M ar 1 99 8 The Resolved Stellar Population of the Post - Starburst Galaxy NGC 1569

We present WFPC2–HST photometry of the resolved stellar population in the post-starburst galaxy NGC 1569. The color-magnitude diagram (CMD) derived in the F439W and F555W photometric bands contains ∼2800 stars with photometric error ≤ 0.2 mag down to mF439, mF555 ≃ 26, and is complete for mF555 ∼ < 23. Adopting the literature distance modulus and reddening, our CMD samples stars more massive than ∼4 M⊙, allowing us to study the star formation (SF) history over the last ∼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 has experienced a global burst of star formation of duration ∼ > 0.1 Gyr, ending ∼ 5−10 Myr ago. During the burst, the SF rate was approximately constant, and, if quiescent periods occurred, they lasted less than ∼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 α = 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. Subject headings: galaxies: evolution — galaxies: individual: NGC 1569 — galaxies: irregular — galaxies: starburst — galaxies: stellar content