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 .
RL AGNs 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 MIR colour separation between RL AGNs and SFGs such that : 95 \% of RL AGNs have [ 4.6 ] - [ 12 ] < 3.0 and 94 \% 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 \alpha ( 90 , 140 ) < -1.4 as a criterion of selecting nearby active galaxies with non-thermal emissions at far-infrared wavelengths .