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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 stillunclear. We carry out non-LTE radiative transfer simulations with RADMC-3D tostudy the [CII] line emission from a young, turbulent molecular cloud beforethe 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 propertiesof the [CII] emitting gas, we treat the [CII] emission as optically thin. Wefind that the [CII] emission originates primarily from cold, moderate densitygas ($40 \lesssim T \lesssim 65$ K and $50 \lesssim n \lesssim 440$ cm$^{-3}$),composed mainly of atomic hydrogen and with an effective visual extinctionbetween $\sim 0.50$ and $\sim 0.91$. Gas dominated by molecular hydrogencontributes only $\lesssim$20% of the total [CII] line emission. Thus, [CII] isnot a good tracer for CO-dark H$_2$ at this early phase in the cloud'slifetime. We also find that the total gas, H and C$^+$ column densities are allcorrelated with the integrated [CII] line emission, with power law slopesranging from 0.5 to 0.7. Further, the median ratio between the total columndensity and the [CII] line emission is $Y_{{\rm CII}}\approx 1.1 \times10^{21}$ 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 inenvironments with a lower or higher radiation field than simulated here.

Synthetic [CII] emission maps of a simulated molecular cloud in formation