  Carbon monoxide (CO) is widely used as a tracer of molecular hydrogen (H2) 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 H2 in
these metal-poor systems, but its suitability remains unproven. To help us to
assess how well atomic carbon traces H2 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 [CI] emission and H2 mass,
$X_{\rm CI}$, scales approximately as $X_{\rm CI} \propto Z^{-1}$. We recover a
similar scaling for the CO-to-H2 conversion factor, $X_{\rm CO}$, but find that
at this point in the evolution of the clouds, $X_{\rm CO}$ is consistently
smaller than $X_{\rm CI}$, by a factor of a few or more. We have also examined
how $X_{\rm CI}$ and $X_{\rm CO}$ evolve with time. We find that $X_{\rm CI}$
does not vary strongly with time, demonstrating that atomic carbon remains a
good tracer of H2 in metal-poor systems even at times significantly before the
onset of star formation. On the other hand, $X_{\rm CO}$ varies very strongly
with time in metal-poor clouds, showing that CO does not trace H2 well in
starless clouds at low metallicity.

Astrophysics of Galaxies