Dissecting the quasar main sequence: insight from host galaxy properties

TL;DR

This study confirms that the diversity in broad-line quasar properties along the main sequence is primarily driven by the Eddington ratio, using host galaxy stellar velocity dispersion measurements from SDSS data.

Contribution

It provides independent evidence linking Eddington ratio to FeII strength in quasars through host galaxy analysis, reinforcing previous clustering-based findings.

Findings

Eddington ratio increases with FeII strength at fixed luminosity.

Sigma* decreases with FeII strength, indicating higher Eddington ratios.

Little dependence of sigma* on broad Hbeta FWHM at fixed luminosity and FeII strength.

Abstract

The diverse properties of broad-line quasars appear to follow a well-defined main sequence along which the optical FeII strength increases. It has been suggested that this sequence is mainly driven by the Eddington ratio (L/L_Edd) of the black hole (BH) accretion. Shen & Ho demonstrated with quasar clustering analysis that the average BH mass decreases with increasing FeII strength when quasar luminosity is fixed, consistent with this suggestion. Here we perform an independent test by measuring the stellar velocity dispersion sigma* (hence the BH mass via the M-sigma* relation) from decomposed host spectra in low-redshift Sloan Digital Sky Survey quasars. We found that at fixed quasar luminosity, sigma* systematically decreases with increasing FeII strength, confirming that Eddington ratio increases with FeII strength. We also found that at fixed luminosity and FeII strength, there is little dependence of sigma* on the broad Hbeta FWHM. These new results reinforce the framework put forward by Shen & Ho that Eddington ratio and orientation govern most of the diversity seen in broad-line quasar properties.