Thursday

Ultra-diffuse galaxies (UDGs) are a class of low surface brightness (LSB) galaxies with stellar masses typical of dwarfs (M*~10^7-10^8) and large sizes (Rh>1.5kpc) for their luminosity. First discovered in the Coma cluster, UDGs are ubiquitous in nearby galaxy clusters. Two opposite paths have been proposed for their formation. UDGs could be massive galaxies that `failed’ to form stars, possibly due to early infall onto the cluster or to extreme feedback. Alternatively, UDGs have low mass haloes (~10^10-10^11 Msun) and simply represent the LSB tail of the abundant dwarf galaxy population.
I will show that the latter scenario well reproduces the global properties of UDG populations in clusters, including their abundance and size distribution. I will also expand on an effort to assess the richness of their GC systems, as a proxy for their virial mass. This has shown that the GC abundances of Coma UDGs are in line with what expected for their stellar mass: the large majority of cluster UDGs are ‘normal’ dwarf galaxies. In agreement with this result, gas-rich UDGs with low rotational velocities have recently been identified in large numbers also in the field.