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

We examine the fine structure lines P-3(1) --> P-3(0) (492 GHz) and P-3(2) --> P-3(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 (CO)-C-12 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 P-3(1) --> P-3(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]/[(CO)-C-12] 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]/ [(CO)-C-12]. The resulting C I lines can be bright and effective H-2 mass tracers especially for diffuse (similar to10(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 it production but nevertheless remains superior to that of (CO)-C-12. 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 greater than or similar to 1).