FINAL RESULTS FROM THE HUBBLE SPACE TELESCOPE KEY PROJECT TO MEASURE THE HUBBLE CONSTANT ¹

Wendy L. Freedman ², Barry F. Madore ^2,3, Brad K. Gibson ⁴, Laura Ferrarese ⁵, Daniel D. Kelson ⁶, Shoko Sakai ⁷, Jeremy R. Mould ⁸, Robert C. Kennicutt, Jr. ⁹, Holland C. Ford ¹⁰, John A. Graham ⁶, John P. Huchra ¹¹, Shaun M.G. Hughes ¹², Garth D. Illingworth ¹³, Lucas M. Macri ¹¹ and Peter B. Stetson ¹⁴,¹⁵

¹ Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA Contract No. NAS 5-26555.
² The Observatories, Carnegie Institution of Washington, Pasadena, CA, USA 91101
³ NASA/IPAC Extragalactic Database, California Institute of Technology, Pasadena, CA, USA 91125
⁴ Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
⁵ Rutgers University, New Brunswick, NJ, 08854
⁶ Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Rd. N.W., Washington, D.C., USA 20015
⁷ National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ, USA 85726
⁸ Research School of Astronomy & Astrophysics, Australian National University, Weston Creek Post Office, Weston, ACT, Australia 2611
⁹ Steward Observatory, University of Arizona, Tucson, AZ, USA 85721
¹⁰ Department of Physics & Astronomy, Bloomberg 501, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD, USA 21218
¹¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA, USA 02138
¹² Institute of Astronomy, Madingley Road., Cambridge, UK CB3~0HA
¹³ Lick Observatory, University of California, Santa Cruz, CA, USA 95064
¹⁴ Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Rd., Victoria, BC, Canada V8X 4M6
¹⁵ Guest User, Canadian Astronomy Data Centre, which is operated by the Herzberg Institute of Astrophysics, National Research Council of Canada.

Abstract. We present here the final results of the Hubble Space Telescope (HST) Key Project to measure the Hubble constant. We summarize our method, the results and the uncertainties, tabulate our revised distances, and give the implications of these results for cosmology. Our results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60 to 400 Mpc. The analysis presented here benefits from a number of recent improvements and refinements, including (1) a larger LMC Cepheid sample to define the fiducial period--luminosity (PL) relations, (2) a more recent HST Wide Field and Planetary Camera 2 (WFPC2) photometric calibration, (3) a correction for Cepheid metallicity, and (4) a correction for incompleteness bias in the observed Cepheid PL samples. We adopt a distance modulus to the LMC (relative to which the more distant galaxies are measured) of µ₀(LMC) = 18.50 ± 0.10 mag, or 50 kpc. New, revised distances are given for the 18 spiral galaxies for which Cepheids have been discovered as part of the Key Project, as well as for 13 additional galaxies with published Cepheid data. The new calibration results in a Cepheid distance to NGC 4258 in better agreement with the maser distance to this galaxy. Based on these revised Cepheid distances, we find values (in km/sec/Mpc) of H₀ = 71 ± 2_r (random) ± 6_s (systematic) (type Ia supernovae), H₀ = 71 ±3_r ± 7_s (Tully-Fisher relation), H₀ = 70 ± 5_r ± 6_s (surface brightness fluctuations), H₀ = 72 ± 9_r ± 7_s (type II supernovae), and 82 ± 6_r ± 9_s (fundamental plane). We combine these results for the different methods with 3 different weighting schemes, and find good agreement and consistency with H₀ = 72 ± 8 km s^-1 Mpc^-1. Finally, we compare these results with other, global methods for measuring H₀.