Radiation hydrodynamics simulations of massive star cluster formation in
  giant molecular clouds
  By performing three-dimensional radiation hydrodynamics simulations, we study
the formation of young massive star clusters (YMCs, $M_{*}>10^4~M_{\odot}$) in
clouds with the surface density ranging from $\Sigma_{\rm cl} = 80$ to
$3200~M_{\odot}\;{\rm pc^{-2}}$. We find that photoionization feedback
suppresses star formation significantly in clouds with low surface density.
Once the initial surface density exceeds $\sim 100~M_{\odot}\;{\rm pc^{-2}}$
for clouds with $M_{\rm cl}=10^{6}~M_{\odot}$ and $Z= Z_{\odot}$, most of the
gas is converted into stars because the photoionization feedback is inefficient
in deep gravitational potential. In this case, the star clusters are massive
and gravitationally bounded as YMCs. The transition surface density increases
as metallicity decreases, and it is $\sim 350~M_{\odot}\;{\rm pc^{-2}}$ for
$Z=10^{-2}~Z_{\odot}$. We show that more than 10 percent of star-formation
efficiency (SFE) is needed to keep a star cluster gravitationally bounded even
after the disruption of a cloud. Also, we develop a semi-analytical model
reproducing the SFEs obtained in our simulations. We find that the SFEs are fit
with a power-law function with the dependency $\propto \Sigma_{\rm cl}^{1/2}$
for low-surface density and rapidly increases at the transition surface
densities. The conditions of the surface density and the metallicity match
recent observations of giant molecular clouds forming YMCs in nearby galaxies.
