Kiloparsec-Scale Simulations of Star Formation in Disk Galaxies. IV.
  Regulation of Galactic Star Formation Rates by Stellar Feedback
  Star formation from the interstellar medium of galactic disks is a basic
process controlling the evolution of galaxies. Understanding the star formation
rate in a local patch of a disk with a given gas mass is thus an important
challenge for theoretical models. Here we simulate a kiloparsec region of a
disk, following the evolution of self-gravitating molecular clouds down to
subparsec scales, as they form stars that then inject feedback energy by
dissociating and ionizing UV photons and supernova explosions. We assess the
relative importance of each feedback mechanism. We find that $\rm
H_2$-dissociating feedback results in the largest absolute reduction in star
formation compared to the run with no feedback. Subsequently adding
photoionization feedback produces a more modest reduction. Our fiducial models
that combine all three feedback mechanisms yield, without fine-tuning, star
formation rates that are in excellent agreement with observations, with $\rm
H_2$-dissociating photons playing a crucial role. Models that only include
supernova feedback---a common method in galaxy evolution simulations---settle
to similar star formation rates, but with very different temperature and
chemical states of the gas, and with very different spatial distributions of
young stars.
