Strong Correlation between Fe ii/Mg ii Ratio and Eddington Ratio of Type 1 Active Galactic Nuclei

The Fe ii/Mg ii 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 Fe ii/Mg ii 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 Fe ii/Mg ii 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.