The Far-Field Hubble Constant^*

We used the Hubble Space Telescope to obtain surface brightness fluctuation (SBF) observations of four nearby brightest cluster galaxies (BCGs) to calibrate the BCG Hubble diagram of Lauer & Postman. This BCG Hubble diagram contains 114 galaxies covering the full celestial sphere and is volume-limited to 15,000 km s^-1, providing excellent sampling of the far-field Hubble flow. The SBF zero point is based on the Cepheid calibration of the ground I_KC method as extended to the WFPC2 F814W filter by Ajhar and coworkers. The BCG globular cluster luminosity functions give distances essentially identical to the SBF results. Using the velocities and SBF distances of the four BCGs alone gives H₀ = 82 ± 8 km s^-1 Mpc^-1 in the CMB frame, valid on ~4500 km s^-1 scales; the error includes both systematic and random contributions. 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 H₀ = 89 ± 10 km s^-1 Mpc^-1 with an effective depth of ~11,000 km s^-1. The larger error in this case is due to the photometric cosmic scatter in using BCGs as redshift estimators; this H₀ is not significantly different from the more local value. The concordance of the present results with other recent H₀ determinations and a review of theoretical treatments 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 with. Further improvements in H₀ depend more on understanding nearby calibrators than on improved sampling of the distant flow.