Simulating the UV escape fractions from molecular cloud populations in star-forming dwarf and spiral galaxies

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, fesc,GMC, from a suite of individual GMC simulations with masses in the range 104–6 M⊙ using the adaptive-mesh refinement code flash. We find significantly different fesc,GMC depending on the GMC mass that can reach >90 per cent in the evolution of 5 × 104 and 105 M⊙ clouds or remain low at ∼5 per cent for most of the lifetime of more massive GMCs. All clouds show fluctuations over short, sub-Myr time-scales produced by flickering H ii regions. We combine our results to calculate the total escape fraction (fesc,tot) from GMC populations in dwarf starburst and spiral galaxies by randomly drawing clouds from a GMC mass distribution (dN/dM ∝ Mα, where α is either −1.5 or −2.5) over fixed time intervals. We find typical fesc,tot values of 8 per cent for both the dwarf and spiral models. The fluctuations of fesc,tot, however, are much larger for the dwarf models with values as high as 90 per cent. The photons escaping from the 5 × 104 and 105 M⊙ GMCs are the dominant contributors to fesc,tot in all cases. We also show that the accompanying star formation rates (SFRs) of our model (∼2 × 10−2 and 0.73 M⊙yr−1) are consistent with observations of SFRs in dwarf starburst and spiral galaxies, respectively.