Competitive feedback in galaxy formation
  It is now well established that many galaxies have nuclear star clusters
(NCs) whose total masses correlate with the velocity dispersion (sigma) of the
galaxy spheroid in a very similar way to the well--known supermassive black
hole (SMBH) M - sigma relation. Previous theoretical work suggested that both
correlations can be explained by a momentum feedback argument. Observations
further show that most known NCs have masses < 10^8 Msun, while SMBHs
frequently have masses > 10^8 Msun, which remained unexplained in previous
work. We suggest here that this changeover reflects a competition between the
SMBH and nuclear clusters in the feedback they produce. When one of the massive
objects reaches its limiting M-sigma value, it drives the gas away and hence
cuts off its own mass and also the mass of the ``competitor''. The latter is
then underweight with respect to the expected M-sigma mass (abridged).
