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

Abstract

Context. Local metal-poor galaxies stand as ideal laboratories to probe the properties of the interstellar medium (ISM) in chemically unevolved conditions. Detailed studies of this primitive ISM can help gain insights on the physics of the first primordial galaxies that may be responsible for the Reionization. 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. The wealth of infrared (IR) tracers available in local galaxies and arising from different ISM phases allows us to constrain complex models in order to estimate physical quantities. Aims. Primitive galaxies with low metal and dust content have been shown to host a more patchy and porous ISM than their highmetallicity counterparts, with numerous density-bounded regions from where ionizing photons might leak out. To what extent this peculiar structure contributes to the leakage of ionizing photons remains to be quantitatively studied. Such effects can only be investigated by accounting for the complexity and inhomogeneity of the ISM. We aim to provide a new statistical framework to quantify various galactic observables by constraining a representative multi-phase and multi-sector topology using a combination of 1D models. Methods. 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 ( f esc,HII ). We apply this new code to a sample of 39 well-know local starbursting dwarf galaxies from the Dwarf Galaxy Survey (Madden et al. 2013). Results. 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 f esc,HII . The predicted f esc,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 f esc,HII . Finally, we provide observational line ratios which could be used as tracers of the photons escaping from density-bounded regions. Among other, we discuss the possible caveats of diagnostics based on [C ii]158µm in low-metallicity environments as we find a strong metallicity dependency of the fraction of [C ii]158µm emitted in the different ISM phases. Conclusions. The new framework presented in this paper allows us to use suites of unresolved IR emission lines to constrain various galactic parameters, including the escape fraction of ionizing photons from H ii regions. Although this multi-sector modelling remains too simple to fully capture the ISM comple...

Detections of [C II] 158 $μ$m and [O III] 88 $μ$m in a Local Lyman Continuum Emitter, Mrk 54, and its Implications to High-redshift ALMA Studies

“…The positive correlation between the two quantities is in qualitative agreement with the predictions of Inoue et al (2016), Katz et al (2022), andRamambason et al (2022).…”

“…The authors found that the parameter mostly affecting the line ratio is the burstiness. More recently, Ramambason et al (2022) examined ISM properties of a local sample of the Herschel Dwarf Galaxy Survey (HDGS, Madden et al 2013;Cormier et al 2015) using a grid of models of H ii regions and photo-dissociated regions. The authors have also found a positive correlation between [O iii]/[C ii]and predicted f esc .…”

Modeling the High-energy Ionizing Output from Simple Stellar and X-Ray Binary Populations

“…There have been a number of works in the literature exploring the efficacy of X-ray sources in the production of high-energy ionizing photons, with a particular recent emphasis on the X-ray contribution to production of He II λ4686 in high-redshift galaxies and their analogs (e.g., Kehrig et al 2018Kehrig et al , 2021Schaerer et al 2019;Senchyna et al 2020;Simmonds et al 2021;Kovlakas et al 2022;Oskinova & Schaerer 2022;Ramambason et al 2022;Umeda et al 2022). The most recent investigations fall broadly into two categories:…”

“…We note that fixing R and varying only ionizing flux means certain geometric effects are not accounted for in these models; such geometrical effects can substantially change both high-ionization and neutral line emission (e.g.,Ramambason et al 2022). …”

Hubble Space Telescope Observations of Tadpole Galaxies Kiso3867, SBS0, SBS1, and UM461

“…The large predicted Hα fluxes compared to the observed values, particularly for SBS1 and Kiso3867, raise the possibility that some of the expected LyC photons are being absorbed by dust or escaping the observed HII regions and diffuse Hα. Low-metallicity galaxies like these typically have more LyC escape (Ramambason et al 2022). Deeper images will be required to determine if escaping LyC is ionizing an extended halo around the galaxies, or is possibly escaping the galaxies altogether.…”