Molecular gas is mostly traced by CO line emission. In particular 12CO(J=1-0) is bright and hence it is commonly used to estimate the molecular gas content of cold clouds. Yet, the CO molecule does not trace all molecular gas. It is more easily dissociated than molecular hydrogen and hence needs visual extinctions higher than 3 mag in order to survive. This leads to the existence of CO-dark molecular hydrogen. Once formed, the CO line becomes quickly optically thick. As a result, only a narrow range of column densities can actually be traced well with 12CO. In the SILCC-Zoom project, we follow the formation of molecular clouds by means of 3D (magneto-) hydrodynamics simulations on adaptive meshes, including a simplified chemical network with H2 and CO formation as well as a treatment for the (self-)shielding of the molecular gas. We post process the simulation data using e.g. the RADMC-3D code to generate synthetic observations. In this way we may study e.g. the XCO-factor (CO-to-H2 conversion factor) as a function of cloud properties, age, as well as numerical resolution. We can show that high resolution simulation data is necessary as otherwise the molecular gas fractions are not converged leaving behind unremovable signatures in the mock observations. The CO-dark gas fraction strongly depends on the magnetisation of the cloud and it is modified by stellar feedback once star formation has set in. Complementary tracers like the [CII] line are hence useful to gather a concise picture of the dense gas. [CII] is generally bright in photo-dissociation regions as there C+ is present in relatively dense gas. I will discuss what gas conditions lead to bright [CII] emission.
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