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

Abstract

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 $I_{[\text{CII}]}>0.5$ 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 $Y_{\mathrm{CII}}\approx 1.1\times 10^{21}$ cm${}^{-2}$ (K km s${}^{-1}$)${}^{-1}$, and $Y_{\mathrm{CII}}$ scales with $I_{[\text{CII}]}^{-0.3}$. We expect $Y_{\mathrm{CII}}$ to change in environments with a lower or higher radiation field than simulated here.

arXiv comment27 pages, 23 figures, accepted for publication in MNRAS

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