Morphology of star formation regions in irregular galaxies

The location of H rr regions, which indicates the locus of present star formation in galaxies, is analysed for a large collection of 110 irregular galaxies (Irr) imaged in Ha! and nearby continuum. The analysis is primarily by visual inspection, although a two-dimensional quantitative measure is also employed. The two different analyses yield essentially identical results. Hn regions appear preferentially at the edges of the light distribution, predominantly on one side of the galaxy, contrary to what is expected from stochastic self-propagating star formation scenarios. This peculiar distribution of star-forming regions cannot be explained by a scenario of star formation triggered by an interaction with extragalactic gas, or by a strong one-armed spiral pattern.