Disentangling the ISM phases of the dwarf galaxy NGC 4214 using [C ii] SOFIA/GREAT observations

Abstract : Context. The [C II] 158 μm fine structure line is one of the dominant cooling lines in the interstellar medium (ISM) and is an important tracer of star formation. Recent velocity-resolved studies with Herschel/HIFI and SOFIA/GREAT showed that the [C II] line can constrain the properties of the ISM phases in star-forming regions. The [C II] line as a tracer of star formation is particularly important in low-metallicity environments where CO emission is weak because of the presence of large amounts of CO-dark gas. Aims: The nearby irregular dwarf galaxy NGC 4214 offers an excellent opportunity to study an actively star-forming ISM at low metallicity. We analyzed the spectrally resolved [C II] line profiles in three distinct regions at different evolutionary stages of NGC 4214 with respect to ancillary H I and CO data in order to study the origin of the [C II] line. Methods: We used SOFIA/GREAT [C II] 158 μm observations, H I data from THINGS, and CO(2 → 1) data from HERACLES to decompose the spectrally resolved [C II] line profiles into components associated with neutral atomic and molecular gas. We use this decomposition to infer gas masses traced by [C II] under different ISM conditions. Results: Averaged over all regions, we associate about 46% of the [C II] emission with the H I emission. However, we can assign only 9% of the total [C II] emission to the cold neutral medium (CNM). We found that about 79% of the total molecular hydrogen mass is not traced by CO emission. Conclusions: On average, the fraction of CO-dark gas dominates the molecular gas mass budget. The fraction seems to depend on the evolutionary stage of the regions: it is highest in the region covering a super star cluster in NGC 4214, while it is lower in a more compact, more metal-rich region. Reduced SOFIA/GREAT data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr ( or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/599/A9
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K. Fahrion, D. Cormier, F. Bigiel, S. Hony, N. P. Abel, et al.. Disentangling the ISM phases of the dwarf galaxy NGC 4214 using [C ii] SOFIA/GREAT observations. Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 599, pp.A9. ⟨10.1051/0004-6361/201629341⟩. ⟨hal-02105251⟩



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