Small Bites: star formation recipes in extreme dwarfs

We study the relationship between the gas column density (Sigma(HI)) and the star formation rate surface density (Sigma(SFR)) for a sample of extremely small (M-B similar to -13, Delta V-50 similar to 30 km s(-1)) dwarf irregular galaxies. We find a clear stochasticity in the relation between the gas column density and star formation. All gas with Sigma(HI) greater than or similar to 10 M-circle dot pc(-2) has some ongoing star formation, but the fraction of the gas with ongoing star formation decreases as the gas column density decreases and falls to about 50 per cent at Sigma(HI) similar to 3 M-circle dot pc(-2). Further, even for the most dense gas, the star formation efficiency is at least a factor of similar to 2 smaller than typical of star-forming regions in spirals. We also find that the ratio of H alpha emission to far-ultraviolet emission increases with the increasing gas column density. This is unlikely to be due to increasing dust extinction because the required dust-to-gas ratios are too high. We suggest instead that this correlation arises because massive (i.e. H alpha-producing) stars are formed preferentially in regions with high gas density.