The quasar main sequence

The Hertzsprung-Russell (H-R) diagram developed in the first half of the XXth century discriminates between the principal classes of normal stars and isolates important states of stellar evolution, using just two parameters: stellar luminosity and color. We are searching for an equivalent diagram that might help clarify the phenomenology of active galactic nuclei (AGNs) and lead to better understanding the physics that drives them. It was expected that more than two dimensions would be required to remove the degeneracy between physics and source orientation. Do we need an H-R diagram for broad-line AGNs or will an average quasar spectrum tell us all that we need to know? We find that AGNs exhibit a large phenomenological diversity, thus a parameter space equivalent to the H-R diagram is invaluable. We introduced a 4D "Eigenvector1" (4DE1) parameter space and defined a quasar main sequence as a surrogate H-R diagram for AGNs. The results are very encouraging and virtually every observational conundrum that has arisen in the past decades concerning the structure, physical condition and dynamics of the broad line emitting region (BLR) of AGNs is simplified in the 4DE1 context (Marziani et al. 2018) . Fig. 1 shows the results of a photoionization model analysis supporting a trend in density along the Population A branch of the quasar main sequence. The trends along the main sequence have even allowed us to isolate a particular spectral type of quasars (radiating close to the Eddington limit) that may be suitable as a distance indicator for cosmology (Dultzin et al. 2020). See also https://web.oapd.inaf.it/marziani/ .