Galaxy And Mass Assembly (GAMA): the environments of high- and low- excitation radio galaxies

We study the environments of low- and high-excitation radio galaxies (LERGs and HERGs, respectively) in the redshift range 0.01 < z < 0.4, using a sample of 399 radio galaxies and environmental measurements from the Galaxy And Mass Assembly (GAMA) survey. In our analysis we use the fifth nearest neighbour density (Σ₅) and the GAMA galaxy groups catalogue (G3Cv6) and construct control samples of galaxies matched in stellar mass and colour to the radio-detected sample. We find that LERGs and HERGs exist in different environments and that this difference is dependent on radio luminosity. High-luminosity LERGs (LNVSS ≳ 10²⁴ W Hz⁻¹) lie in much denser environments than a matched radio-quiet control sample (about three times as dense, as measured by Σ₅), and are more likely to be members of galaxy groups (82^+5_−7per cent of LERGs are in GAMA groups, compared to 58^+3_−3per cent of the control sample.). In contrast, the environments of the HERGs and lower luminosity LERGs are indistinguishable from that of a matched control sample. Our results imply that high-luminosity LERGs lie in more massive haloes than non-radio galaxies of similar stellar mass and colour, in agreement with earlier studies. When we control for the preference of LERGs to be found in groups, both high- and low-luminosity LERGs are found in higher-mass haloes (∼0.2 dex; at least 97 per cent significant) than the non-radio control sample.