SILCC-ZOOM: The early impact of ionizing radiation on forming molecular
  clouds
  As part of the SILCC-ZOOM project we present our first sub-parsec resolution
radiation-hydrodynamic simulations of two molecular clouds self-consistently
forming from a turbulent, multi-phase ISM. The clouds have similar initial
masses of few 10$^4$ M$_{\odot}$, escape velocities of ~5 km s$^{-1}$, and a
similar initial energy budget. We follow the formation of star clusters with a
sink based model and the impact of radiation from individual massive stars with
the tree-based radiation transfer module TreeRay. Photo-ionizing radiation is
coupled to a chemical network to follow gas heating, cooling and molecule
formation and dissociation. For the first 3 Myr of cloud evolution we find that
the overall star formation effciency is considerably reduced by a factor of ~4
to global cloud values of < 10 % as the mass accretion of sinks that host
massive stars is terminated after <1 Myr. Despite the low effciency, star
formation is triggered across the clouds. Therefore, a much larger region of
the cloud is affected by radiation and the clouds begin to disperse. The time
scale on which the clouds are dispersed sensitively depends on the cloud
substructure and in particular on the amount of gas at high visual extinction.
The damage of radiation done to the highly shielded cloud (MC1) is delayed. We
also show that the radiation input can sustain the thermal and kinetic energy
of the clouds at a constant level. Our results strongly support the importance
of ionizing radiation from massive stars for explaining the low observed star
formation effciency of molecular clouds
