Identifying radio active galactic nuclei among radio-emitting galaxies
  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.
