5 Summary

The thermodynamics of the ISM near the Sun are strongly influenced by the disposition of carbon, in grains and in the gas. Spectroscopy of neutral and ionized carbon affords the opportunity to probe the processes which are most basic to the structure of the gaseous medium, nearby and in damped Lyman-$\alpha$ systems. For whatever reason, the very distant gas is easily understood in the same terms as that seen nearby. Here, we have shown that one seemingly disparate aspect of high-z systems, their small fractions of molecular gas, can also be easily understood. Even when cool gas is present, which must be the case for the systems discussed here at z < 2.3, abundances of ${\rm H}_2$ are suppressed by many orders of magnitude at lower metallicity as a result of the sharply non-linear nature of the processes required to maintain substantial columns of ${\rm H}_2$. No wholesale reorganization of the gaseous medium need be hypothesized to account for low ${\rm H}_2$ abundances. Conversely, we showed that the slow gas-phase processes which formed ${\rm H}_2$ in the early Universe provide for a minimum molecular fraction in the range 10^-8-10^-7.

Acknowledgements

The National Radio Astronomy Observatory is operated by AUI, Inc. under a cooperative agreement with the US National Science Foundation. The referee, Mark Wolfire, is thanked for helpful comments.