Molecular and atomic gas in the Local Group galaxy M33

Understanding how stars form in the early universe requires calibrating the relationship between the star formation rate and the gas density or column density. One important step towards doing this is to study this ratio in nearby galaxies, and the Local Group galaxy M33 is an obvious target. In the paper by Gratier et al. highlighted in this issue, the authors present a detailed study of the HI and H2 (inferred from CO) distribution in M33 and use their results to infer a star formation efficiency (star formation rate divided by gas mass) 2-4 times greater than for large spiral galaxies.