We investigate the connection between globular clusters and ultra-compact dwarf galaxies ( UCDs ) by examining the properties of ten compact , high-luminosity ( -11.8 \lesssim M _ { V } \lesssim - 10.8 ) objects associated with M87 ( NGC 4486 , VCC1316 ) , the cD galaxy in the Virgo Cluster .
These objects , most of which were previously classified as M87 globular clusters , were selected from a combination of ground- and space-based imaging surveys .
Our observational database for these objects — which we term DGTOs or “ dwarf-globular transition objects ” — includes Advanced Camera for Survey ( ACS ) F475W and F850LP imaging from ACS Virgo Cluster Survey , integrated-light spectroscopy from Keck/ESI , and archival F606W WFPC2 imaging .
We also present a search for DGTOs associated with other galaxies based on ACS imaging for 100 early-type galaxies in Virgo .
Our main findings can be summarized as follows :

( 1 ) Out of the six DGTOs in M87 with both ground-based spectroscopy and HST imaging , we find two objects to have half-light radii , velocity dispersions and mass-to-light ratios that are consistent with the predictions of population synthesis models for old , metal-rich , high-luminosity globular clusters .

( 2 ) Three other DGTOs are much larger , with half-light radii r _ { h } \sim 20 pc , and have V -band mass-to-light ratios in the range 6 \lesssim { \cal M / L } _ { V } \lesssim 9 .
These objects , which we consider to be UCDs , resemble the nuclei of nucleated dwarf elliptical galaxies in the Virgo Cluster , having similar mass-to-light ratios , luminosities , and colors .

( 3 ) The classification of the sixth object is more uncertain , but it bears a strong resemblance to simulated “ stellar superclusters ” which are presumed to form through the amalgamation of multiple young massive clusters .

( 4 ) In general , the UCDs in M87 are found to follow the extrapolated scaling relations of galaxies more closely than those of globular clusters .
There appears to be a transition between the two types of stellar systems at a mass of \approx 2 \times 10 ^ { 6 } { \cal M } _ { \odot } .
We suggest that the presence of dark matter is the fundamental property distinguishing globular clusters from UCDs .

( 5 ) We identify a sample of 13 DGTO candidates from the complete ACS Virgo Cluster Survey , selecting on the basis of half-light radius , magnitude and color .
For a number of these objects , membership in Virgo can be established through radial velocities or surface brightness fluctuation measurements with our ACS images .
Three of these DGTO candidates are embedded in low-surface brightness envelopes .

( 6 ) Five of the 13 DGTOs in Virgo are associated with a single galaxy : M87 .
This finding suggests that proximity to the Virgo center may be of critical importance for the formation of these objects , although we find M87 to be more abundant in DGTOs than would be expected on the basis of its luminosity , the size of its globular cluster system , or the local galaxy density .

These results show that distinguishing bonafide UCDs from high-luminosity globular clusters requires a careful analysis of their detailed structural and dynamical properties , particularly their mass-to-light ratios .
In general , the properties of the UCDs in our sample are consistent with models in which these objects form through tidal stripping of nucleated dwarf galaxies .