The HST Key Project on the Extragalactic Distance Scale
Session 93 -- HST
Oral presentation, Wednesday, 11, 1995, 2:00pm - 3:30pm

## [93.01] The HST Key Project on the Extragalactic Distance Scale

W. L. Freedman (Carnegie Observatories)

One of the major unresolved problems in observational cosmology is the determination of the Hubble Constant, (H$_0$). The Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale aims to provide a measure of H$_0$ to an accuracy of 10$\%$. Historically the route to H$_0$ has been plagued by systematic errors; hence there is no quick and easy route to a believeable value of H$_0$. Achieving plausible error limits of 10$\%$ requires careful attention to eliminating potential sources of systematic error.

The strategy adopted by the Key Project team is threefold: First, to discover Cepheids in spiral galaxies located in the field and in small groups that are suitable for the calibration of several independent secondary methods. Second, to make direct Cepheid measurements of 3 spiral galaxies in the Virgo cluster and 2 members of the Fornax cluster. Third, to provide a check on the the Cepheid distance scale via independent distance estimates to nearby galaxies, and in addition, to undertake an empirical test of the sensitivity of the zero point of the Cepheid PL relation to heavy-element abundances.

First results from the HST Key Project will be presented. We have now determined Cepheid distances to 4 galaxies using the HST: these are the nearby galaxies M81 and M101, the edge-on galaxy NGC 925, and the face-on spiral galaxy M100 in the Virgo cluster. Recently we have measured a Cepheid distance for M100 of 17 $\pm$ 2 Mpc, which yields a value of H$_0$ = 80 $\pm$ 17 km/sec/Mpc.

This work was carried out in collaboration with the other members of the HST Key Project team, R. Kennicutt, J. Mould, F. Bresolin, S. Faber, L. Ferrarese, H. Ford, J. Graham, J. Gunn, M. Han, P. Harding, J. Hoessel, R. Hill, J. Huchra, S. Hughes, G. Illingworth, D. Kelson, B. Madore, R. Phelps, A. Saha, N. Silbermann, P. Stetson, and A. Turner.