The HST Key Project on the Extragalactic Distance Scale. XV. A Cepheid Distance to the Fornax Cluster and Its Implications

摘要

Using the Hubble Space Telescope (HST) 37 long-period Cepheid variables havebeen discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting Vand I period-luminosity relations yield a true distance modulus of 31.35 +/-0.07 mag, which corresponds to a distance of 18.6 +/- 0.6 Mpc. This measurementprovides several routes for estimating the Hubble Constant. (1) Assuming thisdistance for the Fornax Cluster as a whole yields a local Hubble Constant of 70+/-18_{random} [+/-7]_{systematic} km/s/Mpc. (2) Nine Cepheid-based distancesto groups of galaxies out to and including the Fornax and Virgo clusters yieldHo = 73 (+/-16)_r [+/-7]_s km/s/Mpc. (3) Recalibrating the I-band Tully-Fisherrelation using NGC 1365 and six nearby spiral galaxies, and applying it to 15galaxy clusters out to 100 Mpc gives Ho = 76 (+/-3)_r [+/-8]_s km/s/Mpc. (4)Using a broad-based set of differential cluster distance moduli ranging fromFornax to Abell 2147 gives Ho = 72 (+/-)_r [+/-6]_s km/s/Mpc. And finally, (5)Assuming the NGC 1365 distance for the two additional Type Ia supernovae inFornax and adding them to the SnIa calibration (correcting for light curveshape) gives Ho = 67 (+/-6)_r [+/-7]_s km/s/Mpc out to a distance in excess of500 Mpc. All five of these Ho determinations agree to within their statisticalerrors. The resulting estimate of the Hubble Constant combining all thesedeterminations is Ho = 72 (+/-5)_r [+/-12]_s km/s/Mpc.