Ultra Violet Escape Fractions from Giant Molecular Clouds During Early
  Cluster Formation
  The UV photon escape fraction from molecular clouds is a key parameter for
understanding the ionization of the Interstellar Medium (ISM), and
extragalactic processes, such as cosmic reionization. We present the ionizing
photon flux and the corresponding photon escape fraction (f$_{esc}$) arising as
a consequence of star cluster formation in a turbulent, 10$^6$ M$_{\odot}$ GMC,
simulated using the code FLASH. We make use of sink particles to represent
young, star-forming clusters coupled with a radiative transfer scheme to
calculate the emergent UV flux. We find that the ionizing photon flux across
the cloud boundary is highly variable in time and space due to the turbulent
nature of the intervening gas. The escaping photon fraction remains at $\sim$5%
for the first 2.5 Myr, followed by two pronounced peaks at 3.25 and 3.8 Myr
with a maximum f$_{esc}$ of 30% and 37%, respectively. These peaks are due to
the formation of large HII regions, that expand into regions of lower density
and some of which reach the cloud surface. However, these phases are short
lived and f$_{esc}$ drops sharply as the HII regions are quenched by the
central cluster passing through high-density material due to the turbulent
nature of the cloud. We find an average f$_{esc}$ of 15% with factor of two
variations over 1 Myr timescales. Our results suggest that assuming a single
value for f$_{esc}$ from a molecular cloud is in general a poor approximation,
and that the dynamical evolution of the system leads to large temporal
variation.
