The CaFe Project: Optical Fe II and Near-Infrared Ca II triplet emission
  in active galaxies -- simulated EWs, the co-dependence of cloud sizes and
  metal content
  Modelling the low ionization lines (LIL) in active galactic nuclei still
faces problems in explaining the observed equivalent widths (EWs). We examine
the optical Fe II and near-infrared Ca II triplet (CaT) emission strengths
using the photoionization code CLOUDY. Using an incident continuum for I Zw 1 -
a prototypical Type-1 narrow-line Seyfert galaxy, we can recover the line
ratios for the optical Fe II (i.e. R$_{\rm{Fe II}}$) and the NIR CaT (i.e.
R$_{\rm{CaT}}$) in agreement to the observed estimates. Although, the pairs of
(U,$\rm{n_{H}}$) that reproduce the conforming line ratios, unfortunately, do
not relate to agreeable line EWs. We thus propose that the LIL region of the
BLR cloud doesn't see the same continuum seen by a distant observer that is
emanated from the accretion disk, rather it sees a filtered version of the
original continuum. The assumption of the filtered continuum as the source of
BLR irradiation recovers realistic EWs for LIL species. However, our study
finds that to account for the adequate R$_{\rm{Fe II}}$ (Fe II/H$\beta$ flux
ratio) emission, the BLR needs to be selectively overabundant in iron. On the
other hand, the R$_{\rm{CaT}}$ (CaT/H$\beta$ flux ratio) emission spans a
broader range from solar to super-solar metallicities. In all these models the
BLR cloud density is found to be consistent with our conclusions from prior
works, i.e. $\rm{n_{H}} \sim 10^{12}$ cm$^{-3}$. An interesting result obtained
here is the reduction in the value of the metallicity by up to a factor 10 for
the R$_{\rm{Fe II}}$ cases when the microturbulence is invoked, suggesting that
microturbulence can act as an apparent metallicity controller for the Fe II. On
the contrary, the R$_{\rm{CaT}}$ cases are rather unaffected by the effect of
microturbulence.
