Strong correlation between FeII/MgII ratio and Eddington ratio of type 1 Active galactic nuclei

TL;DR

This study finds a strong correlation between the FeII/MgII emission line ratio and the Eddington ratio in type 1 AGNs, indicating a link between BLR metallicity and accretion activity rather than galaxy properties.

Contribution

It demonstrates that FeII/MgII ratios are primarily correlated with Eddington ratio, revealing a connection between BLR metallicity and AGN accretion activity, based on extensive UV spectral data.

Findings

FeII/MgII ratio strongly correlates with Eddington ratio

No significant correlation with black hole mass or luminosity

BLR metallicity linked to accretion activity, not galaxy properties

Abstract

The FeII/MgII line flux ratio has been used as an indicator of the Fe/Mg abundance ratio in the broad line region (BLR) of active galactic nuclei (AGNs). On the basis of archival rest-frame UV spectra obtained via the Hubble Space Telescope and the Sloan Digital Sky Survey, we investigate the FeII/MgII ratios of type 1 AGNs at z < 2. Over wide dynamic ranges of AGN properties (i.e., black hole mass, AGN luminosity, and Eddington ratio), we confirm that the FeII/MgII ratio strongly correlates with Eddington ratio but not with black hole mass, AGN luminosity, or redshift. Our results suggest that the metallicity in the BLR are physically related to the accretion activity of AGNs, but not to the global properties of galaxies (i.e., galaxy mass and luminosity). With regard to the relation between the BLR metallicity and the accretion rate of AGNs, we discuss that metal cooling may play an important role in enhancing the gas inflow into the central region of host galaxies, resulting in the high accretion rate of AGNs.