Disentangling the ISM phases of the dwarf galaxy NGC 4214 using [CII]
  SOFIA/GREAT observations
  The [CII] 158 um 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 [CII] line can constrain the properties of the ISM phases in star-forming
regions. The [CII] 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. 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 [CII] line profiles in
three distinct regions at different evolutionary stages of NGC 4214 with
respect to ancillary HI and CO data in order to study the origin of the [CII]
line. We used SOFIA/GREAT [CII] 158 um observations, HI data from THINGS, and
CO(2-1) data from HERACLES to decompose the spectrally resolved [CII] line
profiles into components associated with neutral atomic and molecular gas. We
use this decomposition to infer gas masses traced by [CII] under different ISM
conditions. Averaged over all regions, we associate about 46% of the [CII]
emission with the HI emission. However, we can assign only around 9% of the
total [CII] emission to the cold neutral medium (CNM). We found that about 79%
of the total molecular hydrogen mass is not traced by CO emission. 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.
