C II RADIATIVE COOLING OF THE GALATIC DIFFUSE INTERSTELLAR MEDIUM: INSIGHT INTO THE STAR FORMATION IN DAMPED Ly alpha SYSTEMS

The far-infrared C II] 158 mu m fine structure transition is considered to be a dominant coolant in the interstellar medium (ISM). For this reason, under the assumption of a thermal steady state, it may be used to infer the heating rate and, in turn, the star formation rate (SFR) in local as well as in high redshift systems. In this work, radio and ultraviolet observations of the Galactic ISM are used to understand whether C II is indeed a good tracer of the SFR. For a sample of high Galactic latitude sightlines, direct measurements of the temperature indicate the presence of C II in both the cold and the warm phases of the diffuse interstellar gas. The cold gas fraction (similar to 10%-50% of the total neutral gas column density) is not negligible even at high Galactic latitude. It is shown that to correctly estimate the SFR, C II cooling in both phases should hence be considered. The simple assumption, that the C II] line originates only from either the cold or the warm phase, significantly underpredicts or overpredicts the SFR, respectively. These results are particularly important in the context of Damped Ly alpha systems for which a similar method is often used to estimate the SFR. The derived SFRs in such cases may not be reliable if the temperature of the gas under consideration is not constrained independently