Infrared colour properties of nearby radio-luminous galaxies

By combining the data of the Two Micron All Sky Survey, the Wide-Field Infrared Survey Explorer and the AKARI satellite, we study the infrared colour properties of a sample of 2712 nearby radio-luminous galaxies (RLGs). These RLGs are divided into radio-loud (RL) active galactic nuclei (AGNs), mainly occurring at redshifts of 0.05 < z < 0.3 and star-forming-dominated RLGs (SFGs), mainly occurring at redshifts of 0.01 < z < 0.15. RLAGNs and SFGs are separately distributed in the ([3.4]-[4.6])-([4.6]-[12]) two-colour diagram, in which the RL AGNs display a double-core distribution, and the SFGs display a single-core distribution. SFGs have a redder [4.6]-[12] colour than RL AGNs due to the significant contribution from the dust component of SFGs. We find simple criteria of mid-infrared (MIR) colour separation between RL AGNs and SFGs such that: 95 per cent of RL AGNs have [4.6]-[12] < 3.0 and 94 per cent of SFGs have [4.6]-[12] > 3.0. We also analyse the MIR colours of RL AGNs divided into low-and high-excitation radio galaxies (LERGs and HERGs, respectively). The ([3.4]-[4.6])-([4.6]-[12]) diagram clearly shows separate distributions of LERGs and HERGs and a region of overlap, which suggests that LERGs and HERGs have different MIR properties. LERGs are responsible for the double-core distribution of RL AGNs on the ([3.4]-[4.6])-([4.6]-[12]) diagram. In addition, we also suggest 90-140 mu m band spectral index a(90, 140) < -1.4 as a criterion of selecting nearby active galaxies with non-thermal emissions at far-infrared wavelengths.