A main sequence for quasars

TL;DR

This paper reviews the development of a main sequence framework for quasars based on optical and UV spectroscopic data, highlighting physical parameters, structural implications, and potential standard candle applications.

Contribution

It synthesizes 25 years of research into the quasar main sequence, emphasizing empirical trends, physical interpretations, and the significance of FeII emitters as Eddington standard candles.

Findings

Identification of systematic trends in quasar properties.

Correlation of the main sequence with physical parameters.

Potential use of FeII emitters as Eddington standard candles.

Abstract

The last 25 years saw a major step forward in the analysis of optical and UV spectroscopic data of large quasar samples. Multivariate statistical approaches have led to the definition of systematic trends in observational properties that are the basis of physical and dynamical modeling of quasar structure. We discuss the empirical correlates of the so-called "main sequence" associated with the quasar Eigenvector 1, its governing physical parameters and several implications on our view of the quasar structure, as well as some luminosity effects associated with the virialized component of the line emitting regions. We also briefly discuss quasars in a segment of the main sequence that includes the strongest FeII emitters. These sources show a small dispersion around a well-defined Eddington ratio value, a property which makes them potential Eddington standard candles.