and radio luminosity at the 97 per cent significance level within our subsample, although further observations are required in order to determine whether this is different for the FRI and FRII radio sources. Assuming that the MBH–σ relation still holds at z∼ 0.5, this implies that radio luminosity scales with black hole mass, in agreement with previous studies.