When feedback fails: the scaling and saturation of star formation efficiency

We present a suite of 3D multiphysics MHD simulations following star formation in isolated turbulent molecular gas discs 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 giant molecular clouds (GMCs) (similar to 10(2) M-circle dot pc(-2)) and extreme ultraluminous infrared galaxy environments (similar to 10(4) M-circle dot 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 discs 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 similar to 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 discs, 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 similar to 1 per cent on the scale of GMCs, but our predictions recover the similar to 1 per cent value for GMC parameters similar to those found in spiral galaxies, including our own.