STAR-FORMATION IN DWARF IRREGULAR GALAXIES - NGC-6822

Our proposed method to study the star-formation histories in nearby irregulars is here applied to NGC 6822. To this purpose we have obtained accurate CCD photometry of three regions in the galaxy, reaching V≃23.5 with the required accuracy of σBVR≤0.1 mag. Major information on the stellar populations, star-formation rates, and initial mass functions of these regions in the last 1 Gyr are derived from the comparison of the observational color-magnitude diagrams and luminosity functions with the corresponding synthetic diagrams and luminosity functions generated by a numerical simulation code based on stellar evolutionary tracks. We find that in the last 1 Gyr the star-formation activity has been rather continuous, possibly occurring in long episodes of moderate activity separated by short quiescent periods, and that the initial mass function exponent α is in the vicinity of Salpeter's value, i.e., slightly flatter than in the solar neighborhood. The studied regions of NGC 6822 appear to contain different stellar populations. The preferred star formation regime for region A is a continuous activity coupled with a moderately flat a. For region C, instead, our best models suggest a discontinuous star formation and a moderately steep a. NGC 6822 is the last object of our original sample of five nearby irregulars. We can then draw the scenario resulting from this sample: most of the observed fields appear to have experienced a gasping regime of star formation, with average exponent α=2.35 for the initial mass function. Despite their small sizes, the majority of the sample galaxies contain different stellar populations in different regions.