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

TL;DR

This study uses hydrodynamical simulations to explore how metal-line cooling and feedback mechanisms influence galaxy properties at redshift 2, highlighting the importance of feedback efficiency and wind velocity in shaping galaxy evolution.

Contribution

It demonstrates the critical role of feedback from star formation and AGN in regulating galaxy growth, emphasizing the effects of wind velocity and metal-line cooling on galaxy properties.

Findings

Feedback efficiency depends on wind velocity and galaxy mass.

Star formation rate inversely correlates with energy injected per stellar mass.

Efficient feedback is necessary to match observed galaxy mass functions.

Abstract

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.