STAR FORMATION IN DWARF IRREGULAR GALAXIES - DDO-210 AND NGC-3109

The method proposed in Paper I [Tosi et al, AJ, 102, 951 (1991)] to study the star formation histories in nearby irregulars is here applied to the galaxies DDO 210 and NGC 3109. To this purpose we have obtained deep CCD photometry of two regions in DDO 210 and three regions in NG-C 3109, reaching V congruent-to 24 with the required accuracy of sigma(V) less-than-or-equal-to 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. In general, for both galaxies 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. According to our models the metallicity should be fairly low (in the range 0.05 less-than-or-equal-to Z/Z. less-than-or-equal-to 0.5) and the initial mass function exponent in the vicinity of Salpeter's value, i.e., slightly flatter than in the solar neighborhood. The studied regions of NGC 3109 appear to contain different stellar populations (i.e., with slightly different metallicity and star formation histories), whereas those of DDO 210 seem to have fairly similar stellar contents, although for this galaxy the interpretation is more uncertain. As a by-product of our method we find that the distance modulus to DDO 210 should be (m-M)0 congruent-to 28, which places this galaxy beyond the edge of the Local Group.