#### Impact of B fields and radiative feedback on CO emission

Within the SILCC-Zoom project, we study the impact of magnetic fields and radiative stellar feedback on the emission of CO using the CO, J=1-0 line. In Seifried et al., 2019 ( arxiv:1906.01015 ), we present a detailed analysis of the CO emission and its dependence on the local cloud properties.
We show that CO-bright gas is associated with gas below a temperature of ~50 K and above a density of ~$300 cm^{-3}$. The amount of CO-dark gas scales with the amount of gas at low visual extinction ($A_V < 1.5$) and a significant fraction (up to 65%) of the $H_2$ can be CO-dark. We also show that the large range of cloud conditions causes the $X_{CO}$ factors of the various clouds to have a large spread of up to a factor of 4 around a fiducial value of about $1.5 \times 10^{20} cm^{-2} (K km s^{-1})^{-1}$. This significantly affects the accuracy of determining the $H_2$ mass of molecular clouds and their virial state.
In order to overcome this problem, we suggest a new method to determine the $H_2$ mass of molecular clouds. It relies on the availability of CO, J=1-0 observations and $A_V$ maps. It reduces the uncertainty of the clouds’ overall $H_2$ mass to a factor of ~1.8 and is also applicable for sub-pc-scaled structures of the clouds.