The local group dwarf irregular galaxy NGC 6822 .3. The recent star formation history

We present a quantitative derivation of the recent star formation history (SFH) in different regions in the main body of NGC 6822. It has been performed using a set of model CM diagrams computed under different assumptions for the recent (similar or equal to 400 Myr) SFH and in which the crowding effects have been simulated using the particular crowding information for each region in the galaxy. The model CM diagrams have been compared with the observed CM diagram using some indicators that account basically for the distribution of stars inside the blue-plume. Also the number of upper AGB stars has been compared in the model and observed CM diagrams to check the goodness of the AGB modelling. The global picture obtained for the last 400 Myr of the SFH of NGC 6822 shows that an enhancement of the star formation activity over the whole main body of the galaxy has occurred in the last 100-200 Myr. The strength of this enhancement has been somewhat different from one region of the galaxy to another, being higher in the bar region than in the outer region and higher still in the northern and southern edges than in the central part of the bar. There is a good correlation between the regions in the galaxy that show a more pronounced enhancement of the recent star formation and the observed H I ridge. On the contrary, these regions not necessarily have an increased number of H II regions. This can be interpreted as a sign of propagation of the star formation across the galaxy, in such a way that star formation taking place in H II regions would produce, when averaged over space and time, the mean observed SFR. An alternate explanation can be that spatial changes of the upper mass limit or the slope of the IMF (or both) depending on environmental conditions (or even stochastically) have occurred. (C) 1996 American Astronomical Society.