Particularly efficient star formation in M 33

¹ Université Bordeaux 1; CNRS; Laboratoire d'Astrophysique, Observatoire de Bordeaux, OASU; UMR 5804, 33270 Floirac, France e-mail: gardan@obs.u-bordeaux1.fr
² IRAM, 300 rue de la piscine, 38406 Saint-Martin-d'Hères, France

Received: 24 April 2007
Accepted: 6 July 2007

Abstract

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 M 33, a small Local Group spiral at a distance of 840 kpc 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 30 m telescope. A region in the northern part of M 33 was observed, reaching a detection threshold of a few 10³ . The correlation in this field between the CO emission and tracers of SF (8 μm, 24 μm, 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 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 M 33, 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 . We find that a simple pressure-based prescription for estimating the molecular to atomic gas fraction does not perform well for M 33, 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 M 33 by the HI, and (ii) the galactocentric distance.