We have used HST to measure Cepheid distances to 18 nearby spiral galaxies. Based on a new, larger sample of calibrating Cepheids in the Large Magellanic Cloud, an improved photometric calibration for the HST Wide Field and Planetary Camera 2, attention to incompleteness bias in the Cepheid period-luminosity relation, and a correction for Cepheid metallicity, we have presented here a set of self-consistent, revised Cepheid distances to 31 galaxies. The total sample includes previously-published ground-based photometry, and additional HST studies. The relative Cepheid distances are determined to ~ ± 5%.
Calibrating 5 secondary methods with these revised Cepheid distances, we find H0 = 72 ± 3 (random) ± 7 (systematic) km s-1 Mpc-1, or H0 = 72 ± 8 km s-1 Mpc-1, if we simply combine the total errors in quadrature. Type Ia supernovae currently extend out to the greatest distances, ~ 400 Mpc. All of the methods are in extremely good agreement: four of the methods yield a value of H0 between 70-72 km s-1 Mpc-1, and the fundamental plane gives H0 = 82 km s-1 Mpc-1. The largest remaining sources of error result from (a) uncertainties in the distance to the Large Magellanic Cloud, (b) photometric calibration of the HST Wide Field and Planetary Camera 2, (c) metallicity calibration of the Cepheid period-luminosity relation, and (d) cosmic scatter in the density (and therefore, velocity) field that could lead to observed variations in H0 on very large scales. A value of H0 = 72 km s-1 Mpc-1 yields an expansion age of ~ 13 Gyr for a flat universe (consistent with the recent cosmic microwave background anisotropy results) if m = 0.3, = 0.7. Combined with the current best estimates of the ages of globular clusters (~ 12.5 Gyr), our results favor a -dominated universe.
There are an enormous number of people who have contributed to and supported this project over the years. It is a pleasure to thank our collaborators Fabio Bresolin, Mingsheng Han, Paul Harding, Robert Hill, John Hoessel, Myung Gyoon Lee, Randy Phelps, Abhijit Saha, Kim Sebo, Nancy Silbermann, and Anne Turner for their contributions to the project. We fondly remember and acknowledge the contributions of Marc Aaronson who led our initial effort until his untimely death in 1987. Daya Rawson and Charles Prosser are also kindly remembered and missed. In particular we thank our HST program coordinator throughout our entire (complicated) observing process, Doug Van Orsow, and former STScI director, Robert Williams. We also thank Andrew Dolphin, Sandy Faber, Riccardo Giacconi, Riccardo Giovanelli, Jim Gunn, Martha Haynes, David Leckrone, Mark Phillips, Brian Schmidt, John Tonry, Brent Tully, and Ed Weiler. We further gratefully acknowledge the many years of assistance of the NASA and STScI support and technical staff. The work presented in this paper is based on observations with the NASA/ESA Hubble Space Telescope, obtained by the Space Telescope Science Institute, which is operated by AURA, Inc. under NASA contract No. 5-26555. Support for this work was provided by NASA through grant GO-2227-87A from STScI. This project could not have been completed in a timely fashion without the generous financial support that we received. SMGH and PBS are grateful to NATO for travel support via a Collaborative Research Grant (CGR960178). LF acknowledges support by NASA through Hubble Fellowship grant HF-01081.01-96A, and through grant number NRA-98-03-LTSA-03. SS acknowledges support from NASA through the Long Term Space Astrophysics Program, NAS-7-1260. BG acknowledges support from the NASA Long-Term Space Astrophysics Program (NAG5-7262), and the FUSE Science Team (NAS5-32985). LMM acknowledges partial support through Gemini Fellowship No. GF-1003-95. WLF acknowledges support from the NSF for the ground-based calibration in the early phases of this project under grants AST-87-13889 and AST-91-16496. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.