Comparison of the active galactic nuclei Baldwin effect with the modified Baldwin effect of the ultraviolet-optical emission lines in a single sample

Abstract

ABSTRACT The inverse relationship between equivalent width (EW) of broad emission lines and the underlying continuum luminosity, known as the Baldwin effect (BE), is an important and well-studied correlation in active galactic nuclei (AGNs). However, the EW is also considered to be anticorrelated with the Eddington ratio, referred to as the modified Baldwin effect (MBE). Investigating whether the BE or MBE is the stronger correlation is important for studying the ionization mechanism and the broad-line region of AGN. Based on a sample of 85 bright quasars with quasi-simultaneous ultraviolet (UV)-optical spectra and a redshift range of 0.03–1.404 from Shang et al., we first present line fitting and measurements of the O vi line that were not previously available and the Si iv + O iv] line. We then investigate and compare the BE and MBE of multiple UV and optical emission lines from this single sample. We find that the BE of most of the broad emission lines is stronger than the MBE. Furthermore, we confirm the previous result that the slope of the BE steepens with the ionization energy of the line, and also find that the MBE slope shows a similar trend. In addition, compared with the C iv line, the O vi shows a broader line width and a larger peak velocity blueshift with respect to the systemic redshift. These results are also in agreement with previous studies and imply that, compared to the C iv gas, the O vi line-emitting gas is closer to the accretion disc and is more strongly influenced by the accretion disc wind.