Quasar main sequence unfolded by 2.5D FRADO

Aims. The quasar main sequence (QMS)–characterized by the Eigenvector 1 (EV1)–serves as a unifying framework for classifying type-1 active galactic nuclei (AGNs) based on their diverse spectral properties. Although it has long eluded a fully self-consistent physical interpretation, our physically motivated 2.5D failed radiatively accelerated dusty outflow (FRADO) model now naturally predicts that the Eddington ratio ( ṁ ) is the underlying physical primary driver of QMS, with the black hole mass ( M • ) and inclination ( i ) acting as secondary contributors. Methods. We recruited a dense grid of FRADO simulations of the geometry and dynamics of the broad-line region covering a representative range of M • and ṁ . For each simulation, we computed the full width at half maximum (FWHM) of the H β line under different i . Results. The resulting FWHM– ṁ diagram strikingly resembles the characteristic trend observed in the EV1 parameter space. Therefore, it establishes the role of ṁ as the true proxy for the Fe II strength parameter ( R Fe ), and vice versa. Our results suggest that ṁ can be the sole underlying physical tracer of R Fe and should therefore scale directly with it. The M • accounts for the virial mass–related scatter in the FWHM. The i then acts as a secondary driver modulating the R Fe and FWHM for a given ṁ and M • , respectively.