The star field age-metallicity relationship of the Small Magellanic Cloud

Abstract

We present the results of the age and metallicity estimates for the unprecedented data base of some 3.3 million stars distributed throughout the entire Small Magellanic Cloud main body, obtained for the first time from CCD Washington CT1 photometry. We produce the first comprehensive star field age–metallicity relationship (AMR) from the birth of the galaxy until ∼1 Gyr ago, independent of any other previous approach. We find that the field stars do not possess gradients in age and metallicity, and that stellar populations formed since ∼2 Gyr ago are more metal rich than [Fe/H] ∼−0.8 dex and are confined to the innermost region (semi-major axis ≲ 1°). For the first time, we compare, homogeneously, the present star field AMR to that of the star cluster population with ages and metallicities in the same star field scales, and find that clusters and star fields share similar chemical evolution histories. Both galaxy components have experienced two enhanced formation processes: the most recent peaked at an age of ∼2 Gyr, and an earlier one detected at an age of ∼5–6 and 7.5 Gyr for clusters and star fields, respectively.