2024ApJ...971....6Z

The complexity of the energy levels in Fe II has made it difficult to theoretically interpret its observational emission spectrum. However, addressing this challenge could provide valuable insights into the main sequence of QSOs using “eigenvector 1” and the chemical evolution of the Universe. Although the templates based on I Zw 1 are widely used, their universality remains unexplained. To address this, we utilized the CLOUDY spectral synthesis code to generate theoretical blends of Fe II optical emission near the Hβ region. Our results indicate that the optimal cloud density and photon flux align with previous studies at gas density 10¹¹ cm^–3 and photon flux 10^20.5 cm^–2 s^–1. We discovered a broad parameter region that fits the data well, encompassing a diverse range of densities and photon fluxes. This helps to explain the general applicability of optical templates. Additionally, we investigated the effects of different spectral energy distributions (SEDs), Eddington ratios, turbulence, and cloud column densities and found that they provided a sufficiently broad range for typical emitting clouds. We found different SEDs had a negligible impact on the template shape, further expanding the usability of empirical templates. However, they significantly affected the absolute intensity of Fe II emission, consistent with previous research. Furthermore, we determined that a turbulence of approximately 100 km s^–1 is necessary to produce sufficiently strong optical Fe II, and either a column density greater than 10²⁴ cm^–2, which is typical for Fe II UV emission, or an abundance higher than solar is preferred.