The Herschel Dwarf Galaxy Survey

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.

Observational and theoretical constraints on the formation and early evolution of the first dust grains in galaxies at 5 < z < 10

“…Distribution of the γ Low−zZ (left panel) and q PAH_Low−zZ (right panel) dust parameters from Draine & Li's models for the Low-zZ galaxy sample. The γ Low−zZ distribution peaks at 0.75 ± 0.07 and most of the q PAH = 0.47 which is the lowest value in Draine & Li's models.γ Low−zZ is assumed to be valid for the Hi-z LBG sample and q PAH_Low−zZ is fully consistent with our assumed q PAH_Hi−z Cormier et al 2019…”

Herschel spectroscopy of massive young stellar objects in the Magellanic Clouds

“…Theoretical and observed [C ii]/[N ii] 122 µm ratios are used to calculate the PDR gas fraction for the Galactic sample (Appendix E); a median of ∼ 60% of the [C ii] emission originates from the PDR gas component, broadly consistent with other estimates (Oberst et al 2011;Bernard-Salas et al 2012). Despite the large scatter for both Galactic and Magellanic samples, these fractions are also consistent with an increased contribution from the ionised gas component for higher metallicity environments (e.g., Cormier et al 2019, and references therein).…”

A dense, solar metallicity ISM in the z = 4.2 dusty star-forming galaxy SPT 0418−47

“…The advantage of the 145 µm line is that it is not affected by extinction and self-absorption. Cormier et al (2019) also showed that adding X-ray dominated regions to their cloudy models leads to a slight increase by a factor 1.2 in the [OI] 145 µm/63 µm luminosity ratio (their Fig. 8).…”

“…7) do not find such a trend in the local luminious infrared galaxies from GOALS. The only galaxies that show clearly higher [OIII] 88 µm / [OI] 63 µm ratios are dwarf galaxies, which has been interpreted as indicating the presence of low density channels allowing the hard UV photons to reach the outer parts of galaxies (Cormier et al 2015(Cormier et al , 2019. As a result, these low metallicity galaxies are likely to be filled with relatively low density (below n e − crit ([OIII] 88 µm)≃500 cm −3 ) gas rather than PDRs.…”