What Regulates Star Formation on Global and Local Scales in Irregular Galaxies?
Or, to put it another way, what distinguishes an Im galaxy with a high star formation rate, but not a starburst, from a galaxy with a low rate. Detailed studies of the stellar populations of irregulars have shown that most irregular galaxies evolve at more or less constant star formation rates with variations of factors of 2-3 in the amplitude of the rate at least over the past few Gyrs (Tolstoy 1996, Tosi et al. 1991, Greggio et al. 1993, Marconi et al. 1995). Therefore, galaxies with relatively high rates today have most likely had relatively high rates for most of their lifetime, and similarly for those forming stars at low rates (but this remains controversial). Plots of star formation rates against other global properties do not yield correlations except for those properties that would be expected to be a consequence of the level of star formation. An example of the latter is surface brightness; the higher the star formation rate per unit area, the higher the surface brightness wo
