Residual spectrum of SDSS J1431+4358 (green symbols in this figure and in Fig. 1) around the [OIII]λ4960 Å, λ5008 Å emission line region. The spectrum is shown in the rest-frame vacuum wavelengths. The gray dashed vertical lines indicate the position of the emission line at the systemic velocity of the host galaxy as derived by the stellar absorption lines. Panel a: the black solid line represents the model obtained by the combination of a power-law continuum plus two Gaussian emission line components for each of the [OIII] lines. Panel b: relevant residuals, shown in terms of sigmas. Panel c: the black solid line indicates the model obtained by adding a Gaussian component at 5020 Å (denoted as Fe II 42) and two broad Gaussian components (denoted as “wing?”, see Sect. 3.1) to the basic model shown in panel a. Panel d: relevant residuals, shown in terms of sigmas.