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Session 108 - Cosmology and Distance Indicators.
Oral session, Thursday, January 16
Frontenac Ballroom,
The Hubble Space Telescope (HST) has been used successfully to discover Cepheid variables in the Fornax cluster spiral galaxy, NGC 1365. A sample of over 50 Cepheids with periods between 12 and 60 days has discovered. The resulting PL relations at V and I yield a preliminary reddening-corrected distance to this galaxy of about 18.4 Mpc with an uncertainty of about 10% in distance. Associating this distance with the Fornax cluster as a whole, and adopting a mean recessional velocity of 1,380 \pm88 km/sec (corrected for Virgocentric flow) gives a local Hubble constant of H_o = 75 km/sec/Mpc. The random error is 6%; while the largest systematic uncertainty is currently the unknown motion of the Fornax cluster with respect to the cosmic microwave background.
We present six Cepheid-based distances to groups of galaxies out to and including the Virgo and Fornax clusters. In addition, the Tully-Fisher relation is recalibrated using NGC 1365 and 6 nearby spiral galaxies, and applied to 15 clusters out to 100 Mpc. A broad-based set of differential moduli established from Fornax out nearly a factor of ten in distance further, to Abell 2147 by Jerjen and Tammann is also recalibrated. The resulting value of the Hubble constant, encompassing all of these determinations, based on Cepheids and tied to secondary distance indicators out to cosmologically significant distances, is found to be 73 \pm 6 (random) \pm 8 (systematic) km/sec/Mpc.
