Spectra of emission lines covered by this programme stacked over the central 1.5 kpc region at 34”. The emission lines are normalised to the maximum in the spectrum. The five bottom spectra are taken from different observing programmes (see Sect. 2.4), while the spectra above come from this project. For the absolute intensities, we refer the reader to Table 3. We note that we employ a hexagonal, half-beam-sized sampling. The angular resolution of 34” corresponds to a physical scale of around 1.4kpc. Therefore, the stacked spectra over the central 1.5kpc region are thus the combination of the spectra of the seven most central sampling points.