The Hubble Space Telescope key project on the extragalactic distance scale. XV. A Cepheid distance to the Fornax cluster and its implications

Using the Hubble Space Telescope, 37 long-period Cepheid variables have bee n discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V and I period-luminosity relations yield a true distance modulus of mu(0) = 31.35 +/- 0.07 mag, which corresponds to a distance of 18.6 +/- 0.6 Mpc. Th is measurement provides several routes for estimating the Hubble constant. (1) Assuming this distance for the Fornax Cluster as a whole yields a local Hubble constant of 70 +/- 18 (random) +/- 7 (systematic) km s(-1) Mpc(-1). (2) Nine Cepheid-based distances to groups of galaxies out to and includin g the Fornax and Virgo Clusters yield H-0 = 73 +/- 16 (random) +/- 7 (syste matic) km s(-1) Mpc(-1). (3) Recalibrating the I-band Tully-Fisher relation using NGC 1365 and six nearby spiral galaxies, and applying it to 15 galax y clusters out to 100 Mpc, give H-0 = 76 +/- 3 (random) is (systematic) km s(-1) Mpc(-1). (4) Using a broad-based set of differential cluster distance moduli ranging from Fornax to Abell 2147 gives H-0 = 72 +/- 3 (random) +/- 6(systematic) km s(-1) Mpc(-1). Finally, (5) assuming the NGC 1365 distanc e for the two additional Type Ia supernovae in Fornax; and adding them to t he SN Ia calibration (correcting for light-curve shape) gives H-0 = 67 +/- 6 (random) +/- 7 (systematic) km s(-1) Mpc(-1) out to a distance in excess of 500 Mpc. All five of these H-0 determinations agree to within their stat istical errors. The resulting estimate of the Hubble constant, combining al l of these determinations, is H-0 = 72 +/- 5 (random) +/- 7 (systematic) km s(-1) Mpc(-1). An extensive tabulation of identified systematic and statis tical errors, and their propagation, is given.