The globular cluster systems in the Coma ellipticals. I. The luminosity function in NGC 4874, and implications for Hubble's constant

We have used deep Hubble Space Telescope WFPC2 images in V (F606W) and I (F 814W) to measure the luminosity distribution of the globular clusters in NG C 4874, the central cD galaxy of the Coma cluster. We find the "turnover" p oint of the globular cluster luminosity function (GCLF) to lie at V = 27.88 +/- 0.12, while the overall GCLF shape matches the standard Gaussian-like form with dispersion sigma(V) = 1.49 +/- 0.12. We use the GCLF as a standar d candle by matching the turnover points in NGC 4874 and another Coma ellip tical, IC 4051, with those of the giant ellipticals in the Virgo cluster (M 87 and five others). The result is Delta(m - M) (Coma-Virgo)= 4.06 +/- 0.11 mag, which converts to a Coma distance d = 102 Mpc if the Virgo distance m odulus is (m - M)(0) = 30.99 +/- 0.04. The Hubble constant which emerges fr om our GCLF measurement is then H-0 = (69 +/- 9) km s(-1) Mpc(-1). We confi rm this H-0 value with a novel presentation of the "Hubble diagram" for GCL Fs in giant E galaxies. Measurements of additional GCLFs in the Coma ellipt icals, as well as calibrating galaxies in Virgo and Fornax, have excellent potential to refine this result in the near future.