Identifying radio active galactic nuclei among radio-emitting galaxies

TL;DR

This paper introduces two new diagnostics, MIRAD and DLM, for identifying radio active galactic nuclei in galaxies using radio, infrared, and optical data, achieving high classification accuracy.

Contribution

It presents novel, empirical diagnostics that classify radio AGNs with high accuracy without requiring optical spectra or emission line data.

Findings

MIRAD and DLM diagnostics classify 99.5% of extended radio sources as AGNs.

Radio AGNs are mostly found in elliptical galaxies with massive black holes.

Most radio-AGN classified sources are optically weak or retired galaxies.

Abstract

Basing our analysis on ROGUE I, a catalog of over 32,000 radio sources associated with optical galaxies, we provide two diagnostics to select the galaxies where the radio emission is due to an active galactic nucleus (AGN). Each of these diagnostics can be applied independently. The first one, dubbed MIRAD, compares the flux $F_{W3}$ in the $W3$ mid-infrared band of the WISE telescope, with the radio flux at 1.4 GHz, $\Frad$. MIRAD requires no optical spectra. The second diagnostic, dubbed DLM, relates the 4000 \AA\ break strength, $D_{\rm n}(4000)$, with the radio luminosity per unit stellar mass. The DLM diagram has already been used in the past, but not as standalone. For these two diagrams, we propose simple, empirical dividing lines that result in the same classification for the objects in common. These lines correctly classify as radio-AGN 99.5 percent of the extended radio sources in the ROGUE~I catalog, and as star-forming (SF) galaxies 98--99 percent of the galaxies identified as such by their emission line ratios. Both diagrams clearly show that radio AGNs are preferentially found among elliptical galaxies and among galaxies hosting the most massive black holes. Most of the radio sources classified as radio-AGNs in the MIRAD or DLM diagrams are either optically weak AGNs or retired galaxies.