The underlying driver for the \civ Baldwin effect in QSOs with $0<z<5$

TL;DR

This study investigates the Baldwin effect in QSOs across a wide redshift range, revealing that the Eddington ratio is the primary driver behind the extciv\ Baldwin effect, rather than SMBH mass.

Contribution

It provides a comprehensive analysis across redshifts and introduces an empirical host fraction formula, establishing the Eddington ratio as the key factor influencing the extciv\ Baldwin effect.

Findings

extciv\ EW correlates strongly with the Eddington ratio.

The Baldwin effect is confirmed across $0<z<5$ in the sample.

Eddington ratio is a better predictor than SMBH mass for extciv\

Abstract

Broad emission lines is a prominent property of type I quasi-stellar objects (QSOs). The origin of the Baldwin effect for \civ $\lambda1549~$\AA\ broad emission lines, i.e., the luminosity dependence of the \civ equivalent width (EW), is not clearly established. Using a sample of 87 low-$z$ Palomar-Green (PG) QSOs and 126 high-$z$ QSOs across the widest possible ranges of redshift ($0<z<5$), we consistently calculate \hb-based single-epoch supermassive black hole (SMBH) mass and the Eddington ratio to investigate the underlying driver of the \civ Baldwin effect. An empirical formula to estimate the host fraction in the continuum luminosity at 5100 \AA\ is presented and used in \hb-based \mbh calculation for low-$z$ PG QSOs. It is found that, for low-$z$ PG QSOs, the Eddington ratio has strong correlations with PC1 and PC2 from the principal component analysis, and \civ EW has a strong correlation with the optical \feii strength or PC1. Expanding the luminosity range with high-$z$ QSOs, it is found that \civ Baldwin effect exists in our QSOs sample. Using \hb-based single-epoch SMBH mass for our QSOs sample, it is found that \civ EW has a strong correlation with the Eddington ratio, which is stronger than that with the SMBH mass. It implies that the Eddington ratio seems to be a better underlying parameter than the SMBH mass to drive the \civ Baldwin effect.