Comparison between the predictions from models of ionization from fast shocks (Allen et al. 2008) and observed line ratios, limited to the regions outside the high-ionization bicone. The black curve shows the line ratios for a gas density of 1 cm⁻³, a metallicity Z = 2 × Z_⊙, and a null magnetic field. The shock velocity ranges from 200 to 1000 km s⁻¹ in steps of 25 km s⁻¹, starting from the bottom left corner. The thick portion of the curve corresponds to the range v_s = 350 − 500 km s⁻¹. The dotted red lines are models with a magnetic field of 0.5 μG and Z = 2 × Z_⊙. The green (dashed) and blue (dot-dashed) curves are the tracks obtained for a solar metallicity and B = 0 μG and B = 0.5 μG, respectively. The five large dots (coded as in Fig. 3) correspond to the compact knots of star formation (the cyan dots are the three knots in the outer filaments).