  We investigate the physics driving the cosmic star formation (SF) history
using the more than fifty large, cosmological, hydrodynamical simulations that
together comprise the OverWhelmingly Large Simulations (OWLS) project. We
systematically vary the parameters of the model to determine which physical
processes are dominant and which aspects of the model are robust. Generically,
we find that SF is limited by the build-up of dark matter haloes at high
redshift, reaches a broad maximum at intermediate redshift, then decreases as
it is quenched by lower cooling rates in hotter and lower density gas, gas
exhaustion, and self-regulated feedback from stars and black holes. The higher
redshift SF is therefore mostly determined by the cosmological parameters and
to a lesser extent by photo-heating from reionization. The location and height
of the peak in the SF history, and the steepness of the decline towards the
present, depend on the physics and implementation of stellar and black hole
feedback. Mass loss from intermediate-mass stars and metal-line cooling both
boost the SF rate at late times. Galaxies form stars in a self-regulated
fashion at a rate controlled by the balance between, on the one hand, feedback
from massive stars and black holes and, on the other hand, gas cooling and
accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF
law. This can be understood in terms of self-regulation: if the SF efficiency
is changed, then galaxies adjust their gas fractions so as to achieve the same
rate of production of massive stars. Self-regulated feedback from accreting
black holes is required to match the steep decline in the observed SF rate
below redshift two, although more extreme feedback from SF, for example in the
form of a top-heavy IMF at high gas pressures, can help.
