An alternative accurate tracer of molecular clouds: the 'X Ci -factor'

We explore the utility of Ci as an alternative high-fidelity gas-mass tracer for galactic molecular clouds. We evaluate the 'X Ci -factor' for the 609 μm carbon line, the analogue of the CO 'X-factor', which is the ratio of the H2 column density to the integrated 12CO(1-0) line intensity. We use 3d-pdr to post-process hydrodynamic simulations of turbulent, star-forming clouds. We compare the emission of Ci and CO for model clouds irradiated by 1 and 10 times the average background and demonstrate that Ci is a comparable or superior tracer of the molecular gas distribution for column densities up to 6 × 1023 cm−2. Our results hold for both reduced and full chemical networks. For our fiducial Galactic cloud, we derive an average X CO of 3.0 × 1020 cm−2 K−1 km−1 s and X Ci of 1.1 × 1021 cm−2 K−1 km−1 s.