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The spectacular success of the Hubble Space Telescope repair mission now makes it possible to realize one of the astrophysical goals for which HST was designed. Recent progress in groundbased observations of galaxy distances has reduced the longstanding factor-of-two uncertainty in the Hubble constant, but lingering inconsistencies between different methods and concerns over the accuracy of the distance scale remain. The incompatibility of high values of H$_0$ with the stellar age scale in an $\Omega$=1, $\Lambda$=0 universe underscores the need for a definitive calibration of the distance scale.
The objective of the HST Extragalactic Distance Scale Key Project is to calibrate H$_0$ to an accuracy of $\pm$10\%. This will be accomplished by measuring Cepheid distances to $\sim$20 galaxies with redshift velocities of $\le$2000 km/s. These galaxies will serve as local benchmarks which can be used to calibrate a wide range of secondary distance indicators in the future. Within the Key Project the data will be used to recalibrate 5 secondary distance indicators, and these will be applied in turn to extend the local distance scale into the region of Hubble flow, and provide a global measurement of H$_0$. Cepheid distances to galaxies in the cores of the Virgo and Fornax clusters will provide another, direct estimate of H$_0$, independent of the secondary indicators. Intercomparison of these 6 distance scales will provide a robust measurement of H$_0$ and a direct determination of its uncertainty. The sample selection, observing and data analysis procedures, and error budget are conservatively designed to minimize susceptibility to the kinds of systematic errors which have frequently plagued distance scale measurements. The recent identification of 20 Cepheids in the Virgo cluster spiral M100 confirms the feasibility of the project, and offers a spectacular demonstration of HST's unprecedented capabilities for this application.
This talk will briefly review the current status of the distance scale problem and describe the ongoing HST programs, with emphasis on the Key Project. Examples of recently obtained data will be shown, and distances determined the first galaxies in the program, including M81, M101, and M100 will be discussed. The ultimate goal of measuring H$_0$ to $\pm$10\% awaits successful completion of the program over the next 2 years, but our M100 distance (17.1$\pm$1.8 Mpc) already constrains its value to the range H$_0 = 80 \pm 17$\ km/s/Mpc. The implications of this result will be discussed.
This work was carried out in collaboration with the other members of the HST Key Project team, W. Freedman, J. Mould, F. Bresolin, S. Faber, L. Ferrarese, H. Ford, J. Graham, J. Gunn, M. Han, P. Harding, R. Hill, J. Hoessel, J. Huchra, S. Hughes, G. Illingworth, D. Kelson, B. Madore, R. Phelps, A. Saha, N. Silbermann, P. Stetson, and A. Turner.
