Collisional excitation of [C II], [O I] and CO in massive galaxies

Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of warm (similar to 10 000 K) and cold (less than or similar to 100 K) gas. In many such low-redshift systems this gas is lifted into the hot interstellar medium in filamentary structures, which are long lived and are typically not forming stars. Two important questions are how far do these reservoirs cool and if cold gas is abundant what is the cause of the low star formation efficiency? Heating and excitation of the filaments from collisions and mixing of hot particles in the surrounding X-ray gas describes well the optical and near infrared line ratios observed in the filaments. In this paper we examine the theoretical properties of dense, cold clouds emitting in the far infrared and sub-millimetre through the bright lines of [C II] lambda 157 mu m, [O I] lambda 63 mu m and CO, exposed to such energetic ionizing particles. We find that optical depth effects and thermal pressure support alone cannot account for the line ratios; however, a very modest additional pressure support can fit the observed [O I] lambda 63 mu m/[C II] lambda 157 mu m line ratios by decreasing the density of the gas. Thismay also help stabilize the filaments against collapse leading to the low rates of star formation. We make predictions for the line ratios expected from cold gas under these conditions and present diagnostic diagrams for comparison with further observations. We provide our code as an Appendix.