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

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 the bulge component of galaxies, the masses of
NSCs correlate much better with the total galaxy stellar mass. In
addition, the mass ratio M-NSC/M-star,M- tot for NSCs in spirals
(at least those with Hubble types Sc and later) is typically an
order of magnitude smaller than the mass ratio M-BH/M-star,M- bul
of 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.