The Effects of Thermal Heating via the Dissipation of Turbulence on Physical Conditions in the Galactic Diffuse Ionized Gas

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Abstract
The observed properties of the diffuse ionized gas (DIG) in our Galaxy are not easily reconcilable with simple photoionization models. Photoionization models, however, can reproduce the observed properties of H II regions. This suggests that there are different or additional physical processes at work in the DIG. We have developed a model of the DIG whereby it is ionized by a relatively soft ionizing spectrum (Teff ≤ 32,000 K) and is also heated by an additional thermal mechanism: the dissipation of turbulence. This model predicts the same electron temperature, [N II] λ6583/Hα ratio, [S II] λ6716/Hα ratio, and He I λ5876/Hα ratio as observed in the DIG. The model suggests that the observed [O III] emission from the diffuse interstellar medium (ISM) does not originate from the oxygen in the DIG. Without the turbulent thermal heating term, this model will not reproduce the observed properties of the DIG. The dissipation of turbulence may also be important in other phases of the ISM.