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A. E. Dolphin (KPNO/NOAO)
The Galaxy's dwarf spheroidal neighbors provide an ideal test case for the study of star formation histories -- they are sufficiently close (m-M \approx 20-21) that even shallow \textit{HST} observations easily reach the ancient main sequence turnoff, and their star formation histories are far simpler than those of irregular or spiral galaxies such as the Magellanic Clouds, IC 1613, or the Galaxy. We also see evidence (most notably the Sagittarius dwarf) that these objects are consumed by the Galaxy, and thus we can implicitly study the formation of a fraction of the Galactic halo by studying these objects. Despite the relative ease of study, the wide variety of recent results (for example, Draco has been claimed to be anywhere from 6-7 Gyr old to older than M68 and M92, based on the same WFPC2 observations) indicates that our photometry and analysis techniques are far from ideal. Additionally, an adequate comprehensive and homogeneous study of our dwarf spheroidal neighbors has yet to be done.
I attempt to address these issues, using new reductions of archival WFPC2 observations of the majority of the Galaxy's dwarf spheroidal neighbors. The photometry is made with HSTphot, a stellar photometry package designed to work despite the undersampled nature of WFPC2 images. This package allows for a uniform, efficient reduction of the data, as well as artificial star test results that allow for the accurate reproduction of the photometry during the star formation history analysis. Finally, I apply a maximum likelihood technique to measure the distance, extinction, and star formation rate (a function of age and metallicity) for each galaxy in the sample. The advantage of this technique is that it provides independent measures of these quantities and avoids the oversimplification of assuming a constant metallicity that has plagued past studies.
Support for this work is provided by NASA through grants GO-07496 and GO-02227.06-A from the Space Telescope Science Institute.
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The author(s) of this abstract have provided an email address for comments about the abstract: dolphin@noao.edu