Do Nuclear Star Clusters and 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
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,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, bulge 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.
