[C ii]/FIR ratio as function of L_FIR. The blue solid line shows a linear fit to the whole data set of M 33 while the green dotted line shows a linear fit to N2 and S2 points of the outer galaxy only. The black dashed-dotted horizontal line shows the lower [C ii]/FIR value found in our data set of M 33. The blue dotted horizontal line shows the average [C ii]/FIR value in normal galaxies (Malhotra et al. 2001, M01). We also show data of Milky Way regions (Stacey et al. 1991, S91) and the low-metallicity objects LMC, SMC, IC 10 (Israel et al. 1996, I96), (Israel & Maloney 2011, I11), (Madden et al. 1997, M97).