Synthetic observations of carbon lines of turbulent flows in the diffuse multiphase interstellar medium

We examine observational characteristics of multiphase turbulent flows in the diffuse interstellar medium (ISM) using a synthetic radiation field of atomic and molecular lines. We consider the multiphase ISM, which is formed by thermal instability under the irradiation of UV photons with moderate visual extinction A_V ∼ 1. Radiation field maps of C⁺, C ⁰, and CO line emissions were generated by calculating the non-local thermodynamic equilibrium (non-LTE) level populations from the results of high-resolution hydrodynamic simulations of diffuse ISM models. By analyzing the synthetic radiation field of carbon lines of [C II] 158 μm, [C I] ³P₂-³P₁ (809 GHz), ³P₁-³P₀ (492 GHz), and CO rotational transitions, we found a high ratio between the lines of high- and low-excitation energies in the diffuse multiphase ISM. This shows that simultaneous observations of the lines of warm- and cold-gas tracers will be useful in examining the thermal structure, and hence the origin, of diffuse interstellar clouds.