The Herschel Dwarf Galaxy Survey

D. Cormier,N. P. Abel,F. L. Polles,S. C. Madden,V. Lebouteiller,M. Galametz,F. Galliano,A. Lambert-Huyghe,S. Hony,I. De Looze

doi:10.1051/0004-6361/201834457

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 (H II 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 activity. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [C II]157 μm 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 [C II] 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 a correlation – though with scatter – between metallicity and both the PDR covering factor and the fraction of [C II] 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.

Highlights

The interstellar medium (ISM) of galaxies is a complex environment composed of phases with inhomogeneous structures and different physical conditions, evolving through dynamical effects such as feedback from massive stars or turbulence (e.g., Hennebelle & Chabrier 2008; Hopkins et al 2012; Naab & Ostriker 2017)
We remind the reader that G0 is the intensity of the ultraviolet radiation field at the photodissociation regions (PDR) front, given in units of the background Habing (1968) value, 1.6 × 10−3 erg cm−2 s−1, as defined by Tielens & Hollenbach (1985)
In the case of single models, we find that the fractions of [C ii] and [Si ii] predicted in the H ii region are small, typically less than 10% for [C ii] and less than 40% for [Si ii]

Summary

Introduction

The interstellar medium (ISM) of galaxies is a complex environment composed of phases with inhomogeneous structures and different physical conditions, evolving through dynamical effects such as feedback from massive stars or turbulence (e.g., Hennebelle & Chabrier 2008; Hopkins et al 2012; Naab & Ostriker 2017). Characterizing the ISM in galaxies is necessary to understand how dense structures are formed and how the environmental properties (metal content, activity, stellar mass) may affect the star-formation process and thereby galaxy evolution. Space and airborne missions in the infrared, with IRAS, ISO, KAO, Spitzer, Herschel, and SOFIA, have been fundamental for unveiling a large part the energy budget of galaxies and enabling for a better census of the cooling of the ISM.

Objectives

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Discussion

Conclusion