The main sequence of quasars: The taming of the extremes

The last few years have seen the confirmation of several trends associated with the quasar main sequence. The idea of a main sequence for quasars is relatively recent, and its full potential for the observational classification and contextualization of quasars' properties has yet to be fully exploited. The main sequence drivers are discussed in terms of the properties of extreme objects. We briefly summarize developments that constrain the viewing angle of the accretion disk in a particular class of quasars (extreme Population B, radiating at low Eddington ratio), as well as inferences on the chemical composition of the broad line emitting gas, and on the nature of radio emission along the quasar main sequence.

Automated summary

In recent years, several trends associated with the quasar main sequence have been confirmed, revealing the relatively recent concept of a main sequence for quasars yet to be fully exploited. The main sequence drivers are discussed in terms of extreme objects, summarizing developments in constraining the viewing angle of accretion disks in specific quasar classes, inferences on the chemical composition of broad line emitting gas, and the nature of radio emission along the quasar main sequence.