Abstract

We derive structural parameters for ~ 2000 globular clusters in the giant Virgo elliptical Messier 87 (M87) using extremely deep Hubble Space Telescope images in F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes (half-light radii rh) and ellipticities are determined from point-spread-function -convolved King-model profile fitting. We find that the rh distribution closely resembles the inner Milky Way clusters, peaking at rh ? 2.5 pc and with virtually no clusters more compact than rh ? 1 pc. The metal-poor clusters have on average an rh 24% larger than the metal-rich ones. The cluster scale size shows a gradual and noticeable increase with galactocentric distance. Clusters are very slightly larger in the bluer waveband V, a possible hint that we may be beginning to see the effects of mass segregation within the clusters.We also derived a color magnitude diagram for the M87 globular cluster system which shows a striking bimodal distribution. © 2009. The American Astronomical Society. All rights reserved.