SO Webloquio: Nuclear star clusters

I review the current knowledge about nuclear star clusters (NSCs), and the spectacularly dense and massive assemblies of stars found at the centers of most galaxies. Understanding the formation, growth, and ultimate fate of NSCs is crucial for a complete picture of galaxy evolution. There is a clear transition mass in galaxies of ∼ 10^9 Msol where the characteristics of NSCs change. I argue that at lower masses, NSCs are formed primarily from globular clusters that inspiral into the center of the galaxy, while at higher masses, star formation within the nucleus forms the bulk of the NSC. I also discuss the co-existence of NSCs and central black holes, and how their growth may be linked. The extreme densities of NSCs and their interaction with massive black holes lead to a wide range of unique phenomena including tidal disruption and gravitational-wave events. Finally, we review the evidence that many NSCs end up in the halos of massive galaxies stripped of the stars that surrounded them, thus providing valuable tracers of the galaxies’ accretion histories.