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

Abstract

Galactic nuclei typically host either a Nuclear Star Cluster (NSC, prevalent\nin galaxies with masses $\\lesssim 10^{10}M_\\odot$) or a Massive Black Hole\n(MBH, common in galaxies with masses $\\gtrsim 10^{12}M_\\odot$). In the\nintermediate mass range, some nuclei host both a NSC and a MBH. In this paper,\nwe explore scaling relations between NSC mass (${\\cal M}_{\\rm NSC}$) and host\ngalaxy total stellar mass (${\\cal M}_{\\star,\\rm gal}$) using a large sample of\nNSCs in late- and early-type galaxies, including a number of NSCs harboring a\nMBH. Such scaling relations reflect the underlying physical mechanisms driving\nthe formation and (co)evolution of these central massive objects. We find\n$\\sim\\!1.5\\sigma$ significant differences between NSCs in late- and early-type\ngalaxies in the slopes and offsets of the relations $r_{\\rm eff,NSC}$--${\\cal\nM}_{\\rm NSC}$, $r_{\\rm eff, NSC}$--${\\cal M}_{\\star,\\rm gal}$ and ${\\cal\nM}_{\\rm NSC}$--${\\cal M}_{\\star,\\rm gal}$, in the sense that $i)$ NSCs in\nlate-types are more compact at fixed ${\\cal M}_{\\rm NSC}$ and ${\\cal\nM}_{\\star,\\rm gal}$; and $ii)$ the ${\\cal M}_{\\rm NSC}$--${\\cal M}_{\\star,\\rm\ngal}$ relation is shallower for NSCs in late-types than in early-types, similar\nto the ${\\cal M}_{\\rm BH}$--${\\cal M}_{\\star,\\rm bulge}$ relation. We discuss\nthese results in the context of the (possibly ongoing) evolution of NSCs,\ndepending on host galaxy type. For NSCs with a MBH, we illustrate the possible\ninfluence of a MBH on its host NSC, by considering the ratio between the radius\nof the MBH sphere of influence and $r_{\\rm eff, NSC}$. NSCs harbouring a\nsufficiently massive black hole are likely to exhibit surface brightness\nprofile deviating from a typical King profile.\n