Comparison of GS-14 (red star), literature AGN (cyan diamonds), and nebular theoretical predictions of the N IV]/N V and O VI/N V flux ratios. The contours represent isoproportions of the density (i.e., 8% of the probability mass lies outside of the contour drawn for 0.08). Brown contours refer to shock predictions from the Morisset et al. (2015) 3MDB^S database, green contours to SF models from Gutkin et al. (2016), and blue contours to AGN emission as computed by Feltre et al. (2016). Literature AGN are the sources described in Sect. 2.3 and in Appendix A with all the three lines detected. No shock model is able to reproduce the GS-14 line ratios; SF and AGN can both be at the origin of the observed nebular emission. As the GS-14 O VI line is not corrected for the IGM attenuation, we have only a lower limit on its flux, thus a lower limit on the O VI/N V ratio (black arrow).