The Case for Thermalization as a Contributor to the [CII] Deficit
  The [CII] deficit, which describes the observed decrease in the ratio of
[CII] 158 micron emission to continuum infrared emission in galaxies with high
star formation surface densities, places a significant challenge to the
interpretation of [CII] detections from across the observable universe. In an
attempt to further decode the cause of the [CII] deficit, the [CII] and dust
continuum emission from 18 Local Volume galaxies has been split based on
conditions within the interstellar medium where it originated. This is
completed using the Key Insights in Nearby Galaxies: a Far-Infrared Survey with
Herschel (KINGFISH) and Beyond the Peak (BtP) surveys and the wide-range of
wavelength information, from UV to far-infrared emission lines, available for a
selection of star-forming regions within these samples. By comparing these
subdivided [CII] emissions to isolated infrared emission and other properties,
we find that the thermalization (collisional de-excitation) of the [CII] line
in HII regions plays a significant role in the deficit observed in our sample.
