Modelling of the quasar main sequence in the optical plane

TL;DR

This paper presents a physically motivated model explaining the distribution of quasars in the optical plane, linking accretion disk properties and emission line features, and explores the factors influencing the quasar main sequence.

Contribution

The study introduces a comprehensive model with six key parameters that successfully reproduces the observed quasar main sequence in the optical plane, highlighting the roles of metallicity and cloud properties.

Findings

Model reproduces the observed quasar distribution in the optical plane.

Super-Solar metallicity is needed to match strong Fe II emitters.

Viewing angle has limited impact on the sequence dispersion.

Abstract

The concept of the quasar main sequence is very attractive since it stresses correlations between various parameters and implies the underlying simplicity. In the optical plane defined by the width of the H\{beta} line and the ratio of the equivalent width of the Fe II to H\{beta} observed objects form a characteristic pattern. In this paper, we use a physically motivated model to explain the distribution of quasars in the optical plane. Continuum is modelled as an accretion disk with a hard X-ray power law uniquely tight to the disk at the basis of observational scaling, and the Broad Line Region distance is determined also from observational scaling. We perform the computations of the FeII and H\{beta} line production with the code CLOUDY. We have only six free parameters for an individual source: maximum temperature of the accretion disk, Eddington ratio, cloud density, cloud column density, microturbulence, and iron abundance, and only the last four remain as global parameters in our modelling of the whole sequence. Our theoretically computed points cover well the optical plane part populated with the observed quasars, particularly if we allow for super-Solar abundance of heavy elements. Explanation of the exceptionally strong Fe II emitter requires a stronger contribution from the dark sides of the clouds. Analyzing the way how our model covers the optical plane we conclude that there is no single simple driver behind the sequence, as neither the Eddington ratio nor broad band spectrum shape plays the dominant role. Also, the role of the viewing angle in providing the dispersion of the quasar main sequence is apparently not as strong as expected.