Galaxies on FIRE (Feedback In Realistic Environments): Stellar Feedback Explains Cosmologically Inefficient Star Formation

We present a series of high-resolution cosmological zoom-in simulations1 of galaxy formation to z = 0, spanning halo masses Mhalo ∼ 108 −1013 M and stellar masses M∗ ∼ 104 −1011 M . Our simulations include a fully explicit treatment of both the multi-phase ISM (molecular through hot) and stellar feedback. The stellar feedback inputs (energy, momentum, mass, and metal fluxes) are taken directly from stellar population models. These sources of stellar feedback, with zero adjusted parameters, reproduce the observed relation between stellar and halo mass up to Mhalo ∼ 1012 M (including dwarfs, satellites, MW-mass disks, and small groups). By extension, this leads to reasonable agreement with the stellar mass function for M∗ . 1011 M . We predict weak redshift evolution in the M∗−Mhalo relation, consistent with current constraints up to z > 6. We find that the M∗−Mhalo relation in our calculations is relatively insensitive to numerical details, but is sensitive to the feedback physics. Simulations with only supernova feedback fail to reproduce the observed stellar masses, particularly for dwarf and/or high-redshift galaxies: radiative feedback (photo-heating and radiation pressure) is necessary to disrupt molecular clouds and enable efficient coupling of later supernovae explosions to the gas. Instantaneous star formation rates agree well with the observed Kennicutt relation, with weak redshift evolution. The galaxy-averaged Kennicutt relation is very different from the numerically imposed law for converting gas into stars on small scales in the simulation and is instead determined by self-regulation via stellar feedback. We find that feedback reduces star formation rates considerably and produces a reservoir of gas that leads to flatter or rising late-time star formation histories significantly different from the halo accretion history. Feedback also produces large short-timescale variability in galactic star formation rates, especially in dwarf galaxies. Many of these properties of galaxy formation with explicit feedback are not captured by common “sub-grid” galactic wind models.