2007A&A...473...91G

The Star Formation (SF) rate in galaxies is an important parameter at all redshifts and evolutionary stages of galaxies. In order to understand the increased SF rates in intermediate redshift galaxies one possibility is to study star formation in local galaxies with properties frequently found at this earlier epoch like low metallicity and small size. We present sensitive observations of the molecular gas in M33, a small Local Group spiral at a distance of 840kpc which shares many of the characteristics of the intermediate redshift galaxies. The observations were carried out in the CO(2-1) line with the HERA heterodyne array on the IRAM 30m telescope. A 11'x22' region in the northern part of M33 was observed, reaching a detection threshold of a few 10³M_☉. The correlation in this field between the CO emission and tracers of SF (8µm, 24µm, H_α, FUV) is excellent and CO is detected very far North, showing that molecular gas forms far out in the disk even in a small spiral with a subsolar metallicity. One major molecular cloud was discovered in an interarm region with no HI peak and little if any signs of SF - without a complete survey this cloud would never have been found. The radial dependence of the CO emission has a scale length similar to the dust emission, less extended than the H_α or FUV. If, however, the N(H₂)/ I_CO ratio varies inversely with metallicity, then the scale length of the H₂ becomes similar to that of the H_α or FUV. Comparing the SF rate to the H₂ mass shows that M33, like the intermediate redshift galaxies it resembles, has a significantly higher SF efficiency than large local universe spirals. The data presented here also provide an ideal test for theories of molecular cloud formation and cover a new region in parameter space, where Σ_stars<Σ_gas. We find that a simple pressure-based prescription for estimating the molecular to atomic gas fraction does not perform well for M33, at least in the outer parts. On the other hand, we show that the molecular gas fraction is influenced by (i) the total Hydrogen column density, dominated in M33 by the HI, and (ii) the galactocentric distance.