Simulating the UV Escape Fractions from Molecular Cloud Populations in Star-forming Dwarf and Spiral Galaxies

Abstract

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.

15 pages, 7 figures, accepted to MNRAS

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