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We aim to better understand the heating of the gas by observing the prominentgas cooling line [CII] at 158um in the low-metallicity environment of the LocalGroup spiral galaxy M33 at scales of 280pc. In particular, we aim at describingthe variation of the photoelectric heating efficiency with galacticenvironment. In this unbiased study, we used ISO/LWS [CII] observations alongthe major axis of M33, in combination with Herschel PACS and SPIRE continuummaps, IRAM 30m CO 2-1 and VLA HI data to study the variation of velocityintegrated intensities. The ratio of [CII] emission over the far-infraredcontinuum is used as a proxy for the heating efficiency, and models ofphoton-dominated regions are used to study the local physical densities, FUVradiation fields, and average column densities of the molecular clouds. Theheating efficiency stays constant at 0.8% in the inner 4.5kpc radius of thegalaxy where it starts to increase to reach values of ~3% in the outskirts atabout 6kpc radial distance. The rise of efficiency is explained in theframework of PDR models by lowered volume densities and FUV fields, for opticalextinctions of only a few magnitudes at constant metallicity. In view of thesignificant fraction of HI emission stemming from PDRs, and for typicalpressures found in the Galactic cold neutral medium (CNM) traced by HIemission, the CNM contributes ~15% to the observed [CII] emission in the inner2kpc radius of M33. The CNM contribution remains largely undetermined in thesouth, while positions between 2 and 7.3kpc radial distance in the north of M33show a contribution of ~40%+-20%.

Gas and dust cooling along the major axis of M33 (HerM33es): ISO/LWS CII observations