Globular cluster luminosity functions and the Hubble constant from WFPC2 imaging: The giant elliptical NGC 4365

The turnover, or peak, magnitude in a galaxy's globular cluster luminosity function (GCLF) may provide a standard candle for an independent distance estimator. Here we examine the GCLF of the giant elliptical NGC 4365 using photometry of similar to 350 globular clusters from the Hubble Space Telescope's Wide Field and Planetary Camera 2 (WFPC2). The WFPC2 data have several advantages over equivalent ground-based imaging. The membership of NGC 4365 in the Virgo cluster has been the subject of recent debate. We have fit a Gaussian and t(5) profile to the luminosity function and find that it can be well represented by a turnover magnitude of m(V)(0)=24.2+/-0.3 and a dispersion sigma=1.28+/-0.15. After applying a small metallicity correction to the ''universal'' globular cluster turnover magnitude, we derive a distance modulus of (m-M)=31.6+/-0.3 which is in reasonable agreement with that from surface brightness fluctuation measurements. This result places NGC 4365 about 5 Mpc beyond the Virgo cluster core. For a V-CMB=1592+/-24 km s(-1) the Hubble constant is H-0=72(-12)(+10) km s(-1) Mpc(-1). We also describe our method for estimating a local specific frequency for the GC system within the central 5h(-1) kpc which has fewer uncertain corrections than a total estimate. The resulting value of 6.4+/-1.5 indicates that NGC 4365 has a GC richness similar to other early-type galaxies. (C) 1996 American Astronomical Society.