## Hubble constant from sosie galaxies and HIPPARCOS geometrical calibration
^{1} CRAL-Observatoire de Lyon, UMR 5574, F-69230 Saint-Genis
Laval, France^{2} Tuorla Observatory, FIN-21500 Piikki, Finland^{3} Observatoire de Paris-Meudon, URA 1757, F-92195 Meudon
Principal Cedex, France^{4} Osservatorio di Capodimonte, Via Moiariello 16, I-80131
Napoli, Italy^{5} Université Paris-Sud, F-91405 Orsay, France^{6} Université Claude-Bernard LyonI, F-69622
Villeurbanne Cedex, France
New distances, larger than previous ones, have been obtained for M 31 and M 81 based on the geometrical zero-point of the Cepheid Period-luminosity relation provided by the HIPPARCOS satellite. By combining them with independent determinations we define reasonable ranges for the distances of these important calibrating galaxies. On this basis, we determine the Hubble constant from the method of sosies (look-alike) galaxies, galaxies having the same characteristics than the calibrators. The method is quite secure because it is purely differential and it does not depend on any assumption (apart from the natural one that two sosies galaxies have similar absolute luminosities). Nevertheless, the Malmquist bias has to be taken into account. The observations behave exactly as predicted from the analytical formulation of the bias. Thus, rejecting galaxies which are affected by the Malmquist bias we derive the Hubble constant: If we strictly use the calibration obtained with HIPPARCOS and if the bias found in the Period-Luminosity Relation is considered, the Hubble constant is smaller than this (). This gives arguments in favour of the long-distance scale. We briefly discuss possible improvements aiming at still reducing the uncertainty.
## Contents- 1. Introduction
- 2. Discussion of the distance scale calibration
- 3. Overcoming secondary effects with sosies
- 4. Statistical bias
- 5. Application to M 31 and M 81 sosie galaxies
- 6. Conclusion
- Acknowledgements
- Appendix A: samples of M 31 and M 81 sosie galaxies brighter than BTc=12.5
- References
© European Southern Observatory (ESO) 1998 Online publication: October 22, 1998 |