Simultaneous spectral fit of the [O II]λλ3726, 29 and [S II]λλ6716, 31 doublets to derive the electron density from the integrated spectrum of the main target. The blue curve shows the total best-fit model, the light blue curves show the best-fit Gaussians for the individual emission lines, and the grey curves the uncertainties of the MCMC fit. The bottom panels show the residuals, the dashed magenta line indicates the average 1σ noise level.