Integrated spectra for the emission-line regions surrounding NDWFS J1425+3254 (orange solid), showing the Hβ–[O III] wavelength range (left panel) and the Hα–[N II] wavelength range (right panel). The blue dashed line shows the best model assuming that the physical line width of Hα is fixed to that of [O III] λ 5007: the ‘constrained Hα model’. The black solid line shows the best model allowing the Hα line width to vary: the ‘unconstrained Hα model.’ The dashed orange line shows the residual around Hα after subtracting the constrained Hα model. The orange shaded region shows the ±1σ uncertainty level of the spectra. The individual Gaussian model components of Hβ and [O III] are shown as dashed lines. The [O III] λ 5007/[O III] λ 4959 ratio is constrained to 2.98. The dotted vertical lines mark the emission line locations at the quasar host redshift, z = 5.901.