Structural Parameters of the M87 Globular Cluster System

Abstract

<p>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 r_h) and ellipticities are determined from PSF-convolved King-model profile fitting. We find that the r_h distribution 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 average 24% larger in r_h 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 derive a color magnitude diagram for the M87 globular cluster system which show a clear bimodal distribution. We analyzed in the detail the sub-structure of the CMD by applying statistical fits to the color distribution. We obtain a Globular Cluster Luminosity Function for all M87 clusters and for its red and blue subpopulations. The best statistical fits describing the Luminosity Functions were also calculated. The metal-poor subpopulation has a brighter peak (mean) than the metal-rich subpopulation.</p>