Dark and bright sides of the Broad Line Region clouds as seen in the FeII emission of SDSS RM 102

Résumé fourni par la source

The plot is the key element of the performed analysis addressing the problem of Fe II contamination. Contamination by Fe II emission remains one of the major challenges in accurately measuring emission-line intensities across the UV and optical spectral ranges, particularly in active galactic nuclei with complex broad-line spectra. In this work, we examine the Fe II emission properties of the bright quasar RM 102 using the most recent implementation of the CLOUDY photoionization code, with the aim of constraining the physical conditions and origin of the emitting gas. The modelling approach is based on a constant-pressure description of the line-emitting clouds, in contrast to the more commonly adopted constant-density approximation. A wide range of physical parameters is explored, including metallicities reaching up to 50 times the solar value and turbulent velocities as high as 100 km s⁻¹ for a subset of models. In addition, we consider the impact of geometric effects that may enhance the contribution of the shielded cloud surfaces, as well as scenarios in which extra mechanical heating supplements pure radiative energy input. The results indicate that the broad-line region of RM 102 is consistent with highly metal-enriched gas. The observed Fe II emission is most naturally reproduced in configurations where the geometry favours enhanced visibility of the non-illuminated cloud faces, particularly under predominantly radiative heating conditions that effectively weight the “dark-side” emission. Models including mechanical heating also provide viable solutions, although they require a more self-consistent treatment to fully assess their physical plausibility. Overall, the analysis highlights the importance of cloud geometry—specifically the relative contribution of illuminated versus shielded surfaces—in shaping the observed Fe II spectrum. It also points to significant metal enrichment in the emitting region as a key ingredient. Finally, we note that current versions of CLOUDY still lack a number of atomic transitions relevant to Fe II, which limits the ability to reproduce all observed spectral features in detail. Acknowledgements: Project funded by National Science Centre, Poland under the OPUS call in the Weave programme, UMO-2021/43/I/ST9/01352.

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