The diversity of quasars unified by accretion and orientation

Quasars are rapidly accreting supermassive black holes at the center of massive galaxies. They display a broad range of properties across all wavelengths, reflecting the diversity in the physical conditions of the regions close to the central engine. These properties, however, are not random, but form well-defined trends. The dominant trend is known as Eigenvector 1, where many properties correlate with the strength of optical iron and [OIII] emission. The main physical driver of Eigenvector 1 has long been suspected to be the quasar luminosity normalized by the mass of the hole (the Eddington ratio), an important quantity of the black hole accretion process. But a definitive proof has been missing. Here we report an analysis of archival data that reveals that Eddington ratio indeed drives Eigenvector 1. We also find that orientation plays a significant role in determining the observed kinematics of the gas, implying a flattened, disklike geometry for the fast-moving clouds close to the hole. Our results show that most of the diversity of quasar phenomenology can be unified with two simple quantities, Eddington ratio and orientation.