A Multimessenger View of Supermassive Black Holes and the Quasar Main Sequence

In the last 30 years, we have seen a major step forward in the analysis of optical and UV spectroscopic data of large quasar samples. It is an important achievement that the main empirical correlates can now be organized along what has become known as the Eigenvector 1 “main sequence” (MS) of quasars. The current research is oriented toward the definition and the expansion of the MS correlates, as well as toward the exploitation of the MS systematic trends as a basic input to physical and dynamical modelling of the quasar structure, including the accretion disk structure. Nowadays, it has become widely accepted that the main physical parameter governing the sequence is the Eddington ratio, i.e., the ratio between AGN luminosity and black hole mass or equivalently, between radiation and gravitation forces—a parameter that for AGN takes the role of mass for stars. This means that a number of applications have become possible, among them the estimate of the accretion disk wind properties and of the metal enrichment in large samples of AGN, and, finally, the use of quasars as high-redshift distance indicators. The present Reprint will present several contributions (both theoretical and observational) that deal with diverse aspects related to the main sequence at the frontier of research on AGN.