Plot of the H₂ S(3)–S(1)/PAH (7.7 + 8.6 μm) as a function of PAH 11.3/7.7 (upper left), PAH 6.2/7.7 (upper right), H₂ S(3)/S(1) (lower left) and [Ne iii]/[Ne ii] (lower right). Each data point represents a region of 10 arcsec × 10 arcsec in extent. Red points are the northern outflow, cyan the southern outflow, and green the central 1 kpc of M82 from Beirão et al. (2008). The error bars represent typical uncertainties in the northern (red) and southern (cyan) outflows. The dashed lines represent the average and the 1σ variation of the H₂ S(1)–S(3)/PAH (7.7 + 8.6 μm) ratio in the star-forming galaxy sample of Roussel et al. (2007). The dot–dashed line represents the upper limit in the H₂ S(1)–S(3)/PAH (7.7 + 8.6 μm) ratio predicted by PDR models (Guillard et al. 2012).