The CaFe Project: Optical FeII 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.

In this second paper in the series, we carefully analyze the observational properties of the optical FeII and NIR CaII triplet properties as well as the luminosity, black hole mass, and Eddington ratio in order to define the driving mechanism behind the properties of our sample. Most of the significant correlations are associated with CaII triplet. The CaII shows an inverse Baldwin effect, bringing out the particular behavior of this ion with respect to the other low-ionization lines such as H$\beta$. We also performed a Principal Component Analysis, where $\sim80\%$ of the variance can be explained by the first three principal components drawn from the FWHMs, luminosity, and equivalent widths. The first principal component 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 first eigenvector with the Eddington ratio. Since the observational correlations are better represented by the Eddington ratio, this could be the primary mechanism behind the relations observed in our CaII-FeII sample. Since the calcium belong to the $\alpha$-elements, the FeII/CaII flux ratio can be used as a chemical clock for determining the metal content in AGN and trace the evolution of the host galaxies. We confirm the negative enhancement of the ratio FeII/CaII 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.