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

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 H2 gas mass tracer challenges the long-held view of its distribution over only a relatively narrow layer in the CII/CI/CO transition zone in 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 →3P0 transition, that its surprising ubiquity in molecular clouds has been fully revealed. In the present work we show that under typical ISM conditions such an 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