Molecular cloud formation and the star formation efficiency in M 33

Molecule and star formation in M 33

¹ Laboratoire d'Astrophysique de Bordeaux, Université de Bordeaux, OASU, CNRS/INSU, 33271 Floirac, France e-mail: braine@obs.u-bordeaux1.fr
² IRAM, Avenida Divina Pastora, 7, Nucleo Central, 18012 Granada, Spain
³ IRAM, 300 Rue de la piscine, 38406 St Martin d'Hères, France
⁴ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 31 January 2010
Accepted: 2 July 2010

Abstract

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 H₂ into stars in these objects but (1) a doubt remains about the validity of the H₂ 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 ~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 ¹²CO and ¹³CO, including the only detections (for both lines) beyond the R₂₅ radius in a subsolar metallicity galaxy. The spatial resolution enables mass estimates for the clouds and thus a measure of the N(H₂)/I_CO ratio, which in turn enables a more reliable calculation of the H₂ mass. Our estimate for the outer disk of M 33 is N(H₂)/I_CO(1-0) ~ 5 × 10²⁰ cm^-2/(K km s^-1) with an estimated uncertainty of a factor ≤2. While the ^12/13CO line ratios do not provide a reliable measure of N(H₂)/I_CO, the values we find are slightly greater than Galactic and corroborate a somewhat higher N(H₂)/I_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 Hα peaks, although generally in regions with FIR emission and high HI column density. The results presented here provide support for the quicker transformation of H₂ into stars in M 33 than in large local universe spirals.