Collisional excitation of [C II], [O I] and CO in Massive Galaxies

TL;DR

This paper investigates the excitation mechanisms of far-infrared and submillimeter emission lines in cold gas clouds within massive galaxies, proposing models that include additional pressure support to explain observed line ratios and star formation suppression.

Contribution

It introduces a theoretical model incorporating turbulence or magnetic fields to better match observed emission line ratios in galaxy filaments, advancing understanding of cold gas stability and excitation.

Findings

Additional pressure support explains line ratios better.

Models predict specific line ratios for future observations.

Cold gas stability linked to low star formation rates.

Abstract

Many massive galaxies at the centres of relaxed galaxy clusters and groups have vast reservoirs of cool (~10,000 K) and cold (~100 K) gas. In many low redshift brightest group and cluster galaxies this gas is lifted into the hot ISM 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 infra-red line ratios observed in the filaments. In this paper we examine the theoretical properties of dense, cold clouds emitting in the far infra-red and submillimeter through the bright lines of [C II]157 \mu m , [O I]63 \mu m and CO, exposed to these energetic ionising particles. While some emission lines may be optically thick we find this is not sufficient to model the emission line ratios. Models where the filaments are supported by thermal pressure support alone also cannot account for the cold gas line ratios but a very modest additional pressure support, either from turbulence or magnetic fields can fit the observed [O I]/[C II] line ratios by decreasing the density of the gas. This may also help stabilise the filaments against collapse leading to the low rates of star formation. Finally 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.