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

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  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 ≈60 km s^-1 Mpc^-1. Locally, the corrected Cepheid distances give  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.