Spatial distribution and evolution of the stellar populations and
  candidate star clusters in the Blue Compact Dwarf I Zwicky 18
  The evolutionary properties and spatial distribution of I Zwicky 18 stellar
populations are analyzed by means of HST/ACS deep and accurate photometry. The
comparison of the resulting Colour- Magnitude diagrams with stellar evolution
models indicates that stars of all ages are present in all the system
components, including ob jects possibly up to 13 Gyr old, intermediate age
stars and very young ones. The Colour-Magnitude diagrams 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 ob jects could be unresolved star clusters, and are mostly
concentrated in the North-West (NW) portion of the Main Body (MB). If
interpreted with simple stellar population models, these ob jects indicate a
particularly active star formation over the past hundred Myr in IZw 18. 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 more easily detectable at the
system periphery. The oldest stars are best visible in the Secondary Body (SB)
and in the South East (SE) portion of the MB, where crowding is less severe,
but are present also in the rest of the MB, although measured with larger
uncertainties. The youngest stars are a few Myr old, are located predominantly
in the MB and 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 Secondary Body to the SE portion
of the MB to the NW portion. All our results suggest that IZw18 is not atypical
compared to other BCDs.
