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

TL;DR

This study uses simulations of molecular clouds to estimate ultraviolet photon escape fractions in different galaxy types, revealing significant variability and key contributors to ionization processes.

Contribution

It provides the first detailed simulation-based analysis of UV escape fractions from GMC populations across galaxy types, incorporating cloud mass distributions and short-term fluctuations.

Findings

Escape fractions vary significantly with GMC mass, reaching over 90% in some cases.

Typical total escape fraction is around 8%, with larger fluctuations in dwarf galaxies.

Massive GMCs dominate the photon escape contribution in all models.

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.