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.