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Keywords:

  • galaxies: dwarf;
  • galaxies: irregular;
  • galaxies: star formation;
  • radio lines: galaxies

ABSTRACT

We study the relationship between the gas column density (inline image) and the star formation rate surface density (ΣSFR) for a sample of extremely small (MB∼−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 inline image 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 inline image 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 Hα 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.