Synthetic CO emission and the X_COfactor of young molecular clouds: A convergence study

The properties of synthetic CO emission from 3D simulations of forming molecular clouds are studied within the SILCC-Zoom project. Since the time-scales of cloud evolution and molecule formation are comparable, the simulations include a live chemical network. Two sets of simulations with an increasing spatial resolution (dx = 3.9 pc to dx = 0.06 pc) are used to investigate the convergence of the synthetic CO emission, which is computed by post-processing the simulation data with the radmc-3d radiative transfer code. To determine the excitation conditions, it is necessary to include atomic hydrogen and helium alongside H2, which increases the resulting CO emission by ∼7-26 per cent. Combining the brightness temperature of 12CO and 13CO, we compare different methods to estimate the excitation temperature, the optical depth of the CO line and hence, the CO column density. An intensity-weighted average excitation temperature results in the most accurate estimate of the total CO mass. When the pixel-based excitation temperature is used to calculate the CO mass, it is over-/underestimated at low/high CO column densities where the assumption that 12CO is optically thick while 13CO is optically thin is not valid. Further, in order to obtain a converged total CO luminosity and hence XCO factor, the 3D simulation must have dx pdbl 0.1 pc. The XCO evolves over time and differs for the two clouds; yet pronounced differences with numerical resolution are found. Since high column density regions with a visual extinction larger than 3 mag are not resolved for dx pdbl 1 pc, in this case the H2 mass and CO luminosity both differ significantly from the higher resolution results and the local XCO is subject to strong noise. Our calculations suggest that synthetic CO emission maps are only converged for simulations with dx pdbl 0.1 pc.