An evolutionary continuum from nucleated dwarf galaxies to star clusters.

Systematic studies 1-4 have revealed hundreds of ultra-compact dwarf galaxies (UCDs 5 ) in the nearby Universe. With half-light radii r h of approximately 10-100 parsecs and stellar masses M *  ≈ 10 6 -10 8  solar masses, UCDs are among the densest known stellar systems 6 . Although similar in appearance to massive globular clusters 7 , the detection of extended stellar envelopes 4,8,9 , complex star formation histories 10 , elevated mass-to-light ratio 11,12 and supermassive black holes 13-16 suggest that some UCDs are remnant nuclear star clusters 17 of tidally stripped dwarf galaxies 18,19 , or even ancient compact galaxies 20 . However, only a few objects have been found in the transient stage of tidal stripping 21,22 , and this assumed evolutionary path 19 has never been fully traced by observations. Here we show that 106 galaxies in the Virgo cluster have morphologies that are intermediate between normal, nucleated dwarf galaxies and single-component UCDs, revealing a continuum that fully maps this morphological transition and fills the 'size gap' between star clusters and galaxies. Their spatial distribution and redder colour are also consistent with stripped satellite galaxies on their first few pericentric passages around massive galaxies 23 . The 'ultra-diffuse' tidal features around several of these galaxies directly show how UCDs are forming through tidal stripping and that this evolutionary path can include an early phase as a nucleated ultra-diffuse galaxy 24,25 . These UCDs represent substantial visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and more low-mass remnants probably remain to be found.