Synthetic [CII] emission maps of a simulated molecular cloud in
  formation
  The C$^{+}$ ion is an important coolant of interstellar gas, and so the [CII]
fine structure line is frequently observed in the interstellar medium. However,
the physical and chemical properties of the [CII]-emitting gas are still
unclear. We carry out non-LTE radiative transfer simulations with RADMC-3D to
study the [CII] line emission from a young, turbulent molecular cloud before
the onset of star formation, using data from the SILCC-Zoom project. The [CII]
emission is optically thick over 40% of the observable area with
$I_{[\textrm{CII}]} > 0.5$ K km s$^{-1}$. To determine the physical properties
of the [CII] emitting gas, we treat the [CII] emission as optically thin. We
find that the [CII] emission originates primarily from cold, moderate density
gas ($40 \lesssim T \lesssim 65$ K and $50 \lesssim n \lesssim 440$ cm$^{-3}$),
composed mainly of atomic hydrogen and with an effective visual extinction
between $\sim 0.50$ and $\sim 0.91$. Gas dominated by molecular hydrogen
contributes only $\lesssim$20% of the total [CII] line emission. Thus, [CII] is
not a good tracer for CO-dark H$_2$ at this early phase in the cloud's
lifetime. We also find that the total gas, H and C$^+$ column densities are all
correlated with the integrated [CII] line emission, with power law slopes
ranging from 0.5 to 0.7. Further, the median ratio between the total column
density and the [CII] line emission is $Y_{{\rm CII}}\approx 1.1 \times
10^{21}$ cm$^{-2}$ (K km s$^{-1}$)$^{-1}$, and $Y_{{\rm CII}}$ scales with
$I_{[\textrm{CII}]}^{-0.3}$. We expect $Y_{{\rm CII}}$ to change in
environments with a lower or higher radiation field than simulated here.
