Structural Parameters of the M87 Globular Clusters

We derive structural parameters for ∼ 2000 globular clusters in the giant Virgo elliptical 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 PSF-convolved Kingmodel 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 show a striking bimodal distribution. Subject headings: galaxies: star clusters galaxies: individual (M87) galaxies: elliptical and lenticular, cD globular clusters: general