The resolved stellar population of the poststarburst galaxy NGC 1569

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) photometry of the resolved stellar population in the poststarburst galaxy NGC 1569. The color-magnitude diagram (CMD) derived in the F439W and F555W photometric bands contains similar to 2800 stars with a photometric error of less than or equal to 0.2 mag down to m(F439), m(F555) similar or equal to 26 and is complete for m(F555) less than or similar to 23. Adopting the literature-distance modulus and reddening, our CMD samples stars more massive than similar to 4 M., allowing us to study the star formation (SF) history over the last similar to 0.15 Gyr. The data are interpreted using theoretical simulations based on stellar evolutionary models. The synthetic diagrams include photometric errors and incompleteness factors. Testing various sets of tracks, we find that the ability of the models to reproduce the observed features in the CMD is strictly related to the shape of the blue loops of the sequences with masses around 5 M.. The field of NGC 1569 experienced a global SF burst of greater than or similar to 0.1 Gyr duration, ending similar to 5-10 Myr ago. During the burst, the SF rate was approximately constant, and, if quiescent periods occurred, they lasted less than similar to 10 Myr. The level of the SF rate was very high; for a single-slope initial mass function (IMF) ranging from 0.1 to 120 M., we find values of 3, 1, and 0.5 M. yr(-1) for alpha = 3, 2.6, and 2.35 (Salpeter), respectively. When scaled for the surveyed area, these rates are approximately 100 times larger than found in the most active dwarf irregulars in the Local Group. The data are consistent with a Salpeter IMF, though our best models indicate slightly steeper exponents. We discuss the implications of our results in the general context of the evolution of dwarf galaxies.