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

K. Fahrion , D. Cormier , F. Bigiel (1) , S. Hony (2) , N. P. Abel , P. Cigan , T. Csengeri (3) , U. U. Graf , Vianney Lebouteiller (3) , S. C. Madden (4) , R. Wu , L. Young

1 ZAH - Zentrum für Astronomie der Universität Heidelberg
2 IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers
3 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
4 DAPNIA - Département d'Astrophysique, de physique des Particules, de physique Nucléaire et de l'Instrumentation Associée

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 (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/599/A9

Mots clés: galaxies: dwarf; galaxies: star formation; galaxies: individual: NGC 4214; ISM: lines and bands

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique galactique [astro-ph.GA]; Instrumentation et méthodes pour l'astrophysique [astro-ph.IM]; Phénomènes cosmiques de haute energie [astro-ph.HE]

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, 2017, 599, pp.A9. ⟨10.1051/0004-6361/201629341⟩. ⟨hal-02105251⟩