Emissivity ratio of the optically forbidden Ni emission line, calculated in PyNeb. The dashed curves are the emissivity ratios calculated for each given temperature. The gray region denotes the predicted range of ratios for collisional excitation. The red region denotes the range of values we observe in our data. The solid black line denotes the predicted ratio for collisional+fluorescent excitation. The predicted values are taken from Giannini et al. (2015) assuming T_e = 10 000 Κ and n_e ~ 10³–10⁵ cm⁻³.