  Photoelectric heating has long been recognized as the primary source of
heating for the neutral interstellar medium. Simulations of spiral galaxies
found some indication that photoelectric heating could suppress star formation.
However, simulations that include photoelectric heating have typically found
that it has little effect on the rate of star formation in either spiral
galaxies or dwarfs suggesting that supernovae and not photoelectric heating are
responsible for setting the star formation law in galaxies. This result is in
tension with recent work indicating that a star formation law that depends on
galaxy metallicity, as expected for photoelectric heating but not for
supernovae, reproduces the present-day galaxy population better than a
metallicity-independent one. Here we report a series of simulations of dwarf
galaxies, where the effects of both photoelectric heating and supernovae are
expected to be strongest. We simultaneously include space- and time-dependent
photoelectric heating, and we resolve the Sedov phase of every supernova blast
wave, allowing us to make a direct measurement of the relative importance of
momentum injection by supernovae and dust heating by far ultraviolet (FUV)
photons in suppressing star formation. We find that supernovae are unable to
account for the long observed gas depletion times in dwarf galaxies. Instead,
ordinary photoelectric heating is the dominant means by which dwarf galaxies
regulate their star formation rate at any given time, suppressing the star
formation rate by more than an order of magnitude relative to simulations with
only supernovae.
