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, f_esc,GMC, from a suite of individual GMC simulations with masses in the range 10^4-6 M_⊙ using the adaptive-mesh refinement code FLASH. We find significantly different f_esc,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 (f_esc,tot) from GMC populations in dwarf starburst and spiral galaxies by randomly drawing clouds from a GMC mass distribution (dN/dM ∝ M^α, where α is either -1.5 or -2.5) over fixed time intervals. We find typical f_esc,tot values of 8 per cent for both the dwarf and spiral models. The fluctuations of f_esc,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 f_esc,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.