  Star formation is inefficient. Only a few percent of the available gas in
molecular clouds forms stars, leading to the observed low star formation rate
(SFR). The same holds when averaged over many molecular clouds, such that the
SFR of whole galaxies is again surprisingly low. Indeed, considering the low
temperatures, molecular clouds should be highly gravitationally unstable and
collapse on their global mean freefall timescale. And yet, they are observed to
live about 10-100 times longer, i.e., the SFR per freefall time (SFR_ff) is
only a few percent. Thus, other physical mechanisms must counteract the quick
global collapse. Turbulence, magnetic fields and stellar feedback have been
proposed as regulating agents, but it is still unclear which of these processes
is the most important and what their relative contributions are. Here we run
high-resolution simulations including gravity, turbulence, magnetic fields, and
jet/outflow feedback. We confirm that clouds collapse on a mean freefall time,
if only gravity is considered, producing stars at an unrealistic rate. In
contrast, if turbulence, magnetic fields, and feedback are included
step-by-step, the SFR is reduced by a factor of 2-3 with each additional
physical ingredient. When they all act in concert, we find a constant SFR_ff =
0.04, currently the closest match to observations, but still about a factor of
2-4 higher than the average. A detailed comparison with other simulations and
with observations leads us to conclude that only models with turbulence
producing large virial parameters, and including magnetic fields and feedback
can produce realistic SFRs.
