[Paper Review] The Herschel Dwarf Galaxy Survey: II. Physical conditions, origin of [CII] emission, and porosity of the multiphase low-metallicity ISM

D. Cormier, N. P. Abel+8 more|arXiv (Cornell University)|Apr 17, 2019

Astrophysics and Star Formation StudiesPhysics and Astronomy112 references28 citations

TL;DR

This study models the multiphase interstellar medium (ISM) of low-metallicity dwarf galaxies from the Herschel Dwarf Galaxy Survey using the Cloudy spectral synthesis code to characterize physical conditions, [C II] emission origins, and ISM porosity. It finds that low metallicity drives higher ionization parameters, increased PDR covering factors, and greater ISM porosity—key factors facilitating ionizing photon escape, with [C II] emission predominantly originating from PDRs (not ionized gas), making it a robust SFR tracer in low-metallicity and high-redshift galaxies.

ABSTRACT

The sensitive infrared telescopes, Spitzer and Herschel, have been used to target low-metallicity star-forming galaxies, allowing us to investigate the properties of their interstellar medium (ISM) in unprecedented detail. Interpretation of the observations in physical terms relies on careful modeling of those properties. We have employed a multiphase approach to model the ISM phases (HII region and photodissociation region) with the spectral synthesis code Cloudy. Our goal is to characterize the physical conditions (gas densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel Dwarf Galaxy Survey. We are particularly interested in correlations between those physical conditions and metallicity or star-formation rate. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [CII]157um line, and the characterization of the porosity of the ISM. We find that the lower-metallicity galaxies of our sample tend to have higher ionization parameters and galaxies with higher specific star-formation rates have higher gas densities. The [CII] emission arises mainly from PDRs and the contribution from the ionized gas phases is small, typically less than 30% of the observed emission. We also find correlation - though with scatter - between metallicity and both the PDR covering factor and the fraction of [CII] from the ionized gas. Overall, the low metal abundances appear to be driving most of the changes in the ISM structure and conditions of these galaxies, and not the high specific star-formation rates. These results demonstrate in a quantitative way the increase of ISM porosity at low metallicity. Such porosity may be typical of galaxies in the young Universe.

Motivation & Objective

• To characterize the physical conditions (gas density, ionization parameter, radiation field) in the ISM of low-metallicity dwarf galaxies from the Herschel Dwarf Galaxy Survey.
• To determine the relative contributions of ionized gas and photodissociation regions (PDRs) to [C II] 157 µm emission.
• To investigate the role of metallicity versus specific star-formation rate (sSFR) in shaping ISM structure and porosity.
• To quantify the ISM covering factors and porosity, with implications for ionizing photon escape in high-redshift galaxies.

Proposed method

• Employed the spectral synthesis code Cloudy to model H II regions and PDRs in individual galaxies.
• Used observed FIR fine-structure line fluxes (e.g., [C II], [O III], [N II]) as constraints to tune model parameters.
• Applied a multiphase modeling approach with variable ionization parameters and density profiles to match observed MIR-FIR emission.
• Varied key input parameters (radiation field, dust-to-gas ratio, X-ray luminosity, cosmic ray rate) to assess model sensitivity.
• Calibrated models using ancillary data and previous studies to refine assumptions on abundances and geometry.
• Quantified PDR covering factors and ionized gas filling factors to assess ISM porosity and photon escape potential.

Experimental results

Research questions

• RQ1 What physical conditions (density, ionization parameter, radiation field) characterize the ISM in low-metallicity dwarf galaxies?
• RQ2 What is the dominant ISM phase responsible for [C II] 157 µm emission—ionized gas or PDRs?
• RQ3 How do metallicity and specific star-formation rate correlate with ISM porosity and covering factors?
• RQ4 To what extent does low metallicity drive structural and physical changes in the ISM compared to high-sSFR effects?
• RQ5 Can the observed [O I] 145/63 line ratio be reproduced with standard models, or does it require additional heating mechanisms?

Key findings

• Low-metallicity galaxies exhibit higher ionization parameters (log(U) ≈ −3.0 to −0.3) and higher PDR densities (ne ≈ 10^0.5 to 10^3.0 cm⁻³) than metal-rich galaxies.
• The contribution of ionized gas to [C II] emission is typically less than 30%, confirming that [C II] is predominantly a PDR tracer in low-metallicity environments.
• A clear correlation exists between decreasing metallicity and decreasing PDR covering factor relative to ionized gas, indicating increased ISM porosity.
• The PDR covering factor decreases with decreasing metallicity and decreasing TIR-to-FUV luminosity ratio, suggesting a structural evolution in the ISM driven by metallicity.
• The ionization parameter is most sensitive to the choice of radiation field library, while dust-to-gas ratio and density profile affect PDR density estimates.
• A low-luminosity soft X-ray component and/or increased cosmic ray rate are required to reproduce the observed [O I] 145/63 line ratio in some galaxies.