M31 Globular Clusters in the Hubble Space Telescope Archive. II. Structural Parameters

Abstract

We analyze the structural parameters of the largest available sample of spatially resolved extragalactic globular clusters. The images of M31 GCs were found in a search of Hubble Space Telescope (HST) archival data, described in a companion paper. We measure the ellipticities and position angles of the clusters and conclude that the ellipticities are consistent with being caused by rotation. We find that most clusters' surface brightness distributions are well fitted by two-dimensional single-mass Michie-King models. A few clusters show possible power-law distributions characteristic of core collapse, but the spatial resolution is not high enough to make definitive claims. As has been found for other galaxies, the metal-rich clusters are slightly smaller than the metal-poor clusters. There are strong correlations between structural properties of M31 GCs, as for Milky Way clusters, and the two populations are located close to the same ``fundamental plane'' in parameter space. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.