An alternative accurate tracer of molecular clouds: The 'XCI-factor'

Abstract

We explore the utility of CI as an alternative high-fidelity gas-mass tracer for galactic molecular clouds. We evaluate the 'XCI-factor' for the 609 μm carbon line, the analogue of the CO 'Xfactor', 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 XCO of 3.0 × 1020 cm-2 K-1 km-1 s and XCI of 1.1 × 1021 cm-2 K-1 km-1 s. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.