THE FAR-FIELD HUBBLE CONSTANT

We used the Hubble Space Telescope to obtain surface brightness fluctu ation (SBF) observations of four nearby brightest cluster galaxies (BC Gs) to calibrate the BCG Hubble diagram of Lauer & Postman. This BCG H ubble diagram contains 114 galaxies covering the full celestial sphere and is volume-limited to 15,000 km s(-1), providing excellent samplin g of the far-field Hubble flow. The SBF zero point is based on the Cep heid calibration of the ground I-KC method as extended to the WFPC2 F8 14W filter by Ajhar and coworkers. The BCG globular cluster luminosity functions give distances essentially identical to the SBF results. Us ing the velocities and SBF distances of the four BCGs alone gives H-o = 82 +/- 8 km s(-1) Mpc(-1) in the CMB frame, valid on similar to 4500 km s(-1) scales; the error includes both systematic and random contri butions. Use of BCGs as photometric redshift estimators allows the BCG Hubble diagram to be calibrated independently of recession velocities of the four nearby BCGs, yielding a far-field H-o = 89 +/- 10 km s(-1 ) Mpc(-1) with an effective depth of similar to 11,000 km s(-1). The l arger error in this case is due to the photometric cosmic scatter in u sing BCGs as redshift estimators; this H-o is not significantly differ ent from the more local value. The concordance of the present results with other recent H-o determinations and a review of theoretical treat ments on perturbations in the near-field Hubble flow argue that going to the far field removes an important source of uncertainty, but that there is not a large systematic error to be corrected for to begin wit h. Further improvements in H-o depend more on understanding nearby cal ibrators than on improved sampling of the distant flow.