2020MNRAS.494.4187M

We present near-infrared spectroscopy of the NLS1 galaxy PHL 1092 (z = 0.394), the strongest Fe II emitter ever reported, combined with optical and UV data. We modelled the continuum and the broad emission lines using a power-law plus a blackbody function and Lorentzian functions, respectively. The strength of the Fe II emission was estimated using the latest Fe II templates in the literature. We re-estimate the ratio between the Fe II complex centred at 4570 Å and the broad component of H β, R₄₅₇₀, obtaining a value of 2.58, nearly half of that previously reported (R₄₅₇₀ = 6.2), but still placing PHL 1092 among extreme Fe II emitters. The full width at half-maximum (FWHM) values found for low-ionization lines are very similar (FWHM ∼ 1200 km s^–1), but significantly narrower than those of the hydrogen lines (FWHM_H β_ ∼ 1900 km s^–1). Our results suggest that the Fe II emission in PHL 1092 follows the same trend as in normal Fe II emitters, with Fe II being formed in the outer portion of the BLR and co-spatial with Ca II, and O I, while H β is formed closer to the central source. The flux ratio between the UV lines suggests high densities, log(n_H) ∼ 13.0 cm^–3, and a low ionization parameter, log(U) ∼ -3.5. The flux excess found in the Fe II bump at 9200 Å after the subtraction of the NIR Fe II template and its comparison with optical Fe II emission suggests that the above physical conditions optimize the efficiency of the Ly α-fluorescence process, which was found to be the main excitation mechanism in the Fe II production. We discuss the role of PHL 1092 in the eigenvector 1 context.