Connecting MeerKAT radio continuum properties to GAMA optical emission-line and WISE mid-infrared activity

TL;DR

This study combines radio, optical, and mid-infrared data to improve AGN identification, revealing that the infrared-to-radio luminosity ratio can detect additional AGNs missed by traditional methods.

Contribution

It introduces a new classification scheme integrating multi-wavelength data and tests its effectiveness using a large galaxy sample from GAMA, WISE, and MeerKAT.

Findings

q(TIR) detects up to 70% more AGNs than optical/infrared methods.

q(TIR) is less reliable when host galaxy star formation dominates.

Median q(TIR) for star-forming galaxies aligns with previous estimates.

Abstract

The identification of AGN in large surveys has been hampered by seemingly discordant classifications arising from differing diagnostic methods, usually tracing distinct processes specific to a particular wavelength regime. However, as shown in Yao et al. (2020), the combination of optical emission line measurements and mid-infrared photometry can be used to optimise the discrimination capability between AGN and star formation activity. In this paper we test our new classification scheme by combining the existing GAMA-WISE data with high-quality MeerKAT radio continuum data covering 8 deg$^2$ of the GAMA G23 region. Using this sample of 1 841 galaxies (z < 0.25), we investigate the total infrared (derived from 12$\mu$m) to radio luminosity ratio, q(TIR), and its relationship to optical-infrared AGN and star-forming (SF) classifications. We find that while q(TIR) is efficient at detecting AGN activity in massive galaxies generally appearing quiescent in the infrared, it becomes less reliable for cases where the emission from star formation in the host galaxy is dominant. However, we find that the q(TIR) can identify up to 70 % more AGNs not discernible at optical and/or infrared wavelengths. The median q(TIR) of our SF sample is 2.57 $\pm$ 0.23 consistent with previous local universe estimates.