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 been discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V and I period-luminosity relations yield a true distance modulus of μ0=31.35±0.07 mag, which corresponds to a distance of 18.6±0.6 Mpc. This 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 including the Fornax and Virgo Clusters yield H0=73±16 (random) ±7 (systematic) 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 galaxy clusters out to 100 Mpc, give H0=76±3 (random) ±8 (systematic) km s−1 Mpc−1. (4) Using a broad-based set of differential cluster distance moduli ranging from Fornax to Abell 2147 gives H0=72±3 (random) ±6 (systematic) km s−1 Mpc−1. Finally, (5) assuming the NGC 1365 distance for the two additional Type Ia supernovae in Fornax and adding them to the SN Ia calibration (correcting for light-curve shape) gives H0=67±6 (random) ±7 (systematic) km s−1 Mpc−1 out to a distance in excess of 500 Mpc. All five of these H0 determinations agree to within their statistical errors. The resulting estimate of the Hubble constant, combining all of these determinations, is H0=72±5 (random) ±7 (systematic) km s−1 Mpc−1. An extensive tabulation of identified systematic and statistical errors, and their propagation, is given.