The Herschel Dwarf Galaxy Survey - I. Properties of the low-metallicity ISM from PACS spectroscopy
By D. Cormier, S. Madden, V. Lebouteiller, N. Abel, S. Hony, F. Galliano, A. Rémy-Ruyer, F. Bigiel, Maarten Baes, A. Boselli and 10 more. 2015
149 citations recorded by the source corpus.
Context. The far-infrared (FIR) lines are important tracers of the cooling and physical conditions of the interstellar medium (ISM) and are rapidly becoming workhorse diagnostics for galaxies throughout the universe. There are clear indications of a different behavior of these lines at low metallicity that needs to be explored. Aims. Our goal is to explain the main differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies, and how this translates in ISM properties. Methods. We present Herschel/PACS spectroscopic observations of the [C ii] 157 μm, [O i] 63 and 145 μm, [O iii] 88 μm, [N ii] 122 and 205 μm, and [N iii] 57 μm fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-LTIR ratios with LTIR, LTIR/LB, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models. Results. We find that the FIR lines together account for up to 3 percent of LTIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of [O iii]88/[N ii]122 and [N iii]57/[N ii]122 are high, indicative of hard radiation fields. In the photodissociation region (PDR), the [C ii]157/[O i]63 ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the [O i]145/[O i]63 ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The [O iii]88/[O i]63 ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found to be ~4 times higher in the dwarfs than in metal-rich galaxies. The high [C ii]/LTIR, [O i]/LTIR, and [O iii]/LTIR ratios, which decrease with increasing LTIR and LTIR/LB, are interpreted as a combination of moderate far-UV fields and a low PDR covering factor. Harboring compact phases of a low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that of metal-rich galaxies.
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/a42fd185d27b2728cb165c964a0df3b66abab961
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Citations in the corpus
- Big Three Dragons: A z = 7.15 Lyman-break galaxy detected in [O iii] 88 μm, [C ii] 158 μm, and dust continuum with ALMA (2018, background): “In fact, based on Herschel spectroscopy for local dwarf galaxies, Cormier et al. (2015) have demonstrated that [Oiii] 88 µm becomes brighter than [Cii] at low metallic- ity (see also Malhotra et al. 2001).”
- A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies (2017, methodology): “…even though higher [O iii]88/[N ii]122 can be reached at sub-solar metallicities and could explain the location of the sources in the DGS studied by Cormier et al. (2015) (filled stars in Figure 8), this is not likely the reason for the increase in the line ratio observed in dusty systems like…”
- Detection of the Far-infrared [O iii] and Dust Emission in a Galaxy at Redshift 8.312: Early Metal Enrichment in the Heart of the Reionization Era (2018, methodology): “…simulation of galaxy formation (Shimizu et al. 2014) by incorporating an [O III] emission line model as a function of metallicity calibrated by ISO, AKARI, and Herschel observations of local galaxies (Brauher et al. 2008; Kawada et al. 2011; Madden et al. 2012, 2013, see also Cormier et al. 2015).”