The Next Generation Virgo Cluster Survey (NGVS). XXXVI. Caught in the Act of Transformation: Morphological Decomposition and Classification of Ultracompact Dwarf Galaxies in the Virgo Cluster

We present the methods and data for a detailed structural analysis of over 600 ultracompact dwarf (UCD) galaxy candidates in the Virgo Cluster, based on deep ground-based imaging from the Next Generation Virgo Cluster Survey and archival Hubble Space Telescope (HST) Advanced Camera for Surveys imaging. Our goal is to test two main formation scenarios—whether UCDs are massive globular clusters (GCs) or the remnant nuclei of tidally stripped dwarf galaxies (nucleated dwarf elliptical galaxies, dEs)—by analyzing their morphologies and searching for extended stellar envelopes. We find that over 10% of Virgo UCD candidates exhibit two-component profiles with extended outer structures of R_e ∼ 30–300 pc that cannot be well fitted by standard King or Sérsic models, indicating the presence of diffuse envelopes (eUCDs). High-resolution HST imaging is essential for resolving compact eUCDs that are unresolved in ground-based data. We also present results of the Gemini/Gemini Multi-Object Spectrograph spectroscopy follow-up programs to confirm the Virgo Cluster membership for eUCD candidates. The full catalog consists of a near-complete spectroscopic sample of 68 Virgo eUCDs, as well as properties for 51 strongly nucleated dEs that resemble eUCDs. Together, this constitutes the largest and most comprehensive sample thus far of eUCDs and other possible nucleated dE–GC transition objects. This is a companion paper to K. Wang et al., which was based in large part on the data we present here. Future wide-field space-based surveys will offer unprecedented opportunities to identify eUCDs across a range of environments.