AGN dichotomy beyond radio loudness: a Gaussian Mixture Model analysis
  Since the discovery of Quasi-stellar Objects (QSOs), also known as quasars,
they have been traditionally subdivided as radio-loud and radio-quiet sources.
Whether such division is a misleading effect from a highly heterogeneous single
population of objects, or real has yet to be answered. Such dichotomy has been
evidenced by observations of the flux ratio between the optical and radio
emissions (usually $B$-band and 5 GHz). Evidence of two populations in quasars
and samples of a wide diversity of AGNs has been accumulated over the years.
Other quantities beyond radio loudness also seem to show the signature of the
existence of two different populations of AGN. To verify the existence of a
dichotomy through different parameters, we employed a soft clustering scheme,
based on the Gaussian Mixture Model (GMM), to classify these objects
simultaneously using the following parameters: black hole mass, colour and $R$
loudness index, as well as the usual radio and $B$-band luminosity. To
investigate whether different kinds of AGNs manifest any population dichotomy,
we applied GMM to four independent catalogues composed of both optical and
radio information. Our results indicate the persistence of a dichotomy in all
datasets, although the discriminating power differs for different choices of
parameters. Although the Radio Loudness parameter alone does not seem to be
enough to display the dichotomy, the evidence of two populations of AGNs could
persist even if we consider other parameters. Our research suggests that the
dichotomy is not a misleading effect but real.
