New Insights on the QSO Radio-Loud/Radio-Quiet Dichotomy: SDSS Spectra in the Context of the 4D Eigenvector1 Parameter Space

We search for a dichotomy/bimodality between Radio Loud (RL) and Radio Quiet (RQ) Type 1 Active Galactic Nuclei (AGN). We examine several samples of SDSS QSOs with high S/N optical spectra and matching FIRST/NVSS radio observations. We use the radio data to identify the weakest RL sources with FRII structure to define a RL/RQ boundary which corresponds to log L1.4GHz=31.6 ergs s Hz. We measure properties of broad line Hβ and FeII emission to define the optical plane of a 4DE1 spectroscopic diagnostic space. The RL quasars occupy a much more restricted domain in this optical plane compared to the RQ sources, which a 2D Kolmogorov-Smirnov test finds to be highly significant. This tells us that the range of BLR kinematics and structure for RL sources is more restricted than for the RQ QSOs, which supports the notion of dichotomy. FRII and CD RL sources also show significant 4DE1 domain differences that likely reflect differences in line of sight orientation (inclined vs. face-on respectively) for these two classes. The possibility of a distinct Radio Intermediate (RI) population between RQ and RL source is disfavored because a 4DE1 diagnostic space comparison shows no difference between RI and RQ sources. We show that searches for dichotomy in radio vs. bolometric luminosity diagrams will yield ambiguous results mainly because in a reasonably complete sample the radio brightest RQ sources will be numerous enough to blur the gap between RQ and RL sources. Within resolution constraints of NVSS and FIRST we find no FRI sources among the broad line quasar population.