Panchromatic properties of the extreme Fe ii emitter PHL 1092.

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, R ₄₅₇₀, obtaining a value of 2.58, nearly half of that previously reported (R ₄₅₇₀ = 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. [ABSTRACT FROM AUTHOR]