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$_esc,GMC$, from a suite of individual GMC simulations with masses in
the range 10$^4-6$ M$_ødot$ using the adaptive-mesh refinement code FLASH.
We find significantly different f$_esc,GMC$ depending on the GMC mass
which can reach $>$90\% in the evolution of 5$\times$10$^4$ and 10$^5$
M$_ødot$ clouds or remain low at $\sim$5\% for most of the lifetime of more
massive GMCs. All clouds show fluctuations over short, sub-Myr timescales
produced by flickering HII regions. We combine our results to calculate the
total escape fraction (f$_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$_esc,tot$ values of
8\% 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\%. The photons escaping from the 5$\times$10$^4$ and 10$^5$
M$_ødot$ 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 ($\sim$2$\times$10$^-2$ and 0.73 M$_ødot$yr$^-1$) are consistent
with observations of SFRs in dwarf starburst and spiral galaxies, respectively.
Description
[1710.04283] Simulating the UV Escape Fractions from Molecular Cloud Populations in Star-forming Dwarf and Spiral Galaxies
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