The SILCC project --- IV. Impact of dissociating and ionising radiation
  on the interstellar medium and Halpha emission as a tracer of the star
  formation rate
  We present three-dimensional radiation-hydrodynamical simulations of the
impact of stellar winds, photoelectric heating, photodissociating and
photoionising radiation, and supernovae on the chemical composition and star
formation in a stratified disc model. This is followed with a sink-based model
for star clusters with populations of individual massive stars. Stellar winds
and ionising radiation regulate the star formation rate at a factor of ~10
below the simulation with only supernova feedback due to their immediate impact
on the ambient interstellar medium after star formation. Ionising radiation
(with winds and supernovae) significantly reduces the ambient densities for
most supernova explosions to rho < 10^-25 g cm^-3, compared to 10^-23 g cm^-3
for the model with only winds and supernovae. Radiation from massive stars
reduces the amount of molecular hydrogen and increases the neutral hydrogen
mass and volume filling fraction. Only this model results in a molecular gas
depletion time scale of 2 Gyr and shows the best agreement with observations.
In the radiative models, the Halpha emission is dominated by radiative
recombination as opposed to collisional excitation (the dominant emission in
non-radiative models), which only contributes ~1-10 % to the total Halpha
emission. Individual massive stars (M >= 30 M_sun) with short lifetimes are
responsible for significant fluctuations in the Halpha luminosities. The
corresponding inferred star formation rates can underestimate the true
instantaneous star formation rate by factors of ~10.
