# Quasar Main Sequence: a line or a plane?

## TL;DR

This study investigates the quasar main sequence, confirming the presence of two dominant Eigenvectors in high-quality samples and proposing a non-linear model that defines a new plane, potentially linked to black hole mass and luminosity.

## Contribution

It demonstrates that a non-linear decay curve fitting improves the representation of quasar main sequence variance over traditional PCA methods.

## Key findings

- Two dominant Eigenvectors are confirmed in high-quality quasar samples.
- A new plane based on a non-linear decay curve better captures variance.
- The main axis likely correlates with black hole mass, and the secondary with luminosity.

## Abstract

A quasar main sequence is widely believed to reveal itself through objects represented in a plane spanned by two parameters: the full-width at half-maximum (FWHM) of H$\beta{}$, and the ratio of Fe II to H$\beta{}$ equivalent width. This sequence is related to the application to quasar properties of principal component analysis (PCA), which reveals that the main axis of variance (Eigenvector 1) is co-directional with a strong anti-correlation between these two measurements. We aim to determine whether the dominance of two Eigenvectors, originally discovered over two decades ago, is replicated in newer high-quality quasar samples. If so, we aim to test if a non-linear approach is an improvement on the linear PCA method by finding two new parameters which represent a more accurate projection of the variances than the Eigenvectors recovered from PCA. We selected quasars from the XSHOOTER archive and a major quasar catalog to build high quality samples. These samples were tested with PCA. We find that the new high-quality samples do indeed have two dominant Eigenvectors as originally discovered. Subsequently we find that the fitting of a non-linear decay curve to the main sequence allows a new plane spanned by linearly independent axes to be defined, based on the distance along the decay curve as the main axis and the distance of each quasar datapoint from the curve as the secondary axis respectively. The results show that it is possible to define a new plane based on the quasar main sequence which accounts for the majority of the variance. The most likely candidate for the new main axis is an anti-correlation with black hole mass. In this case the secondary axis likely represents luminosity. However, given the results of previous studies, inclination angle likely plays a role in H$\beta{}$ width.