Infrared colour properties of nearby radio-luminous galaxies

TL;DR

This study analyzes infrared color properties of 2712 nearby radio-luminous galaxies using combined survey data, distinguishing between radio-loud AGNs and star-forming galaxies, and proposing criteria for their separation based on infrared colors.

Contribution

It introduces simple MIR color criteria to differentiate RL AGNs from SFGs and explores the MIR properties of LERGs and HERGs, enhancing classification methods for radio-luminous galaxies.

Findings

RL AGNs and SFGs occupy distinct regions in MIR color space.

95% of RL AGNs have [4.6]-[12] < 3.0, while 94% of SFGs have [4.6]-[12] > 3.0.

Lergs and HERGs show different MIR distributions, indicating different MIR properties.

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.