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For many decades,it was widely believed that galaxies and star clusters were completely distinct populations in the manifold of stellar systems,characterized quite different stellar populations,structural and dynamical properties,and formation histories.However,in the late 1990s,a new type of compact stellar system was discovered to have some properties that bridged the gap between compact,low-mass galaxies and globular clusters(GCs).These rare objects came to be known as “Ultra-Compact Dwarf” galaxies(UCDs).More than 15 years after their discovery,the origin of UCDs remains surprisingly obscure.To understand the origin of these puzzling systems,a large and carefully selected sample of UCDs is needed.The Next Generation Virgo cluster Survey(NGVS)is a large program on the 3.6m Canada-France-Hawaii Telescope to carry out deep,multi-band(u,g,r,I,z,K)imaging of the Virgo cluster from its core to its Virial radius.Thanks to the high-quality NGVS images,we can measure the half-light radii for compact stellar systems larger than rh~10 pc.Therefore,a nearly complete sample of UCDs can be selected using a combination of half-light radii and location in the colour-colour(uiK or ugz)diagrams.Here,we present a systematic study UCDs in the Virgo cluster: 1,For the first time,we show the UCD spatial distribution map over a large part of the Virgo cluster(~104 square degrees).2,We present a comparative study of UCDs belonging to three prominent Virgo sub-clusters: i.e.,those centered on the massive,red-sequence galaxies M87,M49 and M60.3,We show for the first time that the UCDs,like the globular clusters in most high-mass galaxies,show a bimodal distribution in color with~80%of the UCDs belonging to the blue sub-population.4,We present evidence that UCDs surrounding M87 may follow a morphological sequence ordered by the prominence of their outer,low surface brightness envelope,ultimately merging with the sequence of nucleated low-mass galaxies,and that envelope prominence correlates with distance from M87.In summary,our photometric analysis provides evidence that tidal stripping of nucleated galaxies is probably the dominant process in the formation of UCDs.