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Carbon Monoxide (CO) is the second most abundant molecule after molecular hydrogen (H2). While H2 is not observed due to a lack of dipole moment, CO is easily observable using ground based telescopes, and its emission is commonly used to determine the H2 mass of molecular clouds by the XCOfactor. 
Simulations of star formation processes include on the fly chemistry, which takes into account the formation of H2 and CO in the course of the simulation. Recently, it was found that the used chemistry prescription needs strict resolution criteria for convergence (Joshi et al. 2019). 
The way how to compare simulations to actual observations is to perform synthetic observations. We investigate the convergence criteria of resolution for synthetic observations of CO emission. The task is addressed by radiative transfer code RADMC-3D. 
The synthetic observations are performed on molecular clouds, which self-consistently condense out of the diffuse medium in the SILCC zoom-in simulations computed by Seifried et al. (2017). The smallest grid cell size varies from 3.9 to 0.06 pc. 
The figure shows the synthetic emission maps in one line of sight for different resolution levels, as indicated above each subplot. The black contour indicates the observable area, where the emission is above 0.1 K km/s. With increasing resolution, the structure of the molecular cloud becomes more defined, and also the CO emission and the XCO factor both increase with increasing resolution. We find, that convergence has not yet been achieved at 0.06 pc. 

Synthetic CO emission maps from the SILCC zoom-in simulations. From top left to lower right, the panels show simulations with increasing resolution from 3.9 to 0.06 pc. The black contours indicate the observable area where the integrated intensity is above 0.1 K km/s. The point of the highest integrated intensity is shown by a cross, the value of which is listed in the legend.