The extremely peculiar globular cluster system of UDG GAMA-526784 and its implications

Recent observations of Ultra-Diffuse Galaxies (UDGs, which have the luminosities of dwarfs but sizes of giant galaxies) have picked up a lot of attention by the community. Two nagging problems that are being discussed in the context of UDGs, are 1) the apparent lack of dark matter of some UDGs, and 2) their anomalously abundant-and-bright globular cluster systems. The former is difficult to reconcile with a LCDM model of structure formation, while the latter is contrary to what we observe in essentially any other galaxy system. We have recently discovered a UDG candidate, that may provide valuable insight towards solving both problems. This object has very similar properties to typical UDGs, apart from a widely spread population of compact star-forming regions. Our ground-based photometry indicates that these regions have stellar masses similar to globular clusters (GCs), and at least three have radial profiles that are unresolved by our ground based imaging - in line with expectations for young massive clusters. The HST brings the high spatial resolution, and imaging capability in the near-UV, that are required to better constrain the physical sizes and formation ages of these compact regions. Interestingly and coincidentally, some properties of this galaxy are strikingly similar to those hypothesised by Silk 2019, who predicts the simultaneous formation of massive GCs, and links this to the formation of UDGs that lack dark matter and exhibit extreme GC systems. This UDG may just provide us a very rare local view on such processes in action.