The CaFe project: Optical Fe II and near-infrared Ca II triplet emission in active galaxies: simulated EWs and the co-dependence of cloud size 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.
Comment: 15 pages, 8 figures, accepted in Astronomy & Astrophysics