Velocity Structure of the ISM as Seen by the Spectral Correlation Function

Abstract

(Abridged) We use the statistical tool known as the ``Spectral Correlation Function" [SCF] to intercompare simulations and observations of the atomic interstellar medium. The simulations considered mimic three distinct sets of physical conditions. One of them (run "ISM") is intended to represent a mixture of cool and warm atomic gas, and includes self-gravity and magnetic fields. For each simulation, H I spectral-line maps are synthesized and intercompared, both with each other, and with observations, using the SCF. We find that, when thermal broadening is large in comparison with fine-scale turbulent velocity structure, it masks sub-thermal velocity sub-structure in the synthesized spectra. The H I observations we use here for comparison are of the North Celestial Pole (NCP) Loop. None of the simulations match the NCP Loop data very well. The most realistic sets of line profiles and SCF statistics comes from artifically rescaling the velocity axis of run ISM. Without rescaling, almost all velocity structure is smeared out by thermal broadening. However, if the velocity axis is expanded by a factor of 6, the SCF distributions of run ISM an the NCP Loop match up fairly well. This means that the ratio of thermal to turbulent pressure in run ISM is much too large as it stands, and that the simulation is deficient in turbulent energy. This is a consequence of run ISM not including the effects of supernovae. We conclude that the SCF is a useful tool for understanding and fine-tuning simulations of interstellar gas, and in particular that realistic simulations of the atomic ISM need to include the effects of energetic stellar winds (e.g. supernovae) in order for the ratio of thermal-to-turbulent pressure to give spectra representative of the observed interstellar medium in our Galaxy.