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

• Published in: Astronomy and astrophysics (Berlin) 2017-03, Vol.599, p.A9
• Main Authors: Fahrion, K., Cormier, D., Bigiel, F., Hony, S., Abel, N. P., Cigan, P., Csengeri, T., Graf, U. U., Lebouteiller, V., Madden, S. C., Wu, R., Young, L.
• Format: Article
• Language: English
• Subjects: Astrophysics; Cosmology and Extra-Galactic Astrophysics; Decomposition; Dwarf galaxies; Emission; Galactic Astrophysics; galaxies: dwarf; galaxies: individual: NGC 4214; galaxies: star formation; High Energy Astrophysical Phenomena; Instrumentation and Methods for Astrophysic; ISM: lines and bands; Phases; Physics; Spectral lines; Star clusters; Star formation; Tracers
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/201629341

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.