Modelling of the broad Hβ (left panel) and Hα line (right panel). The red line represents the best-fit model to the data (black line) composed of individual Gaussians and a straight to approximate the local continuum. Individual Gaussian components for the Balmer lines are shown as blue lines above the continuum for a visual impression of their respective contribution. Two vertical dotted lines mark the wavelengths of the expected [N ii] λλ6548,6583 lines. The residuals of the best-fit model are displayed in the panels below. Some residuals of the bright Na iλ5893 night-sky line are visible above the broad Hβ line. Two telluric absorption bands of the Earth’s atmosphere are clearly visible at 7600–7700 Å and 8150–8350 Å.