Unveiling the quasar main sequence: illuminating the complexity of active galactic nuclei and their evolution

TL;DR

This paper reviews the development and significance of the quasar main sequence, a tool for understanding active galactic nuclei's diversity, evolution, and potential as cosmological distance indicators.

Contribution

It provides a comprehensive overview of the history, current research, and future directions in studying the quasar main sequence and its applications.

Findings

The quasar main sequence links emission line properties to black hole mass and accretion rate.

It has evolved as a key framework for understanding AGN diversity and evolution.

Recent studies explore its potential for cosmological measurements.

Abstract

The Eigenvector 1 schema, or the main sequence of quasars, was introduced as an analogous scheme to the HR diagram that would allow us to understand the more complex, extended sources - active galactic nuclei (AGNs) that harbor accreting supermassive black holes. The study has spanned more than three decades and has advanced our knowledge of the diversity of Type-1 AGNs from both observational and theoretical aspects. The quasar main sequence, in its simplest form, is the plane between the FWHM of the broad H$\beta$ emission line and the strength of the optical FeII emission to the H$\beta$. While the former allows the estimation of the black hole mass, the latter enables direct measurement of the metal content and traces the accretion rate of the AGN. Together, they allow us to track the evolution of AGN in terms of the activity of the central nuclei, its effect on the line-emitting regions surrounding the AGN, and their diversity making them suitable distance indicators to study the expansion of our Universe. This mini-review aims to provide (i) a brief history leading up to the present day in the study of the quasar main sequence, (ii) introduce us to the many possibilities to study AGNs with the main sequence as a guiding tool, and (iii) highlight some recent, exciting lines of researches at the frontier of this ever-growing field.