THE LOCAL GROUP DWARF IRREGULAR GALAXY NGC-6822 .2. THE OLD AND INTERMEDIATE-AGE STAR-FORMATION HISTORY

We present a study of the old and intermediate-age star formation hist ory (SFH) of the Local Group dwarf irregular galaxy (DIG) NGC 6822. It has been quantitatively obtained from the comparison of the observed [(V-I)(0), M(I)] CM diagram with a set of model CM diagrams, computed assuming different evolutionary scenarios. The comparison has been per formed through a number of indicators relative to the stars' distribut ion in two key structures of the CM diagram, the red-tail and the red- tangle. They contain most of the resolved stars of the galaxy which re present a major fraction of its SFH. The reliability of the model CM d iagrams computed is a result of three main components: (i) a reliable set of stellar evolutionary models covering the needed range of ages a nd metallicities, (ii) the interpolation of the stellar evolutionary t racks of fixed mass and metallicity to determine the precise, smooth d istribution of stars of any age, mass, and metallicity on the CM diagr am, and (iii) a realistic simulation of the observational effects on t he synthetic CM diagram. Significant constraints have been put on the SFH of NGC 6822. The main results reached are the following: (i) Star formation beginning later than 6 Gyr ago can be ruled out, Star format ion beginning at about 6 Gyr ago would only be possible if the galaxy started its early star formation from gas already enriched to about ha lf the current metallicity, or if a prompt metal enrichment occurred i n the galaxy. (ii) NGC 6822 has most likely begun forming stars at a v ery early epoch (about 15-12 Gyr ago), from low metallicity gas. (iii) An SFR close to constant or declining in the last few Gyr of the gala xy's lifetime seems best to reproduce the observations. Short-time and small-amplitude fluctuations of the SER may have occurred. This resul t solves a long lasting question about the age of the stars in the Baa de's sheet, confirming, at least for NGC 6822, the original idea of Ba ade (1963) that DIGs contain a considerable amount of old stars. (C) 1 996 American Astronomical Society.