Do Nuclear Star Clusters and Supermassive Black Holes Follow the Same Host-Galaxy Correlations?

Abstract

Studies have suggested that there is a strong correlation between the masses of nuclear star clusters (NSCs) and their host galaxies, a correlation which is said to be an extension of the well-known correlations between supermassive black holes (SMBHs) and their host galaxies. But careful analysis of disk galaxies—including 2D bulge/disk/bar decompositions—shows that while SMBHs correlate with the stellar mass of thebulgecomponent of galaxies, the masses of NSCs correlate much better with thetotalgalaxy stellar mass. In addition, the mass ratioMNSC/M⋆, totfor NSCs in spirals (at least those with Hubble types Sc and later) is typically an order of magnitude smaller than the mass ratioMBH/M⋆, bulof SMBHs. The absence of a universal “central massive object” correlation argues against common formation and growth mechanisms for both SMBHs and NSCs. We also discuss evidence for a break in the NSC-host galaxy correlation, galaxies with Hubble types earlier than Sbc appear to host systematically more massive NSCs than do types Sc and later.