The ionized carbon emission from M33: results from the Herschel/HIFI Key Program HerM33es
(J. Braine,) C. Kramer, P. Gratier, B. Mookerjea
Messier 33 is a spiral galaxy within the Local Group, close enough to resolve individual Giant Molecular clouds and quite similar to the Milky Way but smaller and with a lower metallicity. The Herschel Key project HerM33es aims to understand the cycle of star formation from the atomic gas to the denser molecular phase and finally into stars in a low metallicity environment, not available in our galaxy. In addition to complete imaging in the PACS and SPIRE bands, HerM33es provides ionized carbon [CII] spectra at high spectral resolution with the HIFI instrument all along the major axis in order to determine its origin. We present line profiles of the [CII] emission; comparison with CO and HI profiles at the same angular resolution clearly link the [CII] emission to the molecular component. The difference between [CII] and CO velocities is generally less than 0.5 km s−1 but much greater for the HI. The position-velocity diagrams show similar space-velocity structure between [CII] and CO but not [CII] and HI. The extended warm ionized gas is not detectable at these scales via the [CII] line. The intensity ratios of [CII] with CO and far-infrared emission are typical of low-metallicity sources as found in the Large Magellanic Cloud (LMC). However, a count of carbon atoms yields much less CO-dark molecular gas than in the LMC although some H2 appears to be present above what is accounted for via CO. The intensity ratios and the line profiles indicate that the main contributor to [CII] emission is the photodissociation region at the interface between the HII region and the molecular cloud.