When Feedback Fails: The Scaling and Saturation of Star Formation
  Efficiency
  We present a suite of 3D multi-physics MHD simulations following star
formation in isolated turbulent molecular gas disks ranging from 5 to 500
parsecs in radius. These simulations are designed to survey the range of
surface densities between those typical of Milky Way GMCs ($\sim 10^2
M_\odot\,pc^{-2}}$) and extreme ULIRG environments ($\sim 10^2
M_\odot\,pc^{-2}}$) so as to map out the scaling of the cloud-scale star
formation efficiency (SFE) between these two regimes. The simulations include
prescriptions for supernova, stellar wind, and radiative feedback, which we
find to be essential in determining both the instantaneous per-freefall
($\epsilon_{ff}$) and integrated ($\epsilon_{int}$) star formation
efficiencies. In all simulations, the gas disks form stars until a critical
stellar surface density has been reached and the remaining gas is blown out by
stellar feedback. We find that surface density is a good predictor of
$\epsilon_{int}$, as suggested by analytic force balance arguments from
previous works. SFE eventually saturates to $\sim 1$ at high surface density.
We also find a proportional relationship between $\epsilon_{ff}$ and
$\epsilon_{int}$, implying that star formation is feedback-moderated even over
very short time-scales in isolated clouds. These results have implications for
star formation in galactic disks, the nature and fate of nuclear starbursts,
and the formation of bound star clusters. The scaling of $\epsilon_{ff}$ with
surface density is not consistent with the notion that $\epsilon_{ff}$ is
always $\sim 1\%$ on the scale of GMCs, but our predictions recover the $\sim
1\%$ value for GMC parameters similar to those found in sprial galaxies,
including our own.
