The story of ultra-diffuse galaxies as is told by their globular clusters
Teymoor Saifollahi(University of Groningen)
The nature of very low surface brightness galaxies with large effective radii (recently been dubbed "Ultra Diffuse Galaxies" or "UDGs") has been intensely debated in past years. This debate has focussed on the possible differences between UDGs and the general galaxy population with the same luminosity (i.e. dwarf galaxies) and, in particular, on the amount of dark matter these galaxies may possess. While the vast majority of works point to UDGs having the properties of dwarf galaxies, some UDGs do not fit nicely into the categories of "normal" dwarf galaxies, as their dark matter content has been suggested to be very high or very low. In particular, one of the better-known examples of such an extreme UDG is Dragonfly 44 (DF44). This iconic galaxy, associated with the Coma galaxy cluster, has been claimed to have a dark matter halo comparable with that measured for the Milky Way and posses 99.99% dark matter which is mostly supported by the extensive number of Globular Clusters (GCs) around the galaxy. It is known that the total number of GCs around galaxies correlates with the total mass of the galaxies host dark matter halo. This peculiar connection is the outcome of the hierarchical formation of galaxies in the ΛCDM cosmology. In the first part of this talk, after a review of the properties of UDGs and their globular clusters, I discuss the anomalous behaviour of DF44. I show that the number of GCs around DF44 is smaller than what has been claimed in the past and DF44 is 10 times less massive than what was suggested. I also show that the distribution of GCs around DF44 is more compact that is expected from dwarf galaxies, a feature than may originate from its evolutionary path. In the second part, I explore this finding by investigating a sample of Coma cluster UDGs with large effective radii that resemble DF44 and present my most recent findings. In the end, I based on these findings, I discuss the formation and evolution of UDGs and lessons that we must learn from their globular clusters.