The Hubble space telescope key project on the extragalactic distance scale. XXVI. The calibration of population II secondary distance indicators and the value of the Hubble constant
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A Cepheid-based calibration is derived for four distance indicators that utilize stars in old stellar populations: the tip of the red giant branch (TRGB), the planetary nebula luminosity function (PNLF), the globular cluster luminosity function (GCLF), and the surface brightness fluctuation method (SBF). The calibration is largely based on the Cepheid distances to 18 spiral galaxies within cz = 1500 km s-1 obtained as part of the Hubble Space Telescope (HST) Key Project on the Extragalactic Distance Scale, but relies also on Cepheid distances from separate HST and ground-based efforts. The newly derived calibration of the SBF method is applied to obtain distances to four Abell clusters in the velocity range 3800-5000 km s-1. Combined with cluster velocities corrected for a cosmological flow model, these distances imply a value of the Hubble constant of H0 = 69 4 (random) 6 (systematic) km s-1 Mpc-1. This result assumes that the Cepheid PL relation is independent of the metallicity of the variable stars; adopting a metallicity correction as in Kennicutt et al. would produce a 5% 3% decrease in H0. Finally, the newly derived calibration allows us to investigate systematics in the Cepheid, PNLF, SBF, GCLF, and TRGB distance scales.
