Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of warm (∼10 000 K) and cold (≲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] λ157 μm , [O I] λ63 μm 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] λ63 μm/[C II] λ157 μm line ratios by decreasing the density of the gas. This may 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.

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Published in Monthly Notices of the Royal Astronomical Society, v. 455, issue 3, p. 3042-3057.

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

The copyright holders have granted the permission for posting the article here.

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Support for this work was provided by NASA through award number 1428053 issued by JPL/Caltech. GJF acknowledges support by NSF (1108928, 1109061, and 1412155), NASA (10-ATP10-0053, 10-ADAP10-0073, NNX12AH73G, and ATP13-0153), and STScI (HST-AR-13245, GO-12560, HST-GO-12309, GO-13310.002-A, and HST-AR-13914).