Residual spectrum of SDSS J1431+4358 (green symbols in this figure and in Fig. 1) around the Hα + [NII]λ6550 Å, λ6585 Å emission line region. The spectrum is shown in the rest-frame vacuum wavelengths. The gray dashed vertical lines mark the position of the emission line at the systemic velocity of the host galaxy. Panel a: the black solid line is the model obtained by the combination of a power-law continuum plus two Gaussian emission line components for both the Hα and for each of the [NII] emission lines. Panel b: relevant residuals, shown in terms of sigmas.