STELLAR POPULATIONS AND THE LARGE-SCALE STRUCTURE OF THE SMALL MAGELLANIC CLOUD .4. AGE DISTRIBUTION STUDIES OF THE OUTER REGIONS

The fourth in a series of papers on the large-scale structure and stellar content of the Small Magellanic Cloud (SMC), based on photometric analysis of sets of blue and red UK Schmidt photographic plates, is presented here. The previous papers reviewed the data reduction procedures and presented results on the three-dimensional structure of a large sector of the outer regions of the SMC beyond 2-degrees from the SMC centre. Here, we present a study of the age and spatial distribution of stellar populations in an area covering six UKST survey fields, i.e. virtually the entire outer area of the SMC. The existence of a very old stellar population of age 15-16 Gyr, comprising about 7 per cent of the stellar content of the outer regions by mass, is suggested by the observational results. However, the bulk of the stellar population in the outer regions of the SMC is found to be about 10 Gyr old. The distribution of stellar populations younger than about 2 Gyr is shown to be biased towards the eastern side of the SMC facing the LMC. The star formation history of the outer regions is discussed with particular reference to the role of interactions between the LMC-SMC-Galaxy system.