Fitting results for the SMG spectrum, showing the Narrow 1, Narrow 2, BLR, and Outflow components (dotted purple, dashed red, dashed yellow, and dash-dotted blue lines, respectively). The errors considered in the fitting procedure are displayed in grey and the residuals of the fits are shown at the bottom of each panel. Top panel: Fit of the [O III] λλ4959,5007 Å emission lines. The vertical dotted lines mark the position of [O III] at the systemic redshift determined from [C II]. Middle and bottom panels: Fits of the Hα, [N II] λλ6548,6584 Å, and [S II] λλ6717,6731 Å emission lines for the first (outflow) and second (BLR) scenarios described in Sect. 5. The vertical black lines indicate the position of the emission lines for the Narrow 2 component found in [O III].