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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 HubbleSpace Telescope/Advanced Camera for Surveys mosaic. The metal-poor GCs show amonotonic negative metallicity gradient of (-0.43 +/- 0.10) dex per dex inradius over the full radial range of the data, out to ~ 24 kpc. There isevidence for substantial color substructure among the metal-rich GCs. Themetal-poor GCs have typical sizes ~ 0.4 pc larger than the metal-rich GCs outto large galactocentric distances (~> 20 kpc), favoring an intrinsicexplanation for the size difference rather than projection effects. There is noclear relation between half-light radius and galactocentric distance beyond ~15 kpc, suggesting that the sizes of GCs are not generically set by tidallimitation. Finally, we identify ~ 20 candidate ultra-compact dwarfs thatextend down to surprisingly faint absolute magnitudes (M_z ~ -8.5), and maybridge the gap between this class and "extended clusters" in the Local Group.Three of the brighter candidates have published radial velocities and can beconfirmed as bona fide ultra-compact dwarfs; follow-up spectroscopy willdetermine the nature of the remainder of the candidates.

Deep Chandra Monitoring Observations of NGC 4649: II. Wide-Field Hubble Space Telescope Imaging of the Globular Clusters