Central part of the PACS maps of M 33. From left to right: intensities of total infrared continuum (TIR), [C II], [O I](63 μm), [C II]/TIR, and [C II]/[O I] intensity ratios. Intensities are in units of erg s⁻¹ cm⁻² sr⁻¹. Offsets are in arcminutes relative to the nucleus, after de-rotation of the position angle by 22.5°. The TIR contour is at 2 × 10⁻³ erg s⁻¹ cm⁻² sr⁻¹, the threshold intensity used in the correlation plots to distinguish TIR-bright and TIR-weak regions (Figs. 4–6, 8, 5). The color wedge is the same as in Fig. 1. [C II] contours are the same as in Fig. 1. [O I] contours are 1.4, 3.0, 6.3, 13.8 in units of 10⁻⁶ erg s⁻¹ cm⁻² sr⁻¹. The map of [C II]/TIR is overlayed with contours at the 0.6% and 1% level of the ratio. The map of [C II]/[O I] is overlayed with the contour of 3.5. Long tic marks in the color wedges indicate contour levels.