Cosmological Galaxy Evolution with Superbubble Feedback II: The Limits
  of Supernovae
  We explore when supernovae can (and cannot) regulate the star formation and
bulge growth in galaxies based on a sample of 18 simulated galaxies. The
simulations include key physics such as evaporation and conduction, neglected
in prior work, and required to correctly model superbubbles resulting from
stellar feedback. We show that for galaxies with virial masses
$>10^{12}\;M_\odot$, supernovae alone cannot prevent excessive star formation.
This failure occurs due to a shutdown of galactic winds, with wind mass
loadings falling from $\eta\sim10$ to $\eta<1$. In more massive systems, this
transfer of baryons to the circumgalactic medium falters earlier on and the
galaxies diverge significantly from observed galaxy scaling relations and
morphologies. The decreasing efficiency is simply due to a deepening potential
well preventing gas escape. This implies that non-supernova feedback mechanisms
must become dominant for galaxies with stellar masses greater than
$\sim4\times10^{10}\;M_\odot$. The runaway growth of the central stellar bulge,
strongly linked to black hole growth, suggests that feedback from active
galactic nuclei is the probable mechanism. Below this mass, supernovae alone
are able to produce a realistic stellar mass fraction, star formation history
and disc morphology.
