2004MNRAS.351..147P

We examine the fine structure lines ³P₁⟶³P₀(492 GHz) and ³P₂⟶³P₁(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 ¹²CO transitions, especially at high redshifts. The notion of Ci emission as an H₂gas mass tracer challenges the long-held view of its distribution over only a relatively narrow layer in the Cii/Ci/CO transition zone in far-ultraviolet (FUV) illuminated molecular clouds. Past observations have indeed consistently pointed towards a more extended Ci distribution but it was only recently, with the advent of large-scale imaging of its ³P₁⟶³P₀transition, 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]/[¹²CO] 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]/[¹²CO]. The resulting Ci lines can be bright and effective H₂mass tracers especially for diffuse (∼10²-10³/cm3) gas while in UV-intense and/or metal-poor environments their H₂-tracing capability diminishes because of large-scale Cii production but nevertheless remains superior to that of ¹²CO. The best place to take full advantage of the capacity of Ci to trace H₂is not in the low-z Universe, where large atmospheric absorption at 492 and 809 GHz precludes routine observations, but at high redshifts (z≳ 1).