The escape of ultraviolet photons from the densest regions of the interstellar medium ( ISM ) — Giant Molecular Clouds ( GMCs ) — is a poorly constrained parameter which is vital to understanding the ionization of the ISM and the intergalactic medium .
We characterize the escape fraction , f _ { \text { esc,GMC } } , from a suite of individual GMC simulations with masses in the range 10 ^ { 4 - 6 } M _ { \odot } using the adaptive-mesh refinement code FLASH .
We find significantly different f _ { \text { esc,GMC } } depending on the GMC mass which can reach > 90 % in the evolution of 5 \times 10 ^ { 4 } and 10 ^ { 5 } M _ { \odot } clouds or remain low at \sim 5 % for most of the lifetime of more massive GMCs .
All clouds show fluctuations over short , sub-Myr timescales produced by flickering HII regions .
We combine our results to calculate the total escape fraction ( f _ { \text { esc,tot } } ) from GMC populations in dwarf starburst and spiral galaxies by randomly drawing clouds from a GMC mass distribution ( dN/dM \propto M ^ { \alpha } , where \alpha is either -1.5 or -2.5 ) over fixed time intervals .
We find typical f _ { \text { esc,tot } } values of 8 % for both the dwarf and spiral models .
The fluctuations of f _ { \text { esc,tot } } , however , are much larger for the dwarf models with values as high as 90 % .
The photons escaping from the 5 \times 10 ^ { 4 } and 10 ^ { 5 } M _ { \odot } GMCs are the dominant contributors to f _ { \text { esc,tot } } in all cases .
We also show that the accompanying star formation rates ( SFRs ) of our model ( \sim 2 \times 10 ^ { -2 } and 0.73 M _ { \odot } yr ^ { -1 } ) are consistent with observations of SFRs in dwarf starburst and spiral galaxies , respectively .