PACS spectra of [C II] (blue) and [O I](63 μm) (red) at selected positions in M 33. All spectra are shown at their native angular resolution, before convolution to 12″ resolution. The velocities have been corrected for the systemic velocity of M 33 of −180 km s⁻¹. Dashed, vertical lines mark the residual velocity of the H I line (Warner et al. 1973). Spectra are shown after subtraction of polynomial baselines of up to 3rd order. The two red (blue) fields mark the velocity range, which was used to calculate the baseline rms for the [O I] ([C II]) line. Baselines at more extreme velocities are not shown. [C II] integrated intensities were calculated over the inner range between the blue fields (±538 km s⁻¹), and the integrated intensities for the [O I] line were calculated over the spectral range (in white) between the red fields. The double-peak structure of the weak [O I] line seen in the right spectrum of the lower row is attributed to baseline noise.