Optical Fe II and Near-Infrared Ca II triplet emission in active galaxies: (I) Photoionization modelling

Optical Fe II emission is a strong feature of quasar spectra originating in the broad-line regions (BLRs). The difficulty in understanding the complex Fe II pseudo-continuum has led us in search of other reliable, simpler ionic species such as Ca II. We test the photoionization predictions using CLOUDY and confirm the strong observed correlation between the strengths of two emission features, the optical Fe II and the NIR Ca II. We utilize a broad range of metallicities and extended cloud columns to reinstate this correlation. Our work also explains the emission from the strong Fe II emitting Narrow-Line Seyfert 1 galaxies (NLS1s) and the physical conditions required to produce such strong emission.