Masses and scaling relations for nuclear star clusters, and their co-existence with central black holes

20 Jan

[2016MNRAS.457.2122G]  Georgiev, Iskren Y.; Böker, Torsten; Leigh, Nathan; Lützgendorf, Nora; Neumayer, Nadine

Galactic nuclei typically host either a Nuclear Star Cluster (NSC, prevalent in galaxies with masses 1010M) or a Massive Black Hole (MBH, common in galaxies with masses 1012M). In the intermediate mass range, some nuclei host both a NSC and a MBH. In this paper, we explore scaling relations between NSC mass (MNSC) and host galaxy total stellar mass (M,gal) using a large sample of NSCs in late- and early-type galaxies, including a number of NSCs harboring a MBH. Such scaling relations reflect the underlying physical mechanisms driving the formation and (co)evolution of these central massive objects. We find 1.5σ significant differences between NSCs in late- and early-type galaxies in the slopes and offsets of the relations reff,NSCMNSC, reff,NSCM,gal and MNSCM,gal, in the sense that i) NSCs in late-types are more compact at fixed MNSC and M,gal; and ii) the MNSCM,gal relation is shallower for NSCs in late-types than in early-types, similar to the MBHM,bulge relation. We discuss these results in the context of the (possibly ongoing) evolution of NSCs, depending on host galaxy type. For NSCs with a MBH, we illustrate the possible influence of a MBH on its host NSC, by considering the ratio between the radius of the MBH sphere of influence and reff,NSC. NSCs harbouring a sufficiently massive black hole are likely to exhibit surface brightness profile deviating from a typical King profile.