This work presents high spectral resolution observations of the CII line at 158 micron, one of the major cooling lines of the interstellar medium, taken with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691, an HII region far
north (3.3 kpc) in the disk of M 33, the CII and CO line profiles show similar velocities within $0.5 kms$, while the HI line velocities are systematically shifted towards lower rotation velocities by $sim 5kms$. Observed at the same $12$ angular resolution, the CII lines are broader than those of CO by about 50% but narrower than the HI lines. The CII line to far-infrared continuum ratio suggests a photoelectric heating efficiency of 1.1%. The data, together with published models indicate a UV field $G_0 sim 100$ in units of the solar neighborhood value, a gas density $n_H sim 1000 cc$, and a gas temperature $Tsim 200$ K. Adopting these values, we estimate the C$^+$ column density to be $N_{C^+} approx 1.3 times 10^{17} cmt$. The CII emission comes predominantly from the warm neutral region between the HII region and the cool molecular cloud behind it. From published abundances, the inferred C$^+$ column corresponds to a hydrogen column density of $N_H sim 2 times 10^{21} cmt$. The CO observations suggest that $N_H = 2 N_{H_2} sim 3.2 times 10^{21} cmt$ and 21cm measurements, also at $12$ resolution, yield $N_HI approx 1.2 times 10^{21} cmt$ within the CII velocity range. Thus, some H$_2$ not detected in CO must be present, in agreement with earlier findings based on the SPIRE 250 -- 500 $mu$m emission.
Does star formation proceed in the same way in large spirals such as the Milky Way and in smaller chemically younger galaxies? Earlier work suggests a more rapid transformation of H$_2$ into stars in these objects but (1) a doubt remains about the va
lidity of the H$_2$ mass estimates and (2) there is currently no explanation for why star formation should be more efficient. M~33, a local group spiral with a mass $sim 10$% and a metallicity half that of the Galaxy, represents a first step towards the metal poor Dwarf Galaxies. We have searched for molecular clouds in the outer disk of M~33 and present here a set of detections of both $^{12}$CO and $^{13}$CO, including the only detections (for both lines) beyond the R$_{25}$ radius in a subsolar metallicity galaxy. The spatial resolution enables mass estimates for the clouds and thus a measure of the $N({rm H}_2) / I_{rm CO}$ ratio, which in turn enables a more reliable calculation of the H$_2$ mass. Our estimate for the outer disk of M~33 is $N({rm H}_2) / I_{rm CO(1-0)} sim 5 times 10^{20} ,{rm cm^{-2}/(K{rm km s^{-1}})}$ with an estimated uncertainty of a factor $le 2$. While the $^{12/13}$CO line ratios do not provide a reliable measure of $N({rm H}_2) / I_{rm CO}$, the values we find are slightly greater than Galactic and corroborate a somewhat higher $N({rm H}_2) / I_{rm CO}$ value. Comparing the CO observations with other tracers of the interstellar medium, no reliable means of predicting where CO would be detected was identified. In particular, CO detections were often not directly on local HI or FIR or H$alpha$ peaks, although generally in regions with FIR emission and high HI column density. The results presented here provide support for the quicker transformation of H$_2$ into stars in M~33 than in large local universe spirals.
We present an analysis of the first space-based far-IR-submm observations of M 33, which measure the emission from the cool dust and resolve the giant molecular cloud complexes. With roughly half-solar abundances, M33 is a first step towards young lo
w-metallicity galaxies where the submm may be able to provide an alternative to CO mapping to measure their H$_2$ content. In this Letter, we measure the dust emission cross-section $sigma$ using SPIRE and recent CO and HI observations; a variation in $sigma$ is present from a near-solar neighborhood cross-section to about half-solar with the maximum being south of the nucleus. Calculating the total H column density from the measured dust temperature and cross-section, and then subtracting the HI column, yields a morphology similar to that observed in CO. The H$_2$/HI mass ratio decreases from about unity to well below 10% and is about 15% averaged over the optical disk. The single most important observation to reduce the potentially large systematic errors is to complete the CO mapping of M 33.
