Infrared colour properties of nearby radio-luminous galaxies

Abstract

By combining the data of the Two Micron All Sky Survey, the \textit{Wide Field Infrared Survey Explorer} and the \textit{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.