The CaFe Project: Optical Fe II and Near-infrared Ca II Triplet Emission in Active Galaxies. II. The Driver(s) of the Ca II and Fe II and Its Potential Use as a Chemical Clock

Abstract

In this second paper in the series, we carefully analyze the observational properties of the optical Fe ii and near-IR Ca ii triplet in active galactic nuclei (AGNs), as well as the luminosity, black hole mass, and Eddington ratio in order to define the driving mechanism behind the properties of our sample. The Ca ii shows an inverse Baldwin effect, bringing out the particular behavior of this ion with respect to the other low-ionization lines such as H β. We performed a principal component analysis, where 81.2% of the variance can be explained by the first three principal components drawn from the FWHMs, luminosity, and equivalent widths. The first principal component (PC1) is primarily driven by the combination of black hole mass and luminosity with a significance over 99.9%, which in turn is reflected in the strong correlation of the PC1 with the Eddington ratio. The observational correlations are better represented by the Eddington ratio; thus, it could be the primary mechanism behind the strong correlations observed in the Ca ii–Fe ii sample. Since calcium belongs to the α-elements, the Fe ii/Ca ii flux ratio can be used as a chemical clock for determining the metal content in AGNs and trace the evolution of the host galaxies. We confirm the de-enhancement of the ratio Fe ii/Ca ii by the Eddington ratio, suggesting a metal enrichment of the BLR in intermediate-z with respect to low-z objects. A larger sample, particularly at z > 2, is needed to confirm the present results.