Physical properties of simulated galaxy populations at z=2 - I. Effect
  of metal-line cooling and feedback from star formation and AGN
  We use hydrodynamical simulations from the OWLS project to investigate the
dependence of the physical properties of galaxy populations at redshift 2 on
metal-line cooling and feedback from star formation and active galactic nuclei
(AGN). We find that if the sub-grid feedback from star formation is implemented
kinetically, the feedback is only efficient if the initial wind velocity
exceeds a critical value. This critical velocity increases with galaxy mass and
also if metal-line cooling is included. This suggests that radiative losses
quench the winds if their initial velocity is too low. If the feedback is
efficient, then the star formation rate is inversely proportional to the amount
of energy injected per unit stellar mass formed (which is proportional to the
initial mass loading for a fixed wind velocity). This can be understood if the
star formation is self-regulating, i.e. if the star formation rate (and thus
the gas fraction) increase until the outflow rate balances the inflow rate.
Feedback from AGN is efficient at high masses, while increasing the initial
wind velocity with gas pressure or halo mass allows one to generate galaxy-wide
outflows at all masses. Matching the observed galaxy mass function requires
efficient feedback. In particular, the predicted faint-end slope is too steep
unless we resort to highly mass loaded winds for low-mass objects. Such
efficient feedback from low-mass galaxies (M_* << 10^10 Msun) also reduces the
discrepancy with the observed specific star formation rates, which are higher
than predicted unless the feedback transitions from highly efficient to
inefficient just below the observed stellar mass range.
