Deep Chandra Monitoring Observations of NGC 4649: II. Wide-Field Hubble
  Space Telescope Imaging of the Globular Clusters
  We present g and z photometry and size estimates for globular clusters (GCs)
in the massive Virgo elliptical NGC 4649 (M60) using a five-pointing Hubble
Space Telescope/Advanced Camera for Surveys mosaic. The metal-poor GCs show a
monotonic negative metallicity gradient of (-0.43 +/- 0.10) dex per dex in
radius over the full radial range of the data, out to ~ 24 kpc. There is
evidence for substantial color substructure among the metal-rich GCs. The
metal-poor GCs have typical sizes ~ 0.4 pc larger than the metal-rich GCs out
to large galactocentric distances (~> 20 kpc), favoring an intrinsic
explanation for the size difference rather than projection effects. There is no
clear relation between half-light radius and galactocentric distance beyond ~
15 kpc, suggesting that the sizes of GCs are not generically set by tidal
limitation. Finally, we identify ~ 20 candidate ultra-compact dwarfs that
extend down to surprisingly faint absolute magnitudes (M_z ~ -8.5), and may
bridge the gap between this class and "extended clusters" in the Local Group.
Three of the brighter candidates have published radial velocities and can be
confirmed as bona fide ultra-compact dwarfs; follow-up spectroscopy will
determine the nature of the remainder of the candidates.
