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Issue A&A
Volume 443, Number 3, December I 2005
Page(s) 883 - 889
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20042144



A&A 443, 883-889 (2005)
DOI: 10.1051/0004-6361:20042144

The extragalactic Cepheid bias: significant influence on the cosmic distance scale

G. Paturel1 and P. Teerikorpi2

1  CRAL-Observatoire de Lyon, 69561 Saint-Genis Laval Cedex, France
    e-mail: patu@obs.univ-lyon1.fr
2  Tuorla Observatory, Turku University, Väisäläntie 20, SF21500 Piikkiö, SF, Finland

(Received 8 October 2004 / Accepted 18 June 2005 )

Abstract
The unique measurements with the Hubble Space Telescope of Cepheid variable stars in nearby galaxies led to extragalactic distances that made the HST Key Project conclude that the Hubble constant is H0 = 72 km s-1 Mpc-1. The idea that H0 is now known is widely spread among the astronomical community. Some time ago, we suggested that a strong selection effect may still exist in the Cepheid method, resulting in too short distances. Using a model similar to traditional bias corrections, we deduce here new estimates of distances from HST and previous ground-based observations which are both affected by this effect, showing the same trend which starts at different distances. The recent measurement of M 83 with the VLT is unbiased. Revisiting the calibration of HSTKP's with our new scale, makes long-range distance criteria more concordant and reduces the value of H0 to $\approx$60 km s-1 Mpc-1. Locally, the corrected Cepheid distances give $H_{{\rm local}}=56$ km s-1 Mpc-1 and reduce the velocity dispersion in the Hubble flow. These numbers are indicative of the influence of the suggested Cepheid bias in the context of the HSTKP studies and are not final values.


Key words: stars: variables: Cepheids -- cosmology: distance scale

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© ESO 2005


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