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We use hydrodynamical simulations from the OWLS project to investigate thedependence of the physical properties of galaxy populations at redshift 2 onmetal-line cooling and feedback from star formation and active galactic nuclei(AGN). We find that if the sub-grid feedback from star formation is implementedkinetically, the feedback is only efficient if the initial wind velocityexceeds a critical value. This critical velocity increases with galaxy mass andalso if metal-line cooling is included. This suggests that radiative lossesquench the winds if their initial velocity is too low. If the feedback isefficient, then the star formation rate is inversely proportional to the amountof energy injected per unit stellar mass formed (which is proportional to theinitial mass loading for a fixed wind velocity). This can be understood if thestar formation is self-regulating, i.e. if the star formation rate (and thusthe gas fraction) increase until the outflow rate balances the inflow rate.Feedback from AGN is efficient at high masses, while increasing the initialwind velocity with gas pressure or halo mass allows one to generate galaxy-wideoutflows at all masses. Matching the observed galaxy mass function requiresefficient feedback. In particular, the predicted faint-end slope is too steepunless we resort to highly mass loaded winds for low-mass objects. Suchefficient feedback from low-mass galaxies (M_* << 10^10 Msun) also reduces thediscrepancy with the observed specific star formation rates, which are higherthan predicted unless the feedback transitions from highly efficient toinefficient just below the observed stellar mass range.

Physical properties of simulated galaxy populations at z=2 - I. Effect of metal-line cooling and feedback from star formation and AGN