Hubble Space Telescope Observations of Globular Clusters in M87 and an Estimate of H 0

Abstract

Hubble Space Telescope observations of over 1000 globular clusters in the central region of M87 have been made using the Wide Field and Planetary Camera 2. The limiting magnitude in V is 26 mag, more than 2 mag beyond the turnover of the luminosity function. The distribution is well fitted by a Gaussian profile with a mean of <m⁰_V> = 23.72 +/- 0.06 mag and a width of 1.40 +/- 0.06 mag. Assuming a value of <M⁰_V> = -7.4 +/- 0.25 mag for the globular cluster luminosity function results in a distance modulus of (m - M) = 31.12 mag to the Virgo Cluster. Using a value of Delta Coma-Virgo(m - M) = 3.71 +/- 0.10 mag and adopting a velocity for the Coma Cluster of 7188 km s-1 result in a value for the Hubble constant of H0 = 78 +/- 11 km s-1 Mpc-1. The V - I color distribution is bimodal, with peaks at V - I = 0.95 mag and 1.20 mag. The mean size of the clusters is Reff = 3 pc, with a scatter of 1 pc.