Small bites: star formation recipes in extreme dwarfs

Abstract

We study the relationship between the gas column density (∑_HI) and the star formation rate surface density (∑_SSFR) for a sample of extremely small (M_B~−13, ΔV₅₀^~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 ∑_HI ≳ 10 M_⊙ 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 ∑_HI ~ 3 M_⊙ pc⁻². Further, even for the most dense gas, the star formation efficiency is at least a factor of ^~2 smaller than typical of star-forming regions in spirals. We also find that the ratio of H 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. Hα-producing) stars are formed preferentially in regions with high gas density.