Inferring the HII region escape fraction of ionizing photons from infrared emission lines in metal-poor star-forming dwarf galaxies

TL;DR

This study develops a Bayesian modeling approach to estimate the escape fraction of ionizing photons from H II regions in dwarf galaxies, revealing higher escape fractions in low-metallicity galaxies and identifying spectral tracers for photon leakage.

Contribution

It introduces a new multi-sector Bayesian model incorporating density-bounded regions and X-ray sources to quantitatively predict ionizing photon escape fractions in dwarf galaxies.

Findings

Low-metallicity galaxies have higher ISM porosity and escape fractions.

Predicted escape fractions can reach up to 60% in low-metallicity galaxies.

Specific star-formation rate correlates strongly with escape fraction.

Abstract

(abridged) Quantifying the ISM porosity to ionizing photons in nearby galaxies may improve our understanding of the mechanisms leading to Lyman Continuum photons leakage from galaxies. Primitive galaxies with low metal and dust content have been shown to host a more patchy and porous ISM than their high-metallicity counterparts. To what extent this peculiar structure contributes to the leakage of ionizing photons remains to be quantitatively studied. To address these questions we build a refined grid of models including density-bounded regions and a possible contribution of an X-ray source. Using MULTIGRIS, a new Bayesian code based on Monte Carlo sampling, we combine the models as sectors under various assumptions to extract the probability density distributions of the parameters and infer the corresponding escape fractions from H II regions (fesc,HII). We apply this new code to a sample of 39 well-know local starbursting dwarf galaxies from the Dwarf Galaxy Survey. We confirm previous results hinting at an increased porosity to ionizing photons of the ISM in low-metallicity galaxies and provide, for the first time, quantitative predictions for fesc,HII. The predicted fesc,HII for low-metallicity objects span a large range of values, up to 60%, while the values derived for more metal-rich galaxies are globally lower. We also examine the influence of other parameters on the escape fractions, and find that the specific star-formation rate correlates best with fesc,HII . Finally, we provide observational line ratios which could be used as tracers of the photons escaping from density-bounded regions. Although this multi-sector modelling remains too simple to fully capture the ISM complexity, it can be used to preselect galaxy samples with potential leakage of ionizing photons based on current and up-coming spectral data in unresolved surveys of local and high-redshift galaxies.