Is atomic carbon a good tracer of molecular gas in metal-poor galaxies?

Carbon monoxide (CO) is widely used as a tracer of molecular hydrogen (H-2) in metal-rich galaxies, but is known to become ineffective in low-metallicity dwarf galaxies. Atomic carbon has been suggested as a superior tracer of H-2 in these metal-poor systems, but its suitability remains unproven. To help us to assess how well atomic carbon traces H-2 at low metallicity, we have performed a series of numerical simulations of turbulent molecular clouds that cover a wide range of different metallicities. Our simulations demonstrate that in star-forming clouds, the conversion factor between [C I] emission and H-2 mass, X-CI, scales approximately as X-CI proportional to Z(-1). We recover a similar scaling for the CO-to-H-2 conversion factor, X-CO, but find that at this point in the evolution of the clouds, X-CO is consistently smaller than X-CI, by a factor of a few or more. We have also examined how X-CI and X-CO evolve with time. We find that X-CI does not vary strongly with time, demonstrating that atomic carbon remains a good tracer of H-2 in metal-poor systems even at times significantly before the onset of star formation. On the other hand, X-CO varies very strongly with time in metal-poor clouds, showing that CO does not trace H-2 well in starless clouds at low metallicity.