We have used deep HST/WFPC2 images in V ( F606W ) and I ( F814W ) to measure the luminosity distribution of the globular clusters in NGC 4874 , the central cD galaxy of the Coma cluster .
We find the “ turnover ” point of the globular cluster luminosity function ( GCLF ) to lie at V = 27.88 \pm 0.12 , while the overall GCLF shape matches the standard Gaussian-like form with dispersion \sigma _ { V } = 1.49 \pm 0.12 .
We use the GCLF as a standard candle by matching the turnover points in NGC 4874 and another Coma elliptical , IC 4051 , with those of the giant ellipticals in the Virgo cluster ( M87 and five others ) .
The result is \Delta ( m - M ) ( Coma - Virgo ) = 4.06 \pm 0.11 magnitudes , which converts to a Coma distance d = 102 Mpc if the Virgo distance modulus is ( m - M ) _ { 0 } = 30.99 \pm 0.04 .
The Hubble constant which emerges from our GCLF measurement is then H _ { 0 } = ( 69 \pm 9 ) km s ^ { -1 } Mpc ^ { -1 } .
We confirm this H _ { 0 } value with a novel presentation of the “ Hubble diagram ” for GCLFs in giant E galaxies .
Measurements of additional GCLFs in the Coma ellipticals , as well as calibrating galaxies in Virgo and Fornax , have excellent potential to refine this result in the near future .