Radio-loudness along the quasar main sequence
  Following the established view of the AGNs inner workings, an AGN is
radio-loud (RL) if associated with relativistic ejections emitting a radio
synchrotron spectrum (i.e., a jetted AGN). If large samples of
optically-selected quasars are considered, AGNs are identified as RL if their
Kellermann's radio loudness ratio RK > 10. Our aims are to characterize the
optical properties of different classes based on radio-loudness within the
quasar main sequence (MS) and to test whether the condition RK > 10 is
sufficient for the identification of RL AGNs. A sample of 355 quasars was
selected by cross-correlating the FIRST survey with the SDSS DR14 quasar
catalog. We classified the optical spectra according to their spectral types
along the quasars MS. For each spectral type, we distinguished compact and
extended morphology, and three classes of radio-loudness: detected (specific
flux ratio in the g band and at 1.4GHz, RK' < 10, RD), intermediate (10 < RK' <
70, RI), and radio loud (RK' > 70). The analysis revealed systematic
differences between RD, RI, and RL in each spectral type along the MS. We show
that spectral bins that contain the extreme Population A sources have radio
power compatible with emission by mechanisms ultimately due to star formation
processes. RL sources of Population B are characteristically jetted. Their
broad H-beta profiles can be interpreted as due to a binary broad-line region.
We suggest that RL Population B sources should be preferential targets for the
search of black hole binaries, and present a sample of binary black hole AGN
candidates. The validity of the Kellermann's criterion may be dependent on the
source location along the quasar MS. The consideration of the MS trends allowed
to distinguish between sources whose radio emission mechanisms is jetted from
the ones where the mechanism is likely to be fundamentally different.
