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Tom Shanks (University of Durham, England)
We make a direct test of Tully-Fisher distance estimates to twelve spiral galaxies with HST Cepheid distances and to twelve spiral galaxies with SNIa distances. The HST Cepheid distances come from the work of Freedman et al (1997), Sandage et al (1996) and Tanvir et al (1995). The SNIa distances come from Pierce (1994), calibrated using the Cepheid results of Sandage et al (1996). The Tully-Fisher distances mostly come from the work of Pierce (1994). The results show that the Tully-Fisher distance moduli are too short with respect to the Cepheid distances by 0.46±0.14mag and too short with respect to the SNIa distances by 0.46±0.19mag. Combining the HST Cepheid and SNIa data suggests that, overall, previous Tully-Fisher distances were too short by 0.46±0.11mag, a result which is significant at the 4\sigma level. These data therefore indicate that previous Tully-Fisher distances should be revised upwards by 24±6 distance of 19.3±1.9Mpc. The value of Ho from Tully-Fisher estimates is correspondingly revised downwards from Ho=84±10 to Ho=68±8. Further downward revisions of Ho are possible if it proves that it is Malmquist bias in the TF distance estimates that is causing this discrepancy.