An evolutionary continuum from nucleated dwarf galaxies to star clusters

Systematic studies^1-4 have revealed hundreds of ultra-compact dwarf galaxies (UCDs⁵) in the nearby Universe. With half-light radii r_h of approximately 10-100 parsecs and stellar masses M_*≈ 10⁶-10⁸solar masses, UCDs are among the densest known stellar systems⁶. Although similar in appearance to massive globular clusters⁷, the detection of extended stellar envelopes^4,8,9, complex star formation histories¹⁰, elevated mass-to-light ratio^11,12 and supermassive black holes^13-16 suggest that some UCDs are remnant nuclear star clusters¹⁷ of tidally stripped dwarf galaxies^18,19, or even ancient compact galaxies²⁰. However, only a few objects have been found in the transient stage of tidal stripping^21,22, and this assumed evolutionary path¹⁹ 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 galaxies²³. 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.