C i lines as tracers of molecular gas, and their prospects at high redshifts

We examine the fine structure lines 3 P 1 → 3 P 0 (492 GHz) and 3 P 2 → 3 P 1 (809 GHz) of neutral atomic carbon as bulk molecular gas mass tracers and find that they can be good and on many occasions better than 12 CO transitions, especially at high redshifts. The notion of C  i emission as an H 2 gas mass tracer challenges the long‐held view of its distribution over only a relatively narrow layer in the C  ii /C  i /CO transition zone in far‐ultraviolet (FUV) illuminated molecular clouds. Past observations have indeed consistently pointed towards a more extended C  i distribution but it was only recently, with the advent of large‐scale imaging of its 3 P 1 → 3 P 0 transition, that its surprising ubiquity in molecular clouds has been fully revealed. In the present work we show that under typical interstellar medium conditions such a ubiquity is inevitable because of well‐known dynamic and non‐equilibrium chemistry processes maintaining a significant [C]/[ 12 CO] abundance throughout giant molecular clouds during their lifetime. These processes are more intense in star‐forming environments where a larger ambient cosmic ray flux will also play an important role in boosting [C]/[ 12 CO]. The resulting C  i lines can be bright and effective H 2 mass tracers especially for diffuse (∼10 2 –10 3 cm −3 ) gas while in UV‐intense and/or metal‐poor environments their H 2 ‐tracing capability diminishes because of large‐scale C  ii production but nevertheless remains superior to that of 12 CO. The best place to take full advantage of the capacity of C  i to trace H 2 is not in the low‐ z Universe, where large atmospheric absorption at 492 and 809 GHz precludes routine observations, but at high redshifts ( z ≳ 1) .