Old Stellar Populations of the Small Magellanic Cloud

Abstract

We present WFPC2 and ground-based VI photometry of NGC 121 and a nearbyfield in the outer SMC. For NGC 121, we measure a true distance modulusof {μ}_{0}=18.96+/-0.04 (distance of 61.9+/-1.1 kpc), age of10.6+/-0.5 Gyr, metallicity of [Fe/H]=-1.03+/-0.06, and initial mass of4.1+/-0.4{\times}10^{5} M_{solar}, assuming a SalpeterIMF with lower cutoff at 0.1 M_{solar}. In the outer SMC field,we find evidence of stars covering a wide range of ages-from 2 Gyr to atleast 9-12 Gyr old. We have measured the distance, extinction, and starformation history (past star formation rates and enrichment history)using a CMD-fitting algorithm. The distance modulus of the SMC ismeasured to be {μ}_{0}=18.88+/-0.08, corresponding to adistance of 59.7+/-2.2 kpc. The overall star formation rate appears tohave been relatively constant over this period, although there may besmall gaps in the star-forming activity too small to be resolved. Thelack of current star-forming activity is a selection effect, as thefield was intentionally chosen to avoid recent activity. The meanmetallicity of this field has increased from an average of[Fe/H]=-1.3+/-0.3 for stars older than 8 Gyr to [Fe/H]=-0.7+/-0.2 in thepast 3 Gyr. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal ID 6604.