2025A&A...697A..23F

Context. Contamination from singly ionized iron emission is one of the greatest obstacles to determining the intensity of emission lines in the UV and optical wavelength ranges.
Aims. This study presents a comprehensive analysis of the Fe II emission in the bright quasar RM 102, based on the most recent version of the CLOUDY software, with the goal of understanding the nature and the origin of the emission.
Methods. We employ a constant pressure model for the emitting clouds, instead of the customary constant density assumption. The allowed parameter range is broad, with metallicity up to 50 times the solar value and turbulent velocity up to 100 km s^–1 for a subset of models. We also consider geometrical effects that could enhance the visibility of the non-illuminated faces of the clouds, as well as the presence of additional mechanical heating.
Results. Our investigation reveals that the broad line region of RM 102 is characterized by highly metallic gas. The observed Fe II features provide strong evidence for an inflow pattern geometry that favours the dark sides of clouds over isotropic emission if the heating is predominantly radiative. Solutions with mechanical heating are also an interesting option, but they require further self-consistent analysis.
Conclusions. This study underscores the critical role of the dark versus bright side interpretation for reproducing the strong Fe II features in RM 102, highlighting both the geometry of the emitting region and the presence of chemically enriched gas as fundamental factors. Additionally, we report that CLOUDY currently still lacks certain transitions in its atomic databases, which prevents it from fully reproducing some observed Fe II features in quasar spectra.