2009ApJ...705..237M

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 r_h) and ellipticities are determined from point-spread-function -convolved King-model profile fitting. We find that the r_hdistribution closely resembles the inner Milky Way clusters, peaking at r_h≃ 2.5 pc and with virtually no clusters more compact than r_h≃ 1 pc. The metal-poor clusters have on average an r_h24% 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.