Old Stellar Populations of the Small Magellanic Cloud**Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal ID 6604.
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We present WFPC2 and ground-based VI photometry of NGC 121 and a nearby field in the outer SMC. For NGC 121, we measure a true distance modulus of 0 = 18.96 0.04 (distance of 61.9 1.1 kpc), age of 10.6 0.5 Gyr, metallicity of [Fe/H] = -1.03 0.06, and initial mass of 4.1 0.4 105 M, assuming a Salpeter IMF with lower cutoff at 0.1 M. In the outer SMC field, we find evidence of stars covering a wide range of ages - from 2 Gyr to at least 9-12 Gyr old. We have measured the distance, extinction, and star formation history (past star formation rates and enrichment history) using a CMD-fitting algorithm. The distance modulus of the SMC is measured to be 0 = 18.88 0.08, corresponding to a distance of 59.7 2.2 kpc. The overall star formation rate appears to have been relatively constant over this period, although there may be small gaps in the star-forming activity too small to be resolved. The lack of current star-forming activity is a selection effect, as the field was intentionally chosen to avoid recent activity. The mean metallicity 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 the past 3 Gyr. 2001. The American Astronomical Society. All rights reserved.
