What controls the Fe II strength in active galactic nuclei?

TL;DR

This study analyzes a large sample of active galactic nuclei to determine that the Eddington ratio primarily influences Fe II emission strength, affecting interpretations of chemical evolution in quasars.

Contribution

It reveals that Fe II emission strength correlates strongly with the Eddington ratio, highlighting its role in regulating emission-line variations in active galaxies.

Findings

Fe II emission correlates with Eddington ratio rather than luminosity or black hole mass.

Ultraviolet Fe II EW does not correlate with broad-line width, luminosity, or black hole mass.

Fe II/Mg II ratio must be corrected for L/L_{Edd} when studying chemical evolution.

Abstract

We used a large, homogeneous sample of 4178 z <= 0.8 Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the strength of Fe II emission and its correlation with other emission lines and physical parameters of active galactic nuclei. We find that the strongest correlations of almost all the emission-line intensity ratios and equivalent widths (EWs) are with the Eddington ratio (L/L_{Edd}), rather than with the continuum luminosity at 5100\AA\ (L_{5100}) or black hole mass (M_{BH}); the only exception is the EW of ultraviolet Fe II emission, which does not correlate at all with broad-line width, L_{5100}, M_{BH}, or L/L_{Edd}. By contrast, the intensity ratios of both the ultraviolet and optical Fe II emission to Mg II \lambda 2800 correlate quite strongly with L/L_{Edd}. Interestingly, among all the emission lines in the near-UV and optical, the EW of narrow optical Fe II emission has the strongest correlation with L/L_{Edd}. We suggest that the variation of the emission-line strength in active galaxies is regulated by L/L_{Edd} because it governs the global distribution of the column density of the clouds gravitationally bound in the line-emitting region, as well as its overall gas supply. The systematic dependence on L/L_{Edd} must be corrected when using the FeII/MgII intensity ratio as a measure of the Fe/Mg abundance ratio to study the history of chemical evolution in QSO environments.