The Main Sequence of quasars: the taming of the extremes

TL;DR

This paper discusses the quasar main sequence, highlighting recent trends, the influence of extreme objects, and how properties like viewing angle, chemical composition, and radio emission relate to quasar classification.

Contribution

It reviews recent developments in understanding the quasar main sequence, emphasizing the role of extreme objects and observational parameters in quasar classification.

Findings

Constraints on viewing angles of accretion disks in extreme Population B quasars.

Inferences on chemical composition of broad line emitting gas.

Insights into the nature of radio emission along the quasar main sequence.

Abstract

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 quasar 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.