Old Stellar Populations of the Small Magellanic Cloud
Abstract
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 μ0=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 Msolar, assuming a Salpeter IMF with lower cutoff at 0.1 Msolar. 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 μ0=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. 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2001
- DOI:
- 10.1086/323873
- arXiv:
- arXiv:astro-ph/0107164
- Bibcode:
- 2001ApJ...562..303D
- Keywords:
-
- Galaxies: Evolution;
- Galaxies: Stellar Content;
- Galaxies: Magellanic Clouds;
- Astrophysics
- E-Print:
- 29 pages, 14 figures accepted for publication in ApJ