Top: portion of the proto-BCG 2D spectrum from MOSFIRE, showing emission lines. The spectrum shows the entire slit height but only a part of the spectral range, centred on the emission lines. Middle: same portion of the 1D spectrum. The blue and pink lines show possible decompositions into one and two Hα emission lines respectively. Bottom: same as in the middle panel, but presenting a third possible decomposition (in purple): Hα with the [N II] doublet on each side. The dotted lines trace the individual components of each fit. The shaded zones on the 1D spectra correspond to the regions with the 10% lowest weights in the fits, i.e. regions with subtracted sky lines. The fits are performed on the 1D spectrum.