Main trends of the quasar main sequence - effect of viewing angle

We address the effect of viewing angle of the accretion disk plane and the geometry of the broad line region (BLR) with the goal of interpreting the distribution of quasars along the main sequence (MS). We utilize photoionization code CLOUDY to model the BLR Fe_II emission, incorporating the grossly underestimated role of the form factor (f). We recover the dependence of the strength of the Fe_II emission in the optical (R_FeII) on L_bol/L_Edd ratio and related observational trends - as a function of the spectral energy distribution (SED) shape, cloud density, composition and intra-cloud dynamics, assumed following prior observational constraints. With this approach, we are now able to explain the diversity of quasars and the change of the quasar properties along the Main Sequence (MS). Our approach also explains the rarity of the highest Fe_II emitters known as the extreme xA sources and can be used as a predictive tool in future reverberation mapping studies of Type-1 AGNs. This approach further justifies the use of quasars as `cosmological probes'.