STAR FORMATION IN DWARF IRREGULAR GALAXIES - SEXTANS B

A new method to study the star formation histories of nearby irregular galaxies is presented. We have obtained B, V, and R CCD photometry of 2434 objects down to V congruent-to 26 in two regions of the Local Group galaxy Sextans B. Major information on the stellar population, star formation rate, and initial mass function in the galaxy are derived from the comparison of the color-magnitude diagrams of the observed stars with synthetic diagrams generated by a numerical simulation code based on stellar evolutionary tracks. The observational luminosity function of all the objects and that of only main sequence stars are also compared with the corresponding functions derived from the synthetic diagrams. At our observational limit of reliability (B congruent-to 24), the two regions of Sextans B contain similar stellar populations with ages up to 1 Gyr or more, and metallicity Z = 0.001. The models in better agreement with the observational data suggest that the star formation has proceeded there in long (some 10(8) yr) quiet phases of moderate activity separated by short (not longer than 1 x 10(8) yr) quiescent intervals. Despite the low rates of star formation derived with our method, a continuous activity over the whole galaxy lifetime would provide results inconsistent with the observed values of the present metallicity, gas and total mass of the galaxy. Finally, by testing several initial mass functions, we find that the fraction of massive stars in Sextans B is not much larger than in normal spirals.