2011ApJ...739...74C

The evolutionary properties and spatial distribution of I Zwicky 18 (IZw18) stellar populations are analyzed by means of Hubble Space Telescope/Advanced Camera for Surveys deep and accurate photometry. A comparison of the resulting color-magnitude diagrams (CMDs) with stellar evolution models indicates that stars of all ages are present in all the system's components, including objects possibly up to 13 Gyr old, intermediate-age stars, and very young ones. The CMDs show evidence of thermally pulsing asymptotic giant branch and carbon stars. classical and ultra-long-period Cepheids as well as long-period variables have been measured. About 20 objects could be unresolved star clusters; these are mostly concentrated in the northwest (NW) portion of the main body (MB). If interpreted with simple stellar population models, these objects indicate a particularly active star formation over the past 100 Myr in IZw18. The stellar spatial distribution shows that the younger ones are more centrally concentrated, while old and intermediate-age stars are distributed homogeneously over the two bodies, although they are more easily detectable at the system's periphery. The oldest stars are most visible in the secondary body (SB) and in the southeast (SE) portion of the MB, where crowding is less severe, but are also present in the rest of the MB, where they are measured with larger uncertainties. The youngest stars are a few Myr old, are located predominantly in the MB, and are mostly concentrated in its NW portion. The SE portion of the MB appears to be in a similar, but not as young, evolutionary stage as the NW, while the SB stars are older than at least 10 Myr. There is then a sequence of decreasing age of the younger stars from the SB to the SE portion of the MB to the NW portion. All our results suggest that IZw18 is not atypical compared to other blue compact dwarfs.