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 active galactic nuclei (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 AGNs. 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 data sets, 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.