The Maximum Stellar Surface Density Due to the Failure of Stellar
  Feedback
  A maximum stellar surface density $\Sigma_{max} \sim 3 \times 10^5\,{\rm
M_\odot\,pc^{-2}}$ is observed across all classes of dense stellar systems
(e.g. star clusters, galactic nuclei, etc.), spanning $\sim 8$ orders of
magnitude in mass. It has been proposed that this characteristic scale is set
by some dynamical feedback mechanism preventing collapse beyond a certain
surface density. However, simple analytic models and detailed simulations of
star formation moderated by feedback from massive stars argue that feedback
becomes {\it less} efficient at higher surface densities (with the star
formation efficiency increasing as $\sim \Sigma/\Sigma_{crit}$). We therefore
propose an alternative model wherein stellar feedback becomes ineffective at
moderating star formation above some $\Sigma_{crit}$, so the supply of
star-forming gas is rapidly converted to stars before the system can contract
to higher surface density. We show that such a model -- with $\Sigma_{crit}$
taken directly from the theory -- naturally predicts the observed
$\Sigma_{max}$. $\Sigma_{max}\sim 100\Sigma_{crit}$ because the gas consumption
time is longer than the global freefall time even when feedback is ineffective.
Moreover the predicted $\Sigma_{max}$ is robust to spatial scale and
metallicity, and is preserved even if multiple episodes of star formation/gas
inflow occur. In this context, the observed $\Sigma_{max}$ directly tells us
where feedback fails.
