Stellar Populations in the Wing of the Small Magellanic Cloud fromHubble Space TelescopePhotometry

Abstract

We have used the Wide Field Planetary Camera 2 on the Hubble Space Telescope to image a star field in the wing of the Small Magellanic Cloud (SMC), near the H II region N81. The images were taken in the F336W, F547M, F675W, and F814W filters. From photometry of stars in this field, we construct color-magnitude diagrams (CMDs) for about 4200 stars and compare them with theoretical isochrones. In one CCD frame, we identify an open cluster with an approximate age between 400 and 560 Myr. After statistically subtracting the cluster stars, the remaining field star CMD shows both a strong upper main sequence and a well-developed red giant branch. The brightest main-sequence stars correspond to at most an age of 100 Myr. We also see a substantial number of turnoff stars with V magnitudes between 20 and 22, corresponding to an age range of 1.3 Gyr to at most 12 Gyr. We discuss the effects of the SMC's extended depth on the analysis. From a comparison of the observed CMD with Monte Carlo simulations, we find that the star formation history for this field is not consistent with a constant rate over the last 12 Gyr. We find that the CMD is consistent with increased star formation from 4–12 Gyr ago and over the past 1.7 Gyr. However, we find reduced star formation rates during the period 1.7–4 Gyr ago, in contrast to studies of star clusters that have suggested that the SMC experienced a burst of star formation 2 Gyr ago.